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a range decrease over the last thousand years, surviving only in Central European forests with the last wild population going extinct in Bialowieza forest in 1921. Starting from 1929, reintroduction of animals from zoos allowed the species to recover in the wild. The historical range of Bison bonasus was limited to forested areas, so since at least the sixteenth century conservation measures to preserve the species were based on the assumption that a forest would be the optimal habitat of the species. Ecological, morphological and paleoecological evidences, however, shows that B. bonasus is best adapted to open or mixed environments, indicating that the species was "forced" into a suboptimal habitat due to human influences such as habitat loss, competition with livestock, diseases and hunting. This information has been applied recently to adopt measures more suitable for the conservation of the species. === Deep-time conservation paleobiology === The deep-time approach uses examples of species, communities and ecosystem responses to environmental changes on a longer geologic record, as an archive of natural ecological and evolutionary laboratory. This approach provides examples to infer possible settings concerning climate warming, introduction of invasive species and decline in cultural eutrophication. This also permits the identification of species responses to perturbations of various types and scale to serve as a model for the future scenarios, for example abrupt climate change or volcanic winters. Given its deep-time nature, this approach allows for testing how organisms or ecosystems react to a bigger set of conditions than what is observable in the modern world or in the recent past. ==== Example - Insect damage and increasing temperatures ==== A pressing issue related to current global warming is the potential expansion in the range of tropical and subtropical crop pests, however the signal related to this poleward expansion is not clear.
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{
"page_id": 59573391,
"source": null,
"title": "Conservation paleobiology"
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The analyses of the fossil record from past warm intervals of Earth's history (Paleogene-Eocene Thermal Maximum) provides an adequate comparison to test this hypothesis. Data shows that, during warmer climates, the frequency and diversity of insect damage to North American plants increased significantly, providing support to the hypothesis of pests expansion due to global warming. == Relevance to conservation biology == Over the years, numerous attempts have been made to increase the synergy between paleobiologists and conservation scientists and managers. Despite being recognized as a useful tool to address current biodiversity problems, fossil data is still rarely included in contemporary conservation-related research, with the vast majority of studies focusing on short timescales. However, a few authors have used comparisons of extinction in the geologic past to taxon losses in modern times providing important perspectives on the severity of the modern biodiversity crisis Marine Paleobiology is an interdisciplinary study that utilizes the tools of paleontology and applies them to marine conservation biology. Looking at the deep-time fossil record separates this field from historical ecology. == References ==
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{
"page_id": 59573391,
"source": null,
"title": "Conservation paleobiology"
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In the Babylonian magico-medical tradition, Šulak is the lurker of the bathroom or the demon of the privy. Šulak appears in the Babylonian Diagnostic Handbook (Tablet XXVII), in which various diseases are described and attributed to the hand of a god, goddess, or spirit. A lurker is a type of demon who lies in wait in places where a potential victim is likely to be alone. When a man attends to excretory functions or elimination, he is exposed and hence vulnerable: "Šulak will hit him!" The "hit" may be a type of stroke (mišittu). Ancient folk etymology held that the name Šulak derived from a phrase meaning "dirty hands", due to his dwelling in the bīt musâti - literally "house of rinse-water", i.e. lavatory. Šulak is described in Akkadian sources as a rampant or bipedal but otherwise normal looking lion. The demon referred to as "The Hitter" or "Striker" elsewhere in the handbook may be Šulak identified by an epithet. A much earlier reference to this demon is found in a Hittite diagnostic text. Ancient Mesopotamian medical texts attribute cases of paralysis and stroke to the action of Šulak, a connection possibly due to fears that excessive strain on the toilet could cause such maladies. Protective amulets in the form of the Lion Centaur Urmahlullu, or cuneiform tablets inscribed with spells to ward off Šulak, were often buried in the doorways of lavatories, or in the foundations of the house, or deposited in drainage pipes. == In the Talmud == A similar lavatory demon takes the form of a goat in the Talmud (Shabbat 67a, Berachot 62a). This "demon of the privy" (Sheid beit ha-Kisset) appears also in the Babylonian Talmud: The Rabbis taught: On coming from a privy a man should not have sexual intercourse till he has waited
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"page_id": 22414479,
"source": null,
"title": "Šulak"
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long enough to walk half a mil, Stroke and epilepsy were closely related in ancient medicine. This law is not included in the Mishneh Torah. The "demon of the privy" is the type of unclean spirit that in the early Christian era was regarded as causing both physical and spiritual affliction. == See also == Triptych, May–June 1973 by Francis Bacon Unclean spirit == Notes == == Sources == Geller, M.J. "West Meets East: Early Greek and Babylonian Diagnosis." In Magic and Rationality in Ancient Near Eastern and Graeco-Roman Medicine, Studies in Ancient Medicine 27 (Brill, 2004), p. 19 online. George, A.R. (2015). On Babylonian Lavatories and Sewars. Iraq, 77: pp 75–106. Rosner, Fred. Encyclopedia of Medicine in the Bible and the Talmud. Rowman & Littlefield, 2000, p. 96 online. Stol, Marten. Epilepsy in Babylonia. Brill, 1993, pp. 17, 71, and 76 online. Stol, Marten. Birth in Babylonia and the Bible: Its Mediterranean Setting. Brill, 2000, p. 167 online. == Further reading == *Manekin Bamberger, Avigail. "An Akkadian Demon in the Talmud: Between Šulak and Bar-Širiqa", JSJ 44.2 (2013), 282-287.
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{
"page_id": 22414479,
"source": null,
"title": "Šulak"
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A spectrogram is a visual representation of the spectrum of frequencies of a signal as it varies with time. When applied to an audio signal, spectrograms are sometimes called sonographs, voiceprints, or voicegrams. When the data are represented in a 3D plot they may be called waterfall displays. Spectrograms are used extensively in the fields of music, linguistics, sonar, radar, speech processing, seismology, ornithology, and others. Spectrograms of audio can be used to identify spoken words phonetically, and to analyse the various calls of animals. A spectrogram can be generated by an optical spectrometer, a bank of band-pass filters, by Fourier transform or by a wavelet transform (in which case it is also known as a scaleogram or scalogram). A spectrogram is usually depicted as a heat map, i.e., as an image with the intensity shown by varying the colour or brightness. == Format == A common format is a graph with two geometric dimensions: one axis represents time, and the other axis represents frequency; a third dimension indicating the amplitude of a particular frequency at a particular time is represented by the intensity or color of each point in the image. There are many variations of format: sometimes the vertical and horizontal axes are switched, so time runs up and down; sometimes as a waterfall plot where the amplitude is represented by height of a 3D surface instead of color or intensity. The frequency and amplitude axes can be either linear or logarithmic, depending on what the graph is being used for. Audio would usually be represented with a logarithmic amplitude axis (probably in decibels, or dB), and frequency would be linear to emphasize harmonic relationships, or logarithmic to emphasize musical, tonal relationships. == Generation == Spectrograms of light may be created directly using an optical spectrometer over time.
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"title": "Spectrogram"
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Spectrograms may be created from a time-domain signal in one of two ways: approximated as a filterbank that results from a series of band-pass filters (this was the only way before the advent of modern digital signal processing), or calculated from the time signal using the Fourier transform. These two methods actually form two different time–frequency representations, but are equivalent under some conditions. The bandpass filters method usually uses analog processing to divide the input signal into frequency bands; the magnitude of each filter's output controls a transducer that records the spectrogram as an image on paper. Creating a spectrogram using the FFT is a digital process. Digitally sampled data, in the time domain, is broken up into chunks, which usually overlap, and Fourier transformed to calculate the magnitude of the frequency spectrum for each chunk. Each chunk then corresponds to a vertical line in the image; a measurement of magnitude versus frequency for a specific moment in time (the midpoint of the chunk). These spectrums or time plots are then "laid side by side" to form the image or a three-dimensional surface, or slightly overlapped in various ways, i.e. windowing. This process essentially corresponds to computing the squared magnitude of the short-time Fourier transform (STFT) of the signal s ( t ) {\displaystyle s(t)} — that is, for a window width ω {\displaystyle \omega } , s p e c t r o g r a m ( t , ω ) = | S T F T ( t , ω ) | 2 {\displaystyle \mathrm {spectrogram} (t,\omega )=\left|\mathrm {STFT} (t,\omega )\right|^{2}} . == Limitations and resynthesis == From the formula above, it appears that a spectrogram contains no information about the exact, or even approximate, phase of the signal that it represents. For this reason, it is
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"page_id": 263317,
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"title": "Spectrogram"
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not possible to reverse the process and generate a copy of the original signal from a spectrogram, though in situations where the exact initial phase is unimportant it may be possible to generate a useful approximation of the original signal. The Analysis & Resynthesis Sound Spectrograph is an example of a computer program that attempts to do this. The pattern playback was an early speech synthesizer, designed at Haskins Laboratories in the late 1940s, that converted pictures of the acoustic patterns of speech (spectrograms) back into sound. In fact, there is some phase information in the spectrogram, but it appears in another form, as time delay (or group delay) which is the dual of the instantaneous frequency. The size and shape of the analysis window can be varied. A smaller (shorter) window will produce more accurate results in timing, at the expense of precision of frequency representation. A larger (longer) window will provide a more precise frequency representation, at the expense of precision in timing representation. This is an instance of the Heisenberg uncertainty principle, that the product of the precision in two conjugate variables is greater than or equal to a constant (B*T>=1 in the usual notation). == Applications == Early analog spectrograms were applied to a wide range of areas including the study of bird calls (such as that of the great tit), with current research continuing using modern digital equipment and applied to all animal sounds. Contemporary use of the digital spectrogram is especially useful for studying frequency modulation (FM) in animal calls. Specifically, the distinguishing characteristics of FM chirps, broadband clicks, and social harmonizing are most easily visualized with the spectrogram. Spectrograms are useful in assisting in overcoming speech deficits and in speech training for the portion of the population that is profoundly deaf. The studies
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{
"page_id": 263317,
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"title": "Spectrogram"
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of phonetics and speech synthesis are often facilitated through the use of spectrograms. In deep learning-keyed speech synthesis, spectrogram (or spectrogram in mel scale) is first predicted by a seq2seq model, then the spectrogram is fed to a neural vocoder to derive the synthesized raw waveform. By reversing the process of producing a spectrogram, it is possible to create a signal whose spectrogram is an arbitrary image. This technique can be used to hide a picture in a piece of audio and has been employed by several electronic music artists. See also Steganography. Some modern music is created using spectrograms as an intermediate medium; changing the intensity of different frequencies over time, or even creating new ones, by drawing them and then inverse transforming. See Audio timescale-pitch modification and Phase vocoder. Spectrograms can be used to analyze the results of passing a test signal through a signal processor such as a filter in order to check its performance. High definition spectrograms are used in the development of RF and microwave systems. Spectrograms are now used to display scattering parameters measured with vector network analyzers. The US Geological Survey and the IRIS Consortium provide near real-time spectrogram displays for monitoring seismic stations Spectrograms can be used with recurrent neural networks for speech recognition. Individuals' spectrograms are collected by the Chinese government as part of its mass surveillance programs. For a vibration signal, a spectrogram's color scale identifies the frequencies of a waveform's amplitude peaks over time. Unlike a time or frequency graph, a spectrogram correlates peak values to time and frequency. Vibration test engineers use spectrograms to analyze the frequency content of a continuous waveform, locating strong signals and determining how the vibration behavior changes over time. Spectrograms can be used to analyze speech in two different applications: automatic detection of
|
{
"page_id": 263317,
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"title": "Spectrogram"
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speech deficits in cochlear implant users and phoneme class recognition to extract phone-attribute features. In order to obtain a speaker's pronunciation characteristics, some researchers proposed a method based on an idea from bionics, which uses spectrogram statistics to achieve a characteristic spectrogram to give a stable representation of the speaker's pronunciation from a linear superposition of short-time spectrograms. Researchers explore a novel approach to ECG signal analysis by leveraging spectrogram techniques, possibly for enhanced visualization and understanding. The integration of MFCC for feature extraction suggests a cross-disciplinary application, borrowing methods from audio processing to extract relevant information from biomedical signals. Accurate interpretation of temperature indicating paint (TIP) is of great importance in aviation and other industrial applications. 2D spectrogram of TIP can be used in temperature interpretation. The spectrogram can be used to process the signal for the rate of change of the human thorax. By visualizing respiratory signals using a spectrogram, the researchers have proposed an approach to the classification of respiration states based on a neural network model. == See also == == References == == External links == See an online spectrogram of speech or other sounds captured by your computer's microphone. Generating a tone sequence whose spectrogram matches an arbitrary text, online Further information on creating a signal whose spectrogram is an arbitrary image Article describing the development of a software spectrogram History of spectrograms & development of instrumentation How to identify the words in a spectrogram from a linguistic professor's Monthly Mystery Spectrogram publication. Sonogram Visible Speech GPL Licensed freeware for the Spectrogram generation of Signal Files.
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{
"page_id": 263317,
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"title": "Spectrogram"
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The Odonata Records Committee is the recognised national body which verifies records of rare vagrant dragonflies in Britain. It was set up in 1998 and consists of six members. Its chairman is Adrian Parr. Decisions on records are published in Atropos and the Journal of the British Dragonfly Society. == References ==
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{
"page_id": 2294937,
"source": null,
"title": "Odonata Records Committee"
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Infinite Energy: The Magazine of New Energy Technology, more commonly referred to simply as Infinite Energy, is a bi-monthly magazine published in New Hampshire that details theories and experiments concerning alternative energy, new science and new physics. The phrase "new energy" in the subtitle is a euphemism for perpetual motion. The magazine was founded by the late Eugene Mallove, who was its editor-in-chief, and is owned by the non-profit New Energy Foundation. It was established in 1994 as Cold Fusion magazine and changed its name in March 1995. Topics of interest include "new hydrogen physics," also called cold fusion; vacuum energy, or zero point energy; and so-called "environmental energy" which they define as the attempt to violate the Second Law of Thermodynamics, for example with a perpetual motion machine. This is done in pursuit of the founder's commitment to "unearthing new sources of energy and new paradigms in science." The magazine has also published articles and book reviews that are critical of the Big Bang theory that describes the origin of the universe. The magazine had a print run of 3,000, and is available on U.S. newsstands. The issues ranged in size from 48 to 100 pages. == History == Infinite Energy was founded by Dr. Eugene Mallove, a former chief science writer at the Massachusetts Institute of Technology (MIT), in response to what he and other proponents viewed as the premature dismissal of cold fusion by the mainstream scientific community. The magazine emerged in the aftermath of the 1989 cold fusion controversy, when chemists Martin Fleischmann and Stanley Pons announced they had achieved nuclear fusion at room temperature—an extraordinary claim that drew global attention but was ultimately rejected by most physicists due to irreproducible results and methodological flaws. Mallove, disillusioned by what he perceived as scientific misconduct and suppression
|
{
"page_id": 1115289,
"source": null,
"title": "Infinite Energy (magazine)"
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of promising research, resigned from MIT and became one of the most vocal defenders of cold fusion, or what became known in later years as low-energy nuclear reactions (LENR). He launched Infinite Energy to serve as a platform for the continued exploration of LENR, alternative energy technologies, and unconventional scientific ideas that struggled to find a place in mainstream journals. Backed by the non-profit New Energy Foundation, the magazine was published from Concord, New Hampshire, and quickly became a hub for the cold fusion community, featuring articles, experimental reports, interviews, and editorials advocating for open inquiry and challenging the boundaries of accepted science. Over the years, Infinite Energy also covered topics such as zero-point energy, over-unity devices, and breakthrough propulsion concepts, appealing to a niche readership interested in revolutionary, albeit controversial, scientific developments. Despite widespread skepticism from the broader scientific establishment, Infinite Energy persisted for decades, buoyed by a dedicated community of researchers and enthusiasts. The magazine’s existence reflects the enduring appeal of cold fusion and the broader tension between scientific orthodoxy and fringe innovation. In the 2000s, the editorship was taken over by György Egely; more recently Bill Zebuhr was writing Editorials. Issue 167 (March - June 2024) is the last extant magazine published. == Reception == Charles Platt, writing for Wired' in 1998', described the magazine as "a wild grab bag of eye-popping assertions and evangelistic rants against the establishment", though conceding that "at the same time, buried among the far-fetched claims were rigorous reports from credentialed scientists". == References == == External links == Official website
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{
"page_id": 1115289,
"source": null,
"title": "Infinite Energy (magazine)"
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In botany, an evergreen is a plant which has foliage that remains green and functional throughout the year. This contrasts with deciduous plants, which lose their foliage completely during the winter or dry season. Consisting of many different species, the unique feature of evergreen plants lends itself to various environments and purposes. == Evergreen species == There are many different kinds of evergreen plants, including trees, shrubs, and vines. Evergreens include: Most species of conifers (e.g., pine, hemlock, spruce, and fir), but not all (e.g., larch). Live oak, holly, and "ancient" gymnosperms such as cycads Many woody plants from frost-free climates Rainforest trees All eucalypts Clubmosses and relatives Most bamboos The Latin binomial term sempervirens, meaning "always green", refers to the evergreen nature of the plant, for instance: Cupressus sempervirens (a cypress) Lonicera sempervirens (a honeysuckle) Sequoia sempervirens (a sequoia) The longevity of individual leaves in evergreen plants varies from a few months to several decades (over 30 years in the Great Basin bristlecone pine). === Prominent evergreen families === Japanese umbrella pine is unique in that it has its own family of which it is the only species. == Differences between evergreen and deciduous species == Evergreen and deciduous species vary in a range of morphological and physiological characters. Generally, broad-leaved evergreen species have thicker leaves than deciduous species, with a larger volume of parenchyma and air spaces per unit leaf area. They have larger leaf biomass per unit leaf area, and hence a lower specific leaf area. Construction costs do not differ between the groups. Evergreens have generally a larger fraction of total plant biomass present as leaves (LMF), but they often have a lower rate of photosynthesis. == Reasons for being evergreen or deciduous == Deciduous trees shed their leaves usually as an adaptation to a cold
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"page_id": 66719,
"source": null,
"title": "Evergreen"
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or dry/wet season. Evergreen trees also lose leaves, but each tree loses its leaves gradually and not all at once. Most tropical rainforest plants are considered to be evergreens, replacing their leaves gradually throughout the year as the leaves age and fall, whereas species growing in seasonally arid climates may be either evergreen or deciduous. Most warm temperate climate plants are also evergreen. In cool temperate climates, fewer plants are evergreen. In such climates, there is a predominance of conifers because few evergreen broadleaf plants can tolerate severe cold below about −26 °C (−15 °F). In addition, evergreen foliage experiences significant leaf damage in these cold, dry climates. Root systems are the most vulnerable aspect of many plants. Even though roots are insulated by soil, which tends to be warmer than average air temperatures, soil temperatures that drop too low can kill the plant. The exact temperature which evergreen roots can handle depends on the species, for example, Picea glauca (White Spruce) roots are killed at −10 °F (−23 °C). In areas where there is a reason for being deciduous, e.g. a cold season or dry season, evergreen plants are usually an adaptation of low nutrient levels. Additionally, they usually have hard leaves and have an excellent water economy due to scarce resources in the area in which they reside. The excellent water economy within the evergreen species is due to high abundance when compared to deciduous species, whereas deciduous trees lose nutrients whenever they lose their leaves. In warmer areas, species such as some pines and cypresses grow on poor soils and disturbed ground. In Rhododendron, a genus with many broadleaf evergreens, several species grow in mature forests but are usually found on highly acidic soil where the nutrients are less available to plants. In taiga or boreal forests,
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{
"page_id": 66719,
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"title": "Evergreen"
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it is too cold for the organic matter in the soil to decay rapidly, so the nutrients in the soil are less easily available to plants, thus favoring evergreens. In temperate climates, evergreens can reinforce their own survival; evergreen leaf and needle litter has a higher carbon–nitrogen ratio than deciduous leaf litter, contributing to a higher soil acidity and lower soil nitrogen content. This is the case with Mediterranean evergreen seedlings, which have unique C and N storages that allow stored resources to determine fast growth within the species, limiting competition and bolstering survival. These conditions favor the growth of more evergreens and make it more difficult for deciduous plants to persist. In addition, the shelter provided by existing evergreen plants can make it easier for younger evergreen plants to survive cold and/or drought. == Uses == Evergreen plants can have decorative as well as functional uses. In months where most other plants are dormant, evergreens with their sturdy structure, and vibrant foliage are popular choices to beautify a landscape. Additionally, evergreens can serve as a windbreak, stopping heat loss from buildings during cold months when placed on the northwest side of a structure. == See also == Semi-deciduous (semi-evergreen) == References == == External links == Helen Ingersoll (1920). "Evergreens" . Encyclopedia Americana.
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"page_id": 66719,
"source": null,
"title": "Evergreen"
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The molecular formula C21H24N2O3 may refer to: Ajmalicine 16-Hydroxytabersonine Lochnericine Preakuammicine Raucaffrinoline Vobasine
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{
"page_id": 26608799,
"source": null,
"title": "C21H24N2O3"
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In the fields of horticulture and botany, the term deciduous () means "falling off at maturity" and "tending to fall off", in reference to trees and shrubs that seasonally shed leaves, usually in the autumn; to the shedding of petals, after flowering; and to the shedding of ripe fruit. The antonym of deciduous in the botanical sense is evergreen. Generally, the term "deciduous" means "the dropping of a part that is no longer needed or useful" and the "falling away after its purpose is finished". In plants, it is the result of natural processes. "Deciduous" has a similar meaning when referring to animal parts, such as deciduous antlers in deer, deciduous teeth (baby teeth) in some mammals (including humans); or decidua, the uterine lining that sheds off after birth. == Botany == In botany and horticulture, deciduous plants, including trees, shrubs and herbaceous perennials, are those that lose all of their leaves for part of the year. This process is called abscission. In some cases leaf loss coincides with winter—namely in temperate or polar climates. In other parts of the world, including tropical, subtropical, and arid regions, plants lose their leaves during the dry season or other seasons, depending on variations in rainfall. The converse of deciduous is evergreen, where foliage is shed on a different schedule from deciduous plants, therefore appearing to remain green year round because not all the leaves are shed at the same time. Plants that are intermediate may be called semi-deciduous; they lose old foliage as new growth begins. Other plants are semi-evergreen and lose their leaves before the next growing season, retaining some during winter or dry periods. Many deciduous plants flower during the period when they are leafless, as this increases the effectiveness of pollination. The absence of leaves improves wind transmission of
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{
"page_id": 66722,
"source": null,
"title": "Deciduous"
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pollen for wind-pollinated plants and increases the visibility of the flowers to insects in insect-pollinated plants. This strategy is not without risks, as the flowers can be damaged by frost or, in dry season regions, result in water stress on the plant. Spring leafout and fall leaf drop are triggered by a combination of daylight and air temperatures. The exact conditions required will vary with the species, but generally more cold-tolerant genera such as Salix will leaf-out earlier and lose their leaves later, while genera such as Fraxinus and Juglans can only grow in warm, frost-free conditions so they need at least 13 hours of daylight and air temperatures of around 70 °F (21 °C) to leaf out. They will be among the earliest trees to lose their leaves in the fall. In sub-Arctic climates such as Alaska, leaves begin turning colors as early as August. However, for most temperate regions it takes place in late September through early November and in subtropical climates such as the southern United States, it may be November into December. Leaf drop or abscission involves complex physiological signals and changes within plants. When leafout is completed (marked by the transition from bright green spring leaves to dark green summer ones) the chlorophyll level in the leaves remains stable until cool temperatures arrive in autumn. When autumn arrives and the days are shorter or when plants are drought-stressed, the chlorophyll steadily breaks down, allowing other pigments present in the leaf to become apparent and resulting in non-green colored foliage. The brightest leaf colors are produced when days grow short and nights are cool, but remain above freezing. These other pigments include carotenoids that are yellow, brown, and orange. Anthocyanin pigments produce red and purple colors, though they are not always present in the leaves. Rather,
|
{
"page_id": 66722,
"source": null,
"title": "Deciduous"
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they are produced in the foliage in late summer, when sugars are trapped in the leaves after the process of abscission begins. Parts of the world that have showy displays of bright autumn colors are limited to locations where days become short and nights are cool. The New England region of the United States and southeastern Canada tend to produce particularly good autumn colors for this reason, with Europe producing generally poorer colors due to the humid maritime climate and lower overall species diversity . It is also a factor that the continental United States and southern Canada are at a lower latitude than northern Europe, so the sun during the fall months is higher and stronger. This combination of strong sun and cool temperatures leads to more intense fall colors. The Southern United States also has poor fall colors due to warm temperatures during the fall months and the Western United States as it has more evergreen and fewer deciduous plants, combined with the West Coast and its maritime climate. (See also: Autumn leaf color) Most of the Southern Hemisphere lacks deciduous plants due to its milder winters and smaller landmass, most of which is nearer the equator with only far southern South America and the southern island of New Zealand producing distinct fall colors. The beginnings of leaf drop starts when an abscission layer is formed between the leaf petiole and the stem. This layer is formed in the spring during active new growth of the leaf; it consists of layers of cells that can separate from each other. The cells are sensitive to a plant hormone called auxin that is produced by the leaf and other parts of the plant. When auxin coming from the leaf is produced at a rate consistent with that from the body
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{
"page_id": 66722,
"source": null,
"title": "Deciduous"
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of the plant, the cells of the abscission layer remain connected; in autumn, or when under stress, the auxin flow from the leaf decreases or stops, triggering cellular elongation within the abscission layer. The elongation of these cells breaks the connection between the different cell layers, allowing the leaf to break away from the plant. It also forms a layer that seals the break, so the plant does not lose sap. Some trees, particularly oaks and beeches, exhibit a behavior known as "marcescence" whereby dead leaves are not shed in the fall and remain on the tree until being blown off by the weather. This is caused by incomplete development of the abscission layer. It is mainly seen in the seedling and sapling stage, although mature trees may have marcescence of leaves on the lower branches. A number of deciduous plants remove nitrogen and carbon from the foliage before they are shed and store them in the form of proteins in the vacuoles of parenchyma cells in the roots and the inner bark. In the spring, these proteins are used as a nitrogen source during the growth of new leaves or flowers. === Function === Plants with deciduous foliage have advantages and disadvantages compared to plants with evergreen foliage. Since deciduous plants lose their leaves to conserve water or to better survive winter weather conditions, they must regrow new foliage during the next suitable growing season; this uses resources which evergreens do not need to expend. Evergreens suffer greater water loss during the winter and they also can experience greater predation pressure, especially when small. Deciduous trees experience much less branch and trunk breakage from glaze ice storms when leafless, and plants can reduce water loss due to the reduction in availability of liquid water during cold winter days. Losing
|
{
"page_id": 66722,
"source": null,
"title": "Deciduous"
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leaves in winter may reduce damage from insects; repairing leaves and keeping them functional may be more costly than just losing and regrowing them. Removing leaves also reduces cavitation which can damage xylem vessels in plants. This then allows deciduous plants to have xylem vessels with larger diameters and therefore a greater rate of transpiration (and hence CO2 uptake as this occurs when stomata are open) during the summer growth period. ==== Deciduous woody plants ==== The deciduous characteristic has developed repeatedly among woody plants. Trees include maple, many oaks and nothofagus, elm, beech, aspen, and birch, among others, as well as a number of coniferous genera, such as larch and Metasequoia. Deciduous shrubs include honeysuckle, viburnum, and many others. Most temperate woody vines are also deciduous, including grapes, poison ivy, Virginia creeper, wisteria, etc. The characteristic is useful in plant identification; for instance in parts of Southern California and the American Southeast, deciduous and evergreen oak species may grow side by side. Periods of leaf fall often coincide with seasons: winter in the case of cool-climate plants or the dry-season in the case of tropical plants, however there are no deciduous species among tree-like monocotyledonous plants, e.g. palms, yuccas, and dracaenas. The hydrangea hirta is a deciduous woody shrub found in Japan. === Regions === Forests where a majority of the trees lose their foliage at the end of the typical growing season are called deciduous forests. These forests are found in many areas worldwide and have distinctive ecosystems, understory growth, and soil dynamics. Two distinctive types of deciduous forests are found growing around the world. Temperate deciduous forest biomes are plant communities distributed in North and South America, Asia, Southern slopes of the Himalayas, Europe and for cultivation purposes in Oceania. They have formed under climatic conditions which
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{
"page_id": 66722,
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"title": "Deciduous"
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have great seasonable temperature variability. Growth occurs during warm summers, leaf drop in autumn, and dormancy during cold winters. These seasonally distinctive communities have diverse life forms that are impacted greatly by the seasonality of their climate, mainly temperature and precipitation rates. These varying and regionally different ecological conditions produce distinctive forest plant communities in different regions. Tropical and subtropical deciduous forest biomes have developed in response not to seasonal temperature variations but to seasonal rainfall patterns. During prolonged dry periods the foliage is dropped to conserve water and prevent death from drought. Leaf drop is not seasonally dependent as it is in temperate climates. It can occur any time of year and varies by region of the world. Even within a small local area there can be variations in the timing and duration of leaf drop; different sides of the same mountain and areas that have high water tables or areas along streams and rivers can produce a patchwork of leafy and leafless trees. == References == == External links == The dictionary definition of deciduous at Wiktionary Media related to Deciduous forests at Wikimedia Commons
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{
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An alloy is a mixture of chemical elements of which in most cases at least one is a metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have properties that differ from those of the pure elements from which they are made. The vast majority of metals used for commercial purposes are alloyed to improve their properties or behavior, such as increased strength, hardness or corrosion resistance. Metals may also be alloyed to reduce their overall cost, for instance alloys of gold and copper. A typical example of an alloy is 304 grade stainless steel which is commonly used for kitchen utensils, pans, knives and forks. Sometime also known as 18/8, it as an alloy consisting broadly of 74% iron, 18% chromium and 8% nickel. The chromium and nickel alloying elements add strength and hardness to the majority iron element, but their main function is to make it resistant to rust/corrosion. In an alloy, the atoms are joined by metallic bonding rather than by covalent bonds typically found in chemical compounds. The alloy constituents are usually measured by mass percentage for practical applications, and in atomic fraction for basic science studies. Alloys are usually classified as substitutional or interstitial alloys, depending on the atomic arrangement that forms the alloy. They can be further classified as homogeneous (consisting of a single phase), or heterogeneous (consisting of two or more phases) or intermetallic. An alloy may be a solid solution of metal elements (a single phase, where all metallic grains (crystals) are of the same composition) or a mixture of metallic phases (two or more solutions, forming a microstructure of different crystals within the metal). Examples of alloys include red gold (gold and copper), white gold (gold and silver), sterling
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{
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silver (silver and copper), steel or silicon steel (iron with non-metallic carbon or silicon respectively), solder, brass, pewter, duralumin, bronze, and amalgams. Alloys are used in a wide variety of applications, from the steel alloys, used in everything from buildings to automobiles to surgical tools, to exotic titanium alloys used in the aerospace industry, to beryllium-copper alloys for non-sparking tools. == Characteristics == An alloy is a mixture of chemical elements, which forms an impure substance (admixture) that retains the characteristics of a metal. An alloy is distinct from an impure metal in that, with an alloy, the added elements are well controlled to produce desirable properties, while impure metals such as wrought iron are less controlled, but are often considered useful. Alloys are made by mixing two or more elements, at least one of which is a metal. This is usually called the primary metal or the base metal, and the name of this metal may also be the name of the alloy. The other constituents may or may not be metals but, when mixed with the molten base, they will be soluble and dissolve into the mixture. The mechanical properties of alloys will often be quite different from those of its individual constituents. A metal that is normally very soft (malleable), such as aluminium, can be altered by alloying it with another soft metal, such as copper. Although both metals are very soft and ductile, the resulting aluminium alloy will have much greater strength. Adding a small amount of non-metallic carbon to iron trades its great ductility for the greater strength of an alloy called steel. Due to its very-high strength, but still substantial toughness, and its ability to be greatly altered by heat treatment, steel is one of the most useful and common alloys in modern use.
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By adding chromium to steel, its resistance to corrosion can be enhanced, creating stainless steel, while adding silicon will alter its electrical characteristics, producing silicon steel. Like oil and water, a molten metal may not always mix with another element. For example, pure iron is almost completely insoluble with copper. Even when the constituents are soluble, each will usually have a saturation point, beyond which no more of the constituent can be added. Iron, for example, can hold a maximum of 6.67% carbon. Although the elements of an alloy usually must be soluble in the liquid state, they may not always be soluble in the solid state. If the metals remain soluble when solid, the alloy forms a solid solution, becoming a homogeneous structure consisting of identical crystals, called a phase. If as the mixture cools the constituents become insoluble, they may separate to form two or more different types of crystals, creating a heterogeneous microstructure of different phases, some with more of one constituent than the other. However, in other alloys, the insoluble elements may not separate until after crystallization occurs. If cooled very quickly, they first crystallize as a homogeneous phase, but they are supersaturated with the secondary constituents. As time passes, the atoms of these supersaturated alloys can separate from the crystal lattice, becoming more stable, and forming a second phase that serves to reinforce the crystals internally. Some alloys, such as electrum—an alloy of silver and gold—occur naturally. Meteorites are sometimes made of naturally occurring alloys of iron and nickel, but are not native to the Earth. One of the first alloys made by humans was bronze, which is a mixture of the metals tin and copper. Bronze was an extremely useful alloy to the ancients, because it is much stronger and harder than either of
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its components. Steel was another common alloy. However, in ancient times, it could only be created as an accidental byproduct from the heating of iron ore in fires (smelting) during the manufacture of iron. Other ancient alloys include pewter, brass and pig iron. In the modern age, steel can be created in many forms. Carbon steel can be made by varying only the carbon content, producing soft alloys like mild steel or hard alloys like spring steel. Alloy steels can be made by adding other elements, such as chromium, molybdenum, vanadium or nickel, resulting in alloys such as high-speed steel or tool steel. Small amounts of manganese are usually alloyed with most modern steels because of its ability to remove unwanted impurities, like phosphorus, sulfur and oxygen, which can have detrimental effects on the alloy. However, most alloys were not created until the 1900s, such as various aluminium, titanium, nickel, and magnesium alloys. Some modern superalloys, such as incoloy, inconel, and hastelloy, may consist of a multitude of different elements. An alloy is technically an impure metal, but when referring to alloys, the term impurities usually denotes undesirable elements. Such impurities are introduced from the base metals and alloying elements, but are removed during processing. For instance, sulfur is a common impurity in steel. Sulfur combines readily with iron to form iron sulfide, which is very brittle, creating weak spots in the steel. Lithium, sodium and calcium are common impurities in aluminium alloys, which can have adverse effects on the structural integrity of castings. Conversely, otherwise pure-metals that contain unwanted impurities are often called "impure metals" and are not usually referred to as alloys. Oxygen, present in the air, readily combines with most metals to form metal oxides; especially at higher temperatures encountered during alloying. Great care is often taken
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{
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during the alloying process to remove excess impurities, using fluxes, chemical additives, or other methods of extractive metallurgy. == Theory == Alloying a metal is done by combining it with one or more other elements. The most common and oldest alloying process is performed by heating the base metal beyond its melting point and then dissolving the solutes into the molten liquid, which may be possible even if the melting point of the solute is far greater than that of the base. For example, in its liquid state, titanium is a very strong solvent capable of dissolving most metals and elements. In addition, it readily absorbs gases like oxygen and burns in the presence of nitrogen. This increases the chance of contamination from any contacting surface, and so must be melted in vacuum induction-heating and special, water-cooled, copper crucibles. However, some metals and solutes, such as iron and carbon, have very high melting-points and were impossible for ancient people to melt. Thus, alloying (in particular, interstitial alloying) may also be performed with one or more constituents in a gaseous state, such as found in a blast furnace to make pig iron (liquid-gas), nitriding, carbonitriding or other forms of case hardening (solid-gas), or the cementation process used to make blister steel (solid-gas). It may also be done with one, more, or all of the constituents in the solid state, such as found in ancient methods of pattern welding (solid-solid), shear steel (solid-solid), or crucible steel production (solid-liquid), mixing the elements via solid-state diffusion. By adding another element to a metal, differences in the size of the atoms create internal stresses in the lattice of the metallic crystals; stresses that often enhance its properties. For example, the combination of carbon with iron produces steel, which is stronger than iron, its primary element.
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{
"page_id": 1187,
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The electrical and thermal conductivity of alloys is usually lower than that of the pure metals. The physical properties, such as density, reactivity, Young's modulus of an alloy may not differ greatly from those of its base element, but engineering properties such as tensile strength, ductility, and shear strength may be substantially different from those of the constituent materials. This is sometimes a result of the sizes of the atoms in the alloy, because larger atoms exert a compressive force on neighboring atoms, and smaller atoms exert a tensile force on their neighbors, helping the alloy resist deformation. Sometimes alloys may exhibit marked differences in behavior even when small amounts of one element are present. For example, impurities in semiconducting ferromagnetic alloys lead to different properties, as first predicted by White, Hogan, Suhl, Tian Abrie and Nakamura. Unlike pure metals, most alloys do not have a single melting point, but a melting range during which the material is a mixture of solid and liquid phases (a slush). The temperature at which melting begins is called the solidus, and the temperature when melting is just complete is called the liquidus. For many alloys there is a particular alloy proportion (in some cases more than one), called either a eutectic mixture or a peritectic composition, which gives the alloy a unique and low melting point, and no liquid/solid slush transition. === Heat treatment === Alloying elements are added to a base metal, to induce hardness, toughness, ductility, or other desired properties. Most metals and alloys can be work hardened by creating defects in their crystal structure. These defects are created during plastic deformation by hammering, bending, extruding, et cetera, and are permanent unless the metal is recrystallized. Otherwise, some alloys can also have their properties altered by heat treatment. Nearly all metals
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{
"page_id": 1187,
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can be softened by annealing, which recrystallizes the alloy and repairs the defects, but not as many can be hardened by controlled heating and cooling. Many alloys of aluminium, copper, magnesium, titanium, and nickel can be strengthened to some degree by some method of heat treatment, but few respond to this to the same degree as does steel. The base metal iron of the iron-carbon alloy known as steel, undergoes a change in the arrangement (allotropy) of the atoms of its crystal matrix at a certain temperature (usually between 820 °C (1,500 °F) and 870 °C (1,600 °F), depending on carbon content). This allows the smaller carbon atoms to enter the interstices of the iron crystal. When this diffusion happens, the carbon atoms are said to be in solution in the iron, forming a particular single, homogeneous, crystalline phase called austenite. If the steel is cooled slowly, the carbon can diffuse out of the iron and it will gradually revert to its low temperature allotrope. During slow cooling, the carbon atoms will no longer be as soluble with the iron, and will be forced to precipitate out of solution, nucleating into a more concentrated form of iron carbide (Fe3C) in the spaces between the pure iron crystals. The steel then becomes heterogeneous, as it is formed of two phases, the iron-carbon phase called cementite (or carbide), and pure iron ferrite. Such a heat treatment produces a steel that is rather soft. If the steel is cooled quickly, however, the carbon atoms will not have time to diffuse and precipitate out as carbide, but will be trapped within the iron crystals. When rapidly cooled, a diffusionless (martensite) transformation occurs, in which the carbon atoms become trapped in solution. This causes the iron crystals to deform as the crystal structure tries to
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{
"page_id": 1187,
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change to its low temperature state, leaving those crystals very hard but much less ductile (more brittle). While the high strength of steel results when diffusion and precipitation is prevented (forming martensite), most heat-treatable alloys are precipitation hardening alloys, that depend on the diffusion of alloying elements to achieve their strength. When heated to form a solution and then cooled quickly, these alloys become much softer than normal, during the diffusionless transformation, but then harden as they age. The solutes in these alloys will precipitate over time, forming intermetallic phases, which are difficult to discern from the base metal. Unlike steel, in which the solid solution separates into different crystal phases (carbide and ferrite), precipitation hardening alloys form different phases within the same crystal. These intermetallic alloys appear homogeneous in crystal structure, but tend to behave heterogeneously, becoming hard and somewhat brittle. In 1906, precipitation hardening alloys were discovered by Alfred Wilm. Precipitation hardening alloys, such as certain alloys of aluminium, titanium, and copper, are heat-treatable alloys that soften when quenched (cooled quickly), and then harden over time. Wilm had been searching for a way to harden aluminium alloys for use in machine-gun cartridge cases. Knowing that aluminium-copper alloys were heat-treatable to some degree, Wilm tried quenching a ternary alloy of aluminium, copper, and the addition of magnesium, but was initially disappointed with the results. However, when Wilm retested it the next day he discovered that the alloy increased in hardness when left to age at room temperature, and far exceeded his expectations. Although an explanation for the phenomenon was not provided until 1919, duralumin was one of the first "age hardening" alloys used, becoming the primary building material for the first Zeppelins, and was soon followed by many others. Because they often exhibit a combination of high strength and
|
{
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low weight, these alloys became widely used in many forms of industry, including the construction of modern aircraft. === Mechanisms === When a molten metal is mixed with another substance, there are two mechanisms that can cause an alloy to form, called atom exchange and the interstitial mechanism. The relative size of each element in the mix plays a primary role in determining which mechanism will occur. When the atoms are relatively similar in size, the atom exchange method usually happens, where some of the atoms composing the metallic crystals are substituted with atoms of the other constituent. This is called a substitutional alloy. Examples of substitutional alloys include bronze and brass, in which some of the copper atoms are substituted with either tin or zinc atoms respectively. In the case of the interstitial mechanism, one atom is usually much smaller than the other and can not successfully substitute for the other type of atom in the crystals of the base metal. Instead, the smaller atoms become trapped in the interstitial sites between the atoms of the crystal matrix. This is referred to as an interstitial alloy. Steel is an example of an interstitial alloy, because the very small carbon atoms fit into interstices of the iron matrix. Stainless steel is an example of a combination of interstitial and substitutional alloys, because the carbon atoms fit into the interstices, but some of the iron atoms are substituted by nickel and chromium atoms. == History and examples == === Meteoric iron === The use of alloys by humans started with the use of meteoric iron, a naturally occurring alloy of nickel and iron. It is the main constituent of iron meteorites. As no metallurgic processes were used to separate iron from nickel, the alloy was used as it was. Meteoric iron
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{
"page_id": 1187,
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could be forged from a red heat to make objects such as tools, weapons, and nails. In many cultures it was shaped by cold hammering into knives and arrowheads. They were often used as anvils. Meteoric iron was very rare and valuable, and difficult for ancient people to work. === Bronze and brass === Iron is usually found as iron ore on Earth, except for one deposit of native iron in Greenland, which was used by the Inuit. Native copper, however, was found worldwide, along with silver, gold, and platinum, which were also used to make tools, jewelry, and other objects since Neolithic times. Copper was the hardest of these metals, and the most widely distributed. It became one of the most important metals to the ancients. Around 10,000 years ago in the highlands of Anatolia (Turkey), humans learned to smelt metals such as copper and tin from ore. Around 2500 BC, people began alloying the two metals to form bronze, which was much harder than its ingredients. Tin was rare, however, being found mostly in Great Britain. In the Middle East, people began alloying copper with zinc to form brass. Ancient civilizations took into account the mixture and the various properties it produced, such as hardness, toughness and melting point, under various conditions of temperature and work hardening, developing much of the information contained in modern alloy phase diagrams. For example, arrowheads from the Chinese Qin dynasty (around 200 BC) were often constructed with a hard bronze-head, but a softer bronze-tang, combining the alloys to prevent both dulling and breaking during use. === Amalgams === Mercury has been smelted from cinnabar for thousands of years. Mercury dissolves many metals, such as gold, silver, and tin, to form amalgams (an alloy in a soft paste or liquid form at ambient
|
{
"page_id": 1187,
"source": null,
"title": "Alloy"
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temperature). Amalgams have been used since 200 BC in China for gilding objects such as armor and mirrors with precious metals. The ancient Romans often used mercury-tin amalgams for gilding their armor. The amalgam was applied as a paste and then heated until the mercury vaporized, leaving the gold, silver, or tin behind. Mercury was often used in mining, to extract precious metals like gold and silver from their ores. === Precious metals === Many ancient civilizations alloyed metals for purely aesthetic purposes. In ancient Egypt and Mycenae, gold was often alloyed with copper to produce red-gold, or iron to produce a bright burgundy-gold. Gold was often found alloyed with silver or other metals to produce various types of colored gold. These metals were also used to strengthen each other, for more practical purposes. Copper was often added to silver to make sterling silver, increasing its strength for use in dishes, silverware, and other practical items. Quite often, precious metals were alloyed with less valuable substances as a means to deceive buyers. Around 250 BC, Archimedes was commissioned by the King of Syracuse to find a way to check the purity of the gold in a crown, leading to the famous bath-house shouting of "Eureka!" upon the discovery of Archimedes' principle. === Pewter === The term pewter covers a variety of alloys consisting primarily of tin. As a pure metal, tin is much too soft to use for most practical purposes. However, during the Bronze Age, tin was a rare metal in many parts of Europe and the Mediterranean, so it was often valued higher than gold. To make jewellery, cutlery, or other objects from tin, workers usually alloyed it with other metals to increase strength and hardness. These metals were typically lead, antimony, bismuth or copper. These solutes were
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{
"page_id": 1187,
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sometimes added individually in varying amounts, or added together, making a wide variety of objects, ranging from practical items such as dishes, surgical tools, candlesticks or funnels, to decorative items like ear rings and hair clips. The earliest examples of pewter come from ancient Egypt, around 1450 BC. The use of pewter was widespread across Europe, from France to Norway and Britain (where most of the ancient tin was mined) to the Near East. The alloy was also used in China and the Far East, arriving in Japan around 800 AD, where it was used for making objects like ceremonial vessels, tea canisters, or chalices used in shinto shrines. === Iron === The first known smelting of iron began in Anatolia, around 1800 BC. Called the bloomery process, it produced very soft but ductile wrought iron. By 800 BC, iron-making technology had spread to Europe, arriving in Japan around 700 AD. Pig iron, a very hard but brittle alloy of iron and carbon, was being produced in China as early as 1200 BC, but did not arrive in Europe until the Middle Ages. Pig iron has a lower melting point than iron, and was used for making cast-iron. However, these metals found little practical use until the introduction of crucible steel around 300 BC. These steels were of poor quality, and the introduction of pattern welding, around the 1st century AD, sought to balance the extreme properties of the alloys by laminating them, to create a tougher metal. Around 700 AD, the Japanese began folding bloomery-steel and cast-iron in alternating layers to increase the strength of their swords, using clay fluxes to remove slag and impurities. This method of Japanese swordsmithing produced one of the purest steel-alloys of the ancient world. While the use of iron started to become more
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widespread around 1200 BC, mainly because of interruptions in the trade routes for tin, the metal was much softer than bronze. However, very small amounts of steel, (an alloy of iron and around 1% carbon), was always a byproduct of the bloomery process. The ability to modify the hardness of steel by heat treatment had been known since 1100 BC, and the rare material was valued for the manufacture of tools and weapons. Because the ancients could not produce temperatures high enough to melt iron fully, the production of steel in decent quantities did not occur until the introduction of blister steel during the Middle Ages. This method introduced carbon by heating wrought iron in charcoal for long periods of time, but the absorption of carbon in this manner is extremely slow thus the penetration was not very deep, so the alloy was not homogeneous. In 1740, Benjamin Huntsman began melting blister steel in a crucible to even out the carbon content, creating the first process for the mass production of tool steel. Huntsman's process was used for manufacturing tool steel until the early 1900s. The introduction of the blast furnace to Europe in the Middle Ages meant that people could produce pig iron in much higher volumes than wrought iron. Because pig iron could be melted, people began to develop processes to reduce carbon in liquid pig iron to create steel. Puddling had been used in China since the first century, and was introduced in Europe during the 1700s, where molten pig iron was stirred while exposed to the air, to remove the carbon by oxidation. In 1858, Henry Bessemer developed a process of steel-making by blowing hot air through liquid pig iron to reduce the carbon content. The Bessemer process led to the first large scale manufacture of
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"page_id": 1187,
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steel. Steel is an alloy of iron and carbon, but the term alloy steel usually only refers to steels that contain other elements— like vanadium, molybdenum, or cobalt—in amounts sufficient to alter the properties of the base steel. Since ancient times, when steel was used primarily for tools and weapons, the methods of producing and working the metal were often closely guarded secrets. Even long after the Age of Enlightenment, the steel industry was very competitive and manufacturers went through great lengths to keep their processes confidential, resisting any attempts to scientifically analyze the material for fear it would reveal their methods. For example, the people of Sheffield, a center of steel production in England, were known to routinely bar visitors and tourists from entering town to deter industrial espionage. Thus, almost no metallurgical information existed about steel until 1860. Because of this lack of understanding, steel was not generally considered an alloy until the decades between 1930 and 1970 (primarily due to the work of scientists like William Chandler Roberts-Austen, Adolf Martens, and Edgar Bain), so "alloy steel" became the popular term for ternary and quaternary steel-alloys. After Benjamin Huntsman developed his crucible steel in 1740, he began experimenting with the addition of elements like manganese (in the form of a high-manganese pig-iron called spiegeleisen), which helped remove impurities such as phosphorus and oxygen; a process adopted by Bessemer and still used in modern steels (albeit in concentrations low enough to still be considered carbon steel). Afterward, many people began experimenting with various alloys of steel without much success. However, in 1882, Robert Hadfield, being a pioneer in steel metallurgy, took an interest and produced a steel alloy containing around 12% manganese. Called mangalloy, it exhibited extreme hardness and toughness, becoming the first commercially viable alloy-steel. Afterward, he created
|
{
"page_id": 1187,
"source": null,
"title": "Alloy"
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silicon steel, launching the search for other possible alloys of steel. Robert Forester Mushet found that by adding tungsten to steel it could produce a very hard edge that would resist losing its hardness at high temperatures. "R. Mushet's special steel" (RMS) became the first high-speed steel. Mushet's steel was quickly replaced by tungsten carbide steel, developed by Taylor and White in 1900, in which they doubled the tungsten content and added small amounts of chromium and vanadium, producing a superior steel for use in lathes and machining tools. In 1903, the Wright brothers used a chromium-nickel steel to make the crankshaft for their airplane engine, while in 1908 Henry Ford began using vanadium steels for parts like crankshafts and valves in his Model T Ford, due to their higher strength and resistance to high temperatures. In 1912, the Krupp Ironworks in Germany developed a rust-resistant steel by adding 21% chromium and 7% nickel, producing the first stainless steel. === Others === Due to their high reactivity, most metals were not discovered until the 19th century. A method for extracting aluminium from bauxite was proposed by Humphry Davy in 1807, using an electric arc. Although his attempts were unsuccessful, by 1855 the first sales of pure aluminium reached the market. However, as extractive metallurgy was still in its infancy, most aluminium extraction-processes produced unintended alloys contaminated with other elements found in the ore; the most abundant of which was copper. These aluminium-copper alloys (at the time termed "aluminium bronze") preceded pure aluminium, offering greater strength and hardness over the soft, pure metal, and to a slight degree were found to be heat treatable. However, due to their softness and limited hardenability these alloys found little practical use, and were more of a novelty, until the Wright brothers used an aluminium
|
{
"page_id": 1187,
"source": null,
"title": "Alloy"
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alloy to construct the first airplane engine in 1903. During the time between 1865 and 1910, processes for extracting many other metals were discovered, such as chromium, vanadium, tungsten, iridium, cobalt, and molybdenum, and various alloys were developed. Prior to 1910, research mainly consisted of private individuals tinkering in their own laboratories. However, as the aircraft and automotive industries began growing, research into alloys became an industrial effort in the years following 1910, as new magnesium alloys were developed for pistons and wheels in cars, and pot metal for levers and knobs, and aluminium alloys developed for airframes and aircraft skins were put into use. The Doehler Die Casting Co. of Toledo, Ohio were known for the production of Brastil, a high tensile corrosion resistant bronze alloy. == See also == Alloy broadening CALPHAD Ideal mixture List of alloys == References == == Bibliography == Buchwald, Vagn Fabritius (2005). Iron and steel in ancient times. Det Kongelige Danske Videnskabernes Selskab. ISBN 978-87-7304-308-0. == External links == Roberts-Austen, William Chandler; Neville, Francis Henry (1911). "Alloys" . Encyclopædia Britannica (11th ed.). "Alloy" . The American Cyclopædia. 1879.
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A Cypress forest is a western United States plant association typically dominated by one or more cypress species. Example species comprising the canopy include Cupressus macrocarpa. In some cases these forests have been severely damaged by goats, cattle and other grazing animals. While cypress species are clearly dominant within a Cypress forest, other trees such as California Buckeye, Aesculus californica, are found in some Cypress forests. == Examples == The Guadalupe Island Cypress Forest is situated on Guadalupe Island, offshore from Baja California. This forest of Hesperocyparis guadalupensis trees was devastated by introduced goats, but conservation biology efforts have been conducted to assist in restoring the forest. Another example on the Pacific Coast mainland of Northern California is the Sargent's cypress Forest, located in coastal Marin County, California. == See also == Pygmy forest == Line notes == == References == Marlene A. Rodriguez-Malagon, Alejandro Hinojosa Corona, Alfonso Aguirre-Munoz and Cesar Garcia Gutierrez The Guadalupe Island Forest Recovery Track C.Michael Hogan (2008) Aesculus californica, Globaltwitcher.com, ed. N. Strӧmberg Sargent Forest of Marin County, California USGS Bolinas Quadrangle Map, U.S. Government Printing Office, Washington, DC
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{
"page_id": 19989669,
"source": null,
"title": "Cypress forest"
}
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Blanche Muriel Bristol (21 April 1888 – 15 March 1950) was a British phycologist who worked at Rothamsted Research (then Rothamsted Experimental Station) in 1919. Her research focused on the mechanisms by which algae acquire nutrients. == Statistics and tea == One day at Rothamsted, Ronald Fisher offered Bristol a cup of hot tea that he had just drawn from an urn. Bristol declined it, saying that she preferred the flavour when the milk was poured into the cup before the tea. Fisher scoffed that the order of pouring could not affect the flavour. Bristol insisted that it did and that she could tell the difference. Overhearing this debate, William Roach said, "Let's test her." Fisher and Roach hastily put together an experiment to test Bristol's ability to identify the order in which the two liquids were poured into several cups. At the conclusion of this experiment in which she correctly identified all eight, Roach proclaimed that "Bristol divined correctly more than enough of those cups into which tea had been poured first to prove her case". This incident led Fisher to do important work in the design of statistically valid experiments based on the statistical significance of experimental results. He developed Fisher's exact test to assess the probabilities and statistical significance of experiments. == Family life == Bristol was born on 21 April 1888, the daughter of Alfred Bristol, a commercial traveller, and Annie Eliza, née Davies. She studied botany and completed a PhD on algae at Birmingham, under the tutelage of George Stephen West. Bristol married William Roach in 1923. She died in Bristol on 15 March 1950 of ovarian cancer. == Algae == The green algae species Chlamydomonas muriella is named after her and possibly the genus Muriella. == References ==
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{
"page_id": 5571750,
"source": null,
"title": "Muriel Bristol"
}
|
Orotidine 5'-monophosphate (OMP), also known as orotidylic acid, is a pyrimidine nucleotide which is the last intermediate in the biosynthesis of uridine monophosphate. OMP is formed from orotate and phosphoribosyl pyrophosphate by the enzyme orotate phosphoribosyltransferase. In humans, the enzyme UMP synthase converts OMP into uridine 5'- monophosphate. If UMP synthase is defective, orotic aciduria can result. == References ==
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{
"page_id": 11273383,
"source": null,
"title": "Orotidine 5'-monophosphate"
}
|
The molecular formula C25H29N3O3 may refer to: Adimolol BMS-202
|
{
"page_id": 26608808,
"source": null,
"title": "C25H29N3O3"
}
|
Microwave digestion is a chemical technique used to decompose sample material into a solution suitable for quantitative elemental analysis. It is commonly used to prepare samples for analysis using inductively coupled plasma mass spectrometry (ICP-MS), atomic absorption spectroscopy, and atomic emission spectroscopy (including ICP-AES). To perform the digestion, sample material is combined with a concentrated strong acid or a mixture thereof, most commonly using nitric acid, hydrochloric acid and/or hydrofluoric acid, in a closed PTFE vessel. The vessel and its contents are then exposed to microwave irradiation, raising the pressure and temperature of the solution mixture. The elevated pressures and temperatures within a low pH sample medium increase both the speed of thermal decomposition of the sample and the solubility of elements in solution. Organic compounds are decomposed into gaseous form, effectively removing them from solution. Once these elements are in solution, it is possible to quantify elemental concentrations within samples. Microwaves can be programmed to reach specific temperatures or ramp up to a given temperature at a specified rate. The temperature in the interior of the vessel is monitored by an infrared external sensor or by a optic fiber probe, and the microwave power is regulated to maintain the temperature defined by the active program. The vessel solution must contain at least one solvent that absorbs microwave radiation, usually water. The specific blend of acids (or other reagents) and the temperatures vary depending upon the type of sample being digested. Often a standardized protocol for digestion is followed, such as an Environmental Protection Agency Method. == Comparison between microwave digestion and other sample preparation methods == Before microwave digestion technology was developed, samples were digested using less convenient methods, such as heating vessels in an oven, typically for at least 24 hours. The use of microwave energy allows for
|
{
"page_id": 29623466,
"source": null,
"title": "Microwave digestion"
}
|
fast sample heating, reducing digestion time to as little as one hour. Another common means to decompose samples for elemental analysis is dry-ashing, in which samples are incinerated in a muffle furnace. The resultant ash is then dissolved for analysis, usually into dilute nitric acid. While this method is simple, inexpensive and does not require concentrated acids, it cannot be used for volatile elements such as mercury and can increase the likelihood of background contamination. The incineration will not convert all elements to soluble salts, necessitating an additional digestion step. == Quality control in microwave digestion == In microwave digestion, 100% analyte recovery cannot be assumed. To account for this, scientists perform tests such as fortification recovery, in which a spike (a known amount of the target analyte) is added to test samples. These spiked samples are then analyzed to determine whether the expected increase in analyte concentration occurs. Contamination from improperly cleaned digestion vessels is also a possibility. As such, in any microwave digestion, blank samples need to be digested to determine if there is background contamination. == References == === Footnotes === === Bibliography ===
|
{
"page_id": 29623466,
"source": null,
"title": "Microwave digestion"
}
|
Supermicelle is a hierarchical micelle structure (supramolecular assembly) where individual components are also micelles. Supermicelles are formed via bottom-up chemical approaches, such as self-assembly of long cylindrical micelles into radial cross-, star- or dandelion-like patterns in a specially selected solvent; solid nanoparticles may be added to the solution to act as nucleation centers and form the central core of the supermicelle. The stems of the primary cylindrical micelles are composed of various block copolymers connected by strong covalent bonds; within the supermicelle structure they are loosely held together by hydrogen bonds, electrostatic or solvophobic interactions. == References ==
|
{
"page_id": 48759979,
"source": null,
"title": "Supermicelle"
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|
The molecular formula C15H21NO4 (molar mass: 279.33 g/mol, exact mass: 279.1471 u) may refer to: Afurolol Metalaxyl
|
{
"page_id": 26608813,
"source": null,
"title": "C15H21NO4"
}
|
The molecular formula C19H22N2O3 (molar mass: 326.39 g/mol, exact mass: 326.1630 u) may refer to: Bumadizone Ervatinine 25CN-NBOMe
|
{
"page_id": 61211822,
"source": null,
"title": "C19H22N2O3"
}
|
A model is an informative representation of an object, person, or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin modulus, 'a measure'. Models can be divided into physical models (e.g. a ship model or a fashion model) and abstract models (e.g. a set of mathematical equations describing the workings of the atmosphere for the purpose of weather forecasting). Abstract or conceptual models are central to philosophy of science. In scholarly research and applied science, a model should not be confused with a theory: while a model seeks only to represent reality with the purpose of better understanding or predicting the world, a theory is more ambitious in that it claims to be an explanation of reality. == Types of model == === Model in specific contexts === As a noun, model has specific meanings in certain fields, derived from its original meaning of "structural design or layout": Model (art), a person posing for an artist, e.g. a 15th-century criminal representing the biblical Judas in Leonardo da Vinci's painting The Last Supper Model (person), a person who serves as a template for others to copy, as in a role model, often in the context of advertising commercial products; e.g. the first fashion model, Marie Vernet Worth in 1853, wife of designer Charles Frederick Worth. Model (product), a particular design of a product as displayed in a catalogue or show room (e.g. Ford Model T, an early car model) Model (organism) a non-human species that is studied to understand biological phenomena in other organisms, e.g. a guinea pig starved of vitamin C to study scurvy, an experiment that would be immoral to conduct on a person Model (mimicry), a species that is mimicked by another species Model
|
{
"page_id": 263343,
"source": null,
"title": "Model"
}
|
(logic), a structure (a set of items, such as natural numbers 1, 2, 3,..., along with mathematical operations such as addition and multiplication, and relations, such as < {\displaystyle <} ) that satisfies a given system of axioms (basic truisms), i.e. that satisfies the statements of a given theory Model (CGI), a mathematical representation of any surface of an object in three dimensions via specialized software Model (MVC), the information-representing internal component of a software, as distinct from its user interface === Physical model === A physical model (most commonly referred to simply as a model but in this context distinguished from a conceptual model) is a smaller or larger physical representation of an object, person or system. The object being modelled may be small (e.g., an atom) or large (e.g., the Solar System) or life-size (e.g., a fashion model displaying clothes for similarly-built potential customers). The geometry of the model and the object it represents are often similar in the sense that one is a rescaling of the other. However, in many cases the similarity is only approximate or even intentionally distorted. Sometimes the distortion is systematic, e.g., a fixed scale horizontally and a larger fixed scale vertically when modelling topography to enhance a region's mountains. An architectural model permits visualization of internal relationships within the structure or external relationships of the structure to the environment. Another use is as a toy. Instrumented physical models are an effective way of investigating fluid flows for engineering design. Physical models are often coupled with computational fluid dynamics models to optimize the design of equipment and processes. This includes external flow such as around buildings, vehicles, people, or hydraulic structures. Wind tunnel and water tunnel testing is often used for these design efforts. Instrumented physical models can also examine internal flows, for
|
{
"page_id": 263343,
"source": null,
"title": "Model"
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|
the design of ductwork systems, pollution control equipment, food processing machines, and mixing vessels. Transparent flow models are used in this case to observe the detailed flow phenomenon. These models are scaled in terms of both geometry and important forces, for example, using Froude number or Reynolds number scaling (see Similitude). In the pre-computer era, the UK economy was modelled with the hydraulic model MONIAC, to predict for example the effect of tax rises on employment. === Conceptual model === A conceptual model is a theoretical representation of a system, e.g. a set of mathematical equations attempting to describe the workings of the atmosphere for the purpose of weather forecasting. It consists of concepts used to help understand or simulate a subject the model represents. Abstract or conceptual models are central to philosophy of science, as almost every scientific theory effectively embeds some kind of model of the physical or human sphere. In some sense, a physical model "is always the reification of some conceptual model; the conceptual model is conceived ahead as the blueprint of the physical one", which is then constructed as conceived. Thus, the term refers to models that are formed after a conceptualization or generalization process. === Examples === Conceptual model (computer science), an agreed representation of entities and their relationships, to assist in developing software Economic model, a theoretical construct representing economic processes Language model a probabilistic model of a natural language, used for speech recognition, language generation, and information retrieval Large language models are artificial neural networks used for generative artificial intelligence (AI), e.g. ChatGPT Mathematical model, a description of a system using mathematical concepts and language Statistical model, a mathematical model that usually specifies the relationship between one or more random variables and other non-random variables Model (CGI), a mathematical representation of any
|
{
"page_id": 263343,
"source": null,
"title": "Model"
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|
surface of an object in three dimensions via specialized software Medical model, a proposed "set of procedures in which all doctors are trained" Mental model, in psychology, an internal representation of external reality Model (logic), a set along with a collection of finitary operations, and relations that are defined on it, satisfying a given collection of axioms Model (MVC), information-representing component of a software, distinct from the user interface (the "view"), both linked by the "controller" component, in the context of the model–view–controller software design Model act, a law drafted centrally to be disseminated and proposed for enactment in multiple independent legislatures Standard model (disambiguation) == Properties of models, according to general model theory == According to Herbert Stachowiak, a model is characterized by at least three properties: 1. Mapping A model always is a model of something—it is an image or representation of some natural or artificial, existing or imagined original, where this original itself could be a model. 2. Reduction In general, a model will not include all attributes that describe the original but only those that appear relevant to the model's creator or user. 3. Pragmatism A model does not relate unambiguously to its original. It is intended to work as a replacement for the original a) for certain subjects (for whom?) b) within a certain time range (when?) c) restricted to certain conceptual or physical actions (what for?). For example, a street map is a model of the actual streets in a city (mapping), showing the course of the streets while leaving out, say, traffic signs and road markings (reduction), made for pedestrians and vehicle drivers for the purpose of finding one's way in the city (pragmatism). Additional properties have been proposed, like extension and distortion as well as validity. The American philosopher Michael Weisberg differentiates
|
{
"page_id": 263343,
"source": null,
"title": "Model"
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|
between concrete and mathematical models and proposes computer simulations (computational models) as their own class of models. == Uses of models == According to Bruce Edmonds, there are at least 5 general uses for models: Prediction: reliably anticipating unknown data, including data within the domain of the training data (interpolation), and outside the domain (extrapolation) Explanation: establishing plausible chains of causality by proposing mechanisms that can explain patterns seen in data Theoretical exposition: discovering or proposing new hypotheses, or refuting existing hypotheses about the behaviour of the system being modelled Description: representing important aspects of the system being modelled Illustration: communicating an idea or explanation == See also == == References == == External links == Media related to Physical models at Wikimedia Commons
|
{
"page_id": 263343,
"source": null,
"title": "Model"
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Atomic physics is the field of physics that studies atoms as an isolated system of electrons and an atomic nucleus. Atomic physics typically refers to the study of atomic structure and the interaction between atoms. It is primarily concerned with the way in which electrons are arranged around the nucleus and the processes by which these arrangements change. This comprises ions, neutral atoms and, unless otherwise stated, it can be assumed that the term atom includes ions. The term atomic physics can be associated with nuclear power and nuclear weapons, due to the synonymous use of atomic and nuclear in standard English. Physicists distinguish between atomic physics—which deals with the atom as a system consisting of a nucleus and electrons—and nuclear physics, which studies nuclear reactions and special properties of atomic nuclei. As with many scientific fields, strict delineation can be highly contrived and atomic physics is often considered in the wider context of atomic, molecular, and optical physics. Physics research groups are usually so classified. == Isolated atoms == Atomic physics primarily considers atoms in isolation. Atomic models will consist of a single nucleus that may be surrounded by one or more bound electrons. It is not concerned with the formation of molecules (although much of the physics is identical), nor does it examine atoms in a solid state as condensed matter. It is concerned with processes such as ionization and excitation by photons or collisions with atomic particles. While modelling atoms in isolation may not seem realistic, if one considers atoms in a gas or plasma then the time-scales for atom-atom interactions are huge in comparison to the atomic processes that are generally considered. This means that the individual atoms can be treated as if each were in isolation, as the vast majority of the time they are.
|
{
"page_id": 1200,
"source": null,
"title": "Atomic physics"
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By this consideration, atomic physics provides the underlying theory in plasma physics and atmospheric physics, even though both deal with very large numbers of atoms. == Electronic configuration == Electrons form notional shells around the nucleus. These are normally in a ground state but can be excited by the absorption of energy from light (photons), magnetic fields, or interaction with a colliding particle (typically ions or other electrons). Electrons that populate a shell are said to be in a bound state. The energy necessary to remove an electron from its shell (taking it to infinity) is called the binding energy. Any quantity of energy absorbed by the electron in excess of this amount is converted to kinetic energy according to the conservation of energy. The atom is said to have undergone the process of ionization. If the electron absorbs a quantity of energy less than the binding energy, it will be transferred to an excited state. After a certain time, the electron in an excited state will "jump" (undergo a transition) to a lower state. In a neutral atom, the system will emit a photon of the difference in energy, since energy is conserved. If an inner electron has absorbed more than the binding energy (so that the atom ionizes), then a more outer electron may undergo a transition to fill the inner orbital. In this case, a visible photon or a characteristic X-ray is emitted, or a phenomenon known as the Auger effect may take place, where the released energy is transferred to another bound electron, causing it to go into the continuum. The Auger effect allows one to multiply ionize an atom with a single photon. There are rather strict selection rules as to the electronic configurations that can be reached by excitation by light — however, there
|
{
"page_id": 1200,
"source": null,
"title": "Atomic physics"
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|
are no such rules for excitation by collision processes. === Bohr Model of the Atom === The Bohr model, proposed by Niels Bohr in 1913, is a revolutionary theory describing the structure of the hydrogen atom. It introduced the idea of quantized orbits for electrons, combining classical and quantum physics. Key Postulates of the Bohr Model 1. Electrons Move in Circular Orbits: • Electrons revolve around the nucleus in fixed, circular paths called orbits or energy levels. • These orbits are stable and do not radiate energy. 2. Quantization of Angular Momentum: • The angular momentum of an electron is quantized and given by: L = m e v r = n ℏ , n = 1 , 2 , 3 , … {\displaystyle \ L=m_{e}vr=n_{\hbar },\quad n=1,2,3,\ldots } where: • m e : {\displaystyle m_{e}:} Mass of the electron. • v : {\displaystyle v:} Velocity of the electron. • r : {\displaystyle r:} Radius of the orbit. • ℏ : {\displaystyle \hbar :} Reduced Planck's constant ( ℏ = h 2 π {\displaystyle \hbar ={\frac {h}{2\pi }}} ). • n : {\displaystyle n:} Principal quantum number, representing the orbit. 3. Energy Levels: • Each orbit has a specific energy. The total energy of an electron in the n {\displaystyle n} th orbit is: E n = − 13.6 n 2 eV , {\displaystyle \ E_{n}=-{\frac {13.6}{n^{2}}}\ {\text{eV}},} where 13.6 eV {\displaystyle 13.6\ {\text{eV}}} is the ground-state energy of the hydrogen atom. 4. Emission or Absorption of Energy: • Electrons can transition between orbits by absorbing or emitting energy equal to the difference between the energy levels: Δ E = E f − E i = h ν , {\displaystyle \ \Delta E=E_{f}-E_{i}=h\nu ,} where: • h : {\displaystyle h:} Planck's constant. • ν : {\displaystyle \nu :} Frequency of
|
{
"page_id": 1200,
"source": null,
"title": "Atomic physics"
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emitted/absorbed radiation. • E f , E i : {\displaystyle E_{f},E_{i}:} Final and initial energy levels. == History and developments == One of the earliest steps towards atomic physics was the recognition that matter was composed of atoms. It forms a part of the texts written in 6th century BC to 2nd century BC, such as those of Democritus or Vaiśeṣika Sūtra written by Kaṇāda. This theory was later developed in the modern sense of the basic unit of a chemical element by the British chemist and physicist John Dalton in the 18th century. At this stage, it was not clear what atoms were, although they could be described and classified by their properties (in bulk). The invention of the periodic system of elements by Dmitri Mendeleev was another great step forward. The true beginning of atomic physics is marked by the discovery of spectral lines and attempts to describe the phenomenon, most notably by Joseph von Fraunhofer. The study of these lines led to the Bohr atom model and to the birth of quantum mechanics. In seeking to explain atomic spectra, an entirely new mathematical model of matter was revealed. As far as atoms and their electron shells were concerned, not only did this yield a better overall description, i.e. the atomic orbital model, but it also provided a new theoretical basis for chemistry (quantum chemistry) and spectroscopy. Since the Second World War, both theoretical and experimental fields have advanced at a rapid pace. This can be attributed to progress in computing technology, which has allowed larger and more sophisticated models of atomic structure and associated collision processes. Similar technological advances in accelerators, detectors, magnetic field generation and lasers have greatly assisted experimental work. Beyond the well-known phenomena which can be describe with regular quantum mechanics chaotic processes can
|
{
"page_id": 1200,
"source": null,
"title": "Atomic physics"
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occur which need different descriptions. == Significant atomic physicists == == See also == Particle physics Isomeric shift Atomism Ionisation Quantum Mechanics Electron Correlation Quantum Chemistry Bound State == Bibliography == Will Raven (2025). Atomic Physics for Everyone. Springer Nature. doi:10.1007/978-3-031-69507-0. ISBN 978-3-031-69507-0. Sommerfeld, A. (1923) Atomic structure and spectral lines. (translated from German "Atombau und Spektrallinien" 1921), Dutton Publisher. Foot, CJ (2004). Atomic Physics. Oxford University Press. ISBN 978-0-19-850696-6. Smirnov, B.E. (2003) Physics of Atoms and Ions, Springer. ISBN 0-387-95550-X. Szász, L. (1992) The Electronic Structure of Atoms, John Willey & Sons. ISBN 0-471-54280-6. Herzberg, Gerhard (1979) [1945]. Atomic Spectra and Atomic Structure. New York: Dover. ISBN 978-0-486-60115-1. Bethe, H.A. & Salpeter E.E. (1957) Quantum Mechanics of One- and Two Electron Atoms. Springer. Born, M. (1937) Atomic Physics. Blackie & Son Limited. Cox, P.A. (1996) Introduction to Quantum Theory and Atomic Spectra. Oxford University Press. ISBN 0-19-855916 Condon, E.U. & Shortley, G.H. (1935). The Theory of Atomic Spectra. Cambridge University Press. ISBN 978-0-521-09209-8. {{cite book}}: ISBN / Date incompatibility (help) Cowan, Robert D. (1981). The Theory of Atomic Structure and Spectra. University of California Press. ISBN 978-0-520-03821-9. Lindgren, I. & Morrison, J. (1986). Atomic Many-Body Theory (Second ed.). Springer-Verlag. ISBN 978-0-387-16649-0. == References == == External links == MIT-Harvard Center for Ultracold Atoms Stanford QFARM Initiative for Quantum Science & Enginneering Joint Quantum Institute at University of Maryland and NIST Atomic Physics on the Internet JILA (Atomic Physics) ORNL Physics Division
|
{
"page_id": 1200,
"source": null,
"title": "Atomic physics"
}
|
SqueezeNet is a deep neural network for image classification released in 2016. SqueezeNet was developed by researchers at DeepScale, University of California, Berkeley, and Stanford University. In designing SqueezeNet, the authors' goal was to create a smaller neural network with fewer parameters while achieving competitive accuracy. Their best-performing model achieved the same accuracy as AlexNet on ImageNet classification, but has a size 510x less than it. == Version history == SqueezeNet was originally released on February 22, 2016. This original version of SqueezeNet was implemented on top of the Caffe deep learning software framework. Shortly thereafter, the open-source research community ported SqueezeNet to a number of other deep learning frameworks. On February 26, 2016, Eddie Bell released a port of SqueezeNet for the Chainer deep learning framework. On March 2, 2016, Guo Haria released a port of SqueezeNet for the Apache MXNet framework. On June 3, 2016, Tammy Yang released a port of SqueezeNet for the Keras framework. In 2017, companies including Baidu, Xilinx, Imagination Technologies, and Synopsys demonstrated SqueezeNet running on low-power processing platforms such as smartphones, FPGAs, and custom processors. As of 2018, SqueezeNet ships "natively" as part of the source code of a number of deep learning frameworks such as PyTorch, Apache MXNet, and Apple CoreML. In addition, third party developers have created implementations of SqueezeNet that are compatible with frameworks such as TensorFlow. Below is a summary of frameworks that support SqueezeNet. == Relationship to other networks == === AlexNet === SqueezeNet was originally described in SqueezeNet: AlexNet-level accuracy with 50x fewer parameters and <0.5MB model size. AlexNet is a deep neural network that has 240 MB of parameters, and SqueezeNet has just 5 MB of parameters. This small model size can more easily fit into computer memory and can more easily be transmitted over a
|
{
"page_id": 57410739,
"source": null,
"title": "SqueezeNet"
}
|
computer network. However, it's important to note that SqueezeNet is not a "squeezed version of AlexNet." Rather, SqueezeNet is an entirely different DNN architecture than AlexNet. What SqueezeNet and AlexNet have in common is that both of them achieve approximately the same level of accuracy when evaluated on the ImageNet image classification validation dataset. === Model compression === Model compression (e.g. quantization and pruning of model parameters) can be applied to a deep neural network after it has been trained. In the SqueezeNet paper, the authors demonstrated that a model compression technique called Deep Compression can be applied to SqueezeNet to further reduce the size of the parameter file from 5 MB to 500 KB. Deep Compression has also been applied to other DNNs, such as AlexNet and VGG. == Variants == Some of the members of the original SqueezeNet team have continued to develop resource-efficient deep neural networks for a variety of applications. A few of these works are noted in the following table. As with the original SqueezeNet model, the open-source research community has ported and adapted these newer "squeeze"-family models for compatibility with multiple deep learning frameworks. In addition, the open-source research community has extended SqueezeNet to other applications, including semantic segmentation of images and style transfer. == See also == Convolutional neural network MobileNet EfficientNet You Only Look Once Edge computing == References ==
|
{
"page_id": 57410739,
"source": null,
"title": "SqueezeNet"
}
|
Alais or Allais is the first carbonaceous chondrite meteorite identified. It fell near Alès in 1806 in multiple fragments which together weighed 6 kg (13 lb 4 oz), although only 0.26 kg (9.2 oz) remains. The meteorite contains a number of elements in similar proportions to the Solar System in its primordial state. It also contains organic compounds and water. It has proved to be one of the most important meteorites discovered in France. == History == At 17:00 on 15 March 1806, two detonations were heard near Alès in Gard, France. Shortly afterwards, two soft black stones were discovered in the villages of Saint-Étienne-de-l'Olm and Castelnau-Valence, weighing 4 kg (8 lb 13 oz) and 2 kg (4 lb 7 oz) respectively. The fragments were collected by people who observed the impact and given to two scientists that lived locally. The meteorite was analysed by Louis Jacques Thénard, who published a study in 1807, showing that it had a high carbon content. It was initially doubted that the fragments were of non-terrestrial origins as their attributes were markedly different to existing meteorites. However, it was increasingly realised that this was a new, albeit rare, type of meteorite. The meteorite is also known as Valence. === Curation and distribution === As an early fall (soon after the consensus that meteorites were real, extraterrestrial phenomenon), Alais has largely been dispersed. Few samples have been preserved, less than Orgueil, but more than Tonk and particularly Revelstoke. Source: Grady, M. M. Catalogue of Meteorites, 5th Edition, Cambridge University Press == Description == === Overview === The Alais meteorite is one of the most important meteorites in France. It is black with loose friable textures with a low density of less than 1.7 g/cm3 (0.061 lb/cu in). Originally consisting of fragments that together weighed 6
|
{
"page_id": 63571124,
"source": null,
"title": "Alais meteorite"
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|
kg (13 lb 4 oz), it has been subject to substantial scientific examination and currently only 260 g (9.2 oz) remains. A fragment, weighing 39.3 g (1.39 oz) is held by the National Museum of Natural History, France. === Composition and classification === The meteorite is one of five known meteorites belonging to the CI chondrite group. This group is remarkable for having an elemental distribution that has the strongest similarity to that of the solar nebula. Except for certain volatile elements, like carbon, hydrogen, oxygen, nitrogen and the noble gases, which are not present in the meteorites in the same proportions, the ratios of the elements are very similar. The meteorite contains cubanite, dolomite, fosterite, pyrrhotite and zircon amongst other minerals. == Origin of life controversy == The meteorite has been at the centre of controversial claims about an extraterrestrial origin of life since the discovery of organic matter on the meteorite by Jöns Jacob Berzelius. Organic compounds, amino acids and water have been found in the meteorite. However, studies differentiate between organic and biological matter, the latter not being present. == See also == Glossary of meteoritics == References == === Citations === === Bibliography === Faidit, Jean-Michel. (2006). "Bicentenaire de la météorite d'Alais". L'Astronomie. 120: 162–165. Caillet Comorowski, C.L.V. (2006). "The meteorite collection of the National Museum of Natural History in Paris, France". In McCall, G.J.H.; et al. (eds.). The History of Meteoritics and Key Meteorite Collections: Fireballs, Falls and Finds. London: The Geological Society. pp. 163–204. ISBN 978-1-86239-194-9. Chyba, Christopher F. (1990). "Extraterrestrial amino acids and terrestrial life". Nature. 348 (6297): 113–114. Bibcode:1990Natur.348..113C. doi:10.1038/348113a0. S2CID 4244056. McCall, G.J.H.; et al. (2006). "The history of meteoritics – an overview". In McCall, G.J.H.; et al. (eds.). The History of Meteoritics and Key Meteorite Collections: Fireballs, Falls and Finds.
|
{
"page_id": 63571124,
"source": null,
"title": "Alais meteorite"
}
|
London: The Geological Society. pp. 1–13. ISBN 978-1-86239-194-9. Kerridge, John F.; Macdougall, J. Douglas; Marti, K. (1979). "Clues to the origin of sulfide minerals in CI chondrites". Earth and Planetary Science Letters. 43 (3): 359–367. Bibcode:1979E&PSL..43..359K. doi:10.1016/0012-821X(79)90091-8. Lauretta, Dante S.; McSween, Harry S. (2006). Meteorites and the Early Solar System Volume II. Tucson: University of Arizona Press. ISBN 978-0-81652-562-1. Marvin, Ursula B. (2006). "Meteorites in History: An Overview from the Renaissance to the 20th Century". In McCall, G.J.H.; et al. (eds.). The History of Meteoritics and Key Meteorite Collections: Fireballs, Falls and Finds. London: The Geological Society. pp. 15–72. ISBN 978-1-86239-194-9. Mason, Brian (1963). "The Carbonaceous Chondrites". Space Science Reviews. 1 (4): 621–646. Bibcode:1963SSRv....1..621M. doi:10.1007/BF00212446. S2CID 121308917. Mason, Brian (1967). "Meteorites". American Scientist. 55 (4): 429–455.
|
{
"page_id": 63571124,
"source": null,
"title": "Alais meteorite"
}
|
BOP (benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophosphate) is a reagent commonly used for the synthesis of amides from carboxylic acids and amines in peptide synthesis. It can be prepared from 1-hydroxybenzotriazole and a chlorophosphonium reagent under basic conditions. This reagent has advantages in peptide synthesis since it avoids side reactions like the dehydration of asparagine or glutamine redisues. BOP has used for the synthesis of esters from the carboxylic acids and alcohols. BOP has also been used in the reduction of carboxylic acids to primary alcohols with sodium borohydride (NaBH4). Its use raises safety concerns since the carcinogenic compound HMPA is produced as a stoichiometric by-product. == See also == PyBOP, a related phosphonium reagent for amide bond formation PyAOP, a related phosphonium reagent for amide bond formation == References ==
|
{
"page_id": 12649653,
"source": null,
"title": "BOP reagent"
}
|
H2 producing hydrogenase may refer to: Ferredoxin hydrogenase, an enzyme Hydrogenase (acceptor), an enzyme
|
{
"page_id": 38339763,
"source": null,
"title": "H2 producing hydrogenase"
}
|
In quantum mechanics, an atomic orbital ( ) is a function describing the location and wave-like behavior of an electron in an atom. This function describes an electron's charge distribution around the atom's nucleus, and can be used to calculate the probability of finding an electron in a specific region around the nucleus. Each orbital in an atom is characterized by a set of values of three quantum numbers n, ℓ, and mℓ, which respectively correspond to electron's energy, its orbital angular momentum, and its orbital angular momentum projected along a chosen axis (magnetic quantum number). The orbitals with a well-defined magnetic quantum number are generally complex-valued. Real-valued orbitals can be formed as linear combinations of mℓ and −mℓ orbitals, and are often labeled using associated harmonic polynomials (e.g., xy, x2 − y2) which describe their angular structure. An orbital can be occupied by a maximum of two electrons, each with its own projection of spin m s {\displaystyle m_{s}} . The simple names s orbital, p orbital, d orbital, and f orbital refer to orbitals with angular momentum quantum number ℓ = 0, 1, 2, and 3 respectively. These names, together with their n values, are used to describe electron configurations of atoms. They are derived from description by early spectroscopists of certain series of alkali metal spectroscopic lines as sharp, principal, diffuse, and fundamental. Orbitals for ℓ > 3 continue alphabetically (g, h, i, k, ...), omitting j because some languages do not distinguish between letters "i" and "j". Atomic orbitals are basic building blocks of the atomic orbital model (or electron cloud or wave mechanics model), a modern framework for visualizing submicroscopic behavior of electrons in matter. In this model, the electron cloud of an atom may be seen as being built up (in approximation) in an
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
}
|
electron configuration that is a product of simpler hydrogen-like atomic orbitals. The repeating periodicity of blocks of 2, 6, 10, and 14 elements within sections of periodic table arises naturally from total number of electrons that occupy a complete set of s, p, d, and f orbitals, respectively, though for higher values of quantum number n, particularly when the atom bears a positive charge, energies of certain sub-shells become very similar and so, the order in which they are said to be populated by electrons (e.g., Cr = [Ar]4s13d5 and Cr2+ = [Ar]3d4) can be rationalized only somewhat arbitrarily. == Electron properties == With the development of quantum mechanics and experimental findings (such as the two slit diffraction of electrons), it was found that the electrons orbiting a nucleus could not be fully described as particles, but needed to be explained by wave–particle duality. In this sense, electrons have the following properties: Wave-like properties: Electrons do not orbit a nucleus in the manner of a planet orbiting a star, but instead exist as standing waves. Thus the lowest possible energy an electron can take is similar to the fundamental frequency of a wave on a string. Higher energy states are similar to harmonics of that fundamental frequency. The electrons are never in a single point location, though the probability of interacting with the electron at a single point can be found from the electron's wave function. The electron's charge acts like it is smeared out in space in a continuous distribution, proportional at any point to the squared magnitude of the electron's wave function. Particle-like properties: The number of electrons orbiting a nucleus can be only an integer. Electrons jump between orbitals like particles. For example, if one photon strikes the electrons, only one electron changes state as a result.
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Electrons retain particle-like properties such as: each wave state has the same electric charge as its electron particle. Each wave state has a single discrete spin (spin up or spin down) depending on its superposition. Thus, electrons cannot be described simply as solid particles. An analogy might be that of a large and often oddly shaped "atmosphere" (the electron), distributed around a relatively tiny planet (the nucleus). Atomic orbitals exactly describe the shape of this "atmosphere" only when one electron is present. When more electrons are added, the additional electrons tend to more evenly fill in a volume of space around the nucleus so that the resulting collection ("electron cloud") tends toward a generally spherical zone of probability describing the electron's location, because of the uncertainty principle. One should remember that these orbital 'states', as described here, are merely eigenstates of an electron in its orbit. An actual electron exists in a superposition of states, which is like a weighted average, but with complex number weights. So, for instance, an electron could be in a pure eigenstate (2, 1, 0), or a mixed state 1/2(2, 1, 0) + 1/2 i {\displaystyle i} (2, 1, 1), or even the mixed state 2/5(2, 1, 0) + 3/5 i {\displaystyle i} (2, 1, 1). For each eigenstate, a property has an eigenvalue. So, for the three states just mentioned, the value of n {\displaystyle n} is 2, and the value of l {\displaystyle l} is 1. For the second and third states, the value for m l {\displaystyle m_{l}} is a superposition of 0 and 1. As a superposition of states, it is ambiguous—either exactly 0 or exactly 1—not an intermediate or average value like the fraction 1/2. A superposition of eigenstates (2, 1, 1) and (3, 2, 1) would have an ambiguous
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"page_id": 1206,
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n {\displaystyle n} and l {\displaystyle l} , but m l {\displaystyle m_{l}} would definitely be 1. Eigenstates make it easier to deal with the math. You can choose a different basis of eigenstates by superimposing eigenstates from any other basis (see Real orbitals below). === Formal quantum mechanical definition === Atomic orbitals may be defined more precisely in formal quantum mechanical language. They are approximate solutions to the Schrödinger equation for the electrons bound to the atom by the electric field of the atom's nucleus. Specifically, in quantum mechanics, the state of an atom, i.e., an eigenstate of the atomic Hamiltonian, is approximated by an expansion (see configuration interaction expansion and basis set) into linear combinations of anti-symmetrized products (Slater determinants) of one-electron functions. The spatial components of these one-electron functions are called atomic orbitals. (When one considers also their spin component, one speaks of atomic spin orbitals.) A state is actually a function of the coordinates of all the electrons, so that their motion is correlated, but this is often approximated by this independent-particle model of products of single electron wave functions. (The London dispersion force, for example, depends on the correlations of the motion of the electrons.) In atomic physics, the atomic spectral lines correspond to transitions (quantum leaps) between quantum states of an atom. These states are labeled by a set of quantum numbers summarized in the term symbol and usually associated with particular electron configurations, i.e., by occupation schemes of atomic orbitals (for example, 1s2 2s2 2p6 for the ground state of neon-term symbol: 1S0). This notation means that the corresponding Slater determinants have a clear higher weight in the configuration interaction expansion. The atomic orbital concept is therefore a key concept for visualizing the excitation process associated with a given transition. For example, one
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"page_id": 1206,
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can say for a given transition that it corresponds to the excitation of an electron from an occupied orbital to a given unoccupied orbital. Nevertheless, one has to keep in mind that electrons are fermions ruled by the Pauli exclusion principle and cannot be distinguished from each other. Moreover, it sometimes happens that the configuration interaction expansion converges very slowly and that one cannot speak about simple one-determinant wave function at all. This is the case when electron correlation is large. Fundamentally, an atomic orbital is a one-electron wave function, even though many electrons are not in one-electron atoms, and so the one-electron view is an approximation. When thinking about orbitals, we are often given an orbital visualization heavily influenced by the Hartree–Fock approximation, which is one way to reduce the complexities of molecular orbital theory. === Types of orbital === Atomic orbitals can be the hydrogen-like "orbitals" which are exact solutions to the Schrödinger equation for a hydrogen-like "atom" (i.e., atom with one electron). Alternatively, atomic orbitals refer to functions that depend on the coordinates of one electron (i.e., orbitals) but are used as starting points for approximating wave functions that depend on the simultaneous coordinates of all the electrons in an atom or molecule. The coordinate systems chosen for orbitals are usually spherical coordinates (r, θ, φ) in atoms and Cartesian (x, y, z) in polyatomic molecules. The advantage of spherical coordinates here is that an orbital wave function is a product of three factors each dependent on a single coordinate: ψ(r, θ, φ) = R(r) Θ(θ) Φ(φ). The angular factors of atomic orbitals Θ(θ) Φ(φ) generate s, p, d, etc. functions as real combinations of spherical harmonics Yℓm(θ, φ) (where ℓ and m are quantum numbers). There are typically three mathematical forms for the radial functions R(r)
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which can be chosen as a starting point for the calculation of the properties of atoms and molecules with many electrons: The hydrogen-like orbitals are derived from the exact solutions of the Schrödinger equation for one electron and a nucleus, for a hydrogen-like atom. The part of the function that depends on distance r from the nucleus has radial nodes and decays as e − α r {\displaystyle e^{-\alpha r}} . The Slater-type orbital (STO) is a form without radial nodes but decays from the nucleus as does a hydrogen-like orbital. The form of the Gaussian type orbital (Gaussians) has no radial nodes and decays as e − α r 2 {\displaystyle e^{-\alpha r^{2}}} . Although hydrogen-like orbitals are still used as pedagogical tools, the advent of computers has made STOs preferable for atoms and diatomic molecules since combinations of STOs can replace the nodes in hydrogen-like orbitals. Gaussians are typically used in molecules with three or more atoms. Although not as accurate by themselves as STOs, combinations of many Gaussians can attain the accuracy of hydrogen-like orbitals. == History == The term orbital was introduced by Robert S. Mulliken in 1932 as short for one-electron orbital wave function. Niels Bohr explained around 1913 that electrons might revolve around a compact nucleus with definite angular momentum. Bohr's model was an improvement on the 1911 explanations of Ernest Rutherford, that of the electron moving around a nucleus. Japanese physicist Hantaro Nagaoka published an orbit-based hypothesis for electron behavior as early as 1904. These theories were each built upon new observations starting with simple understanding and becoming more correct and complex. Explaining the behavior of these electron "orbits" was one of the driving forces behind the development of quantum mechanics. === Early models === With J. J. Thomson's discovery of the electron
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in 1897, it became clear that atoms were not the smallest building blocks of nature, but were rather composite particles. The newly discovered structure within atoms tempted many to imagine how the atom's constituent parts might interact with each other. Thomson theorized that multiple electrons revolve in orbit-like rings within a positively charged jelly-like substance, and between the electron's discovery and 1909, this "plum pudding model" was the most widely accepted explanation of atomic structure. Shortly after Thomson's discovery, Hantaro Nagaoka predicted a different model for electronic structure. Unlike the plum pudding model, the positive charge in Nagaoka's "Saturnian Model" was concentrated into a central core, pulling the electrons into circular orbits reminiscent of Saturn's rings. Few people took notice of Nagaoka's work at the time, and Nagaoka himself recognized a fundamental defect in the theory even at its conception, namely that a classical charged object cannot sustain orbital motion because it is accelerating and therefore loses energy due to electromagnetic radiation. Nevertheless, the Saturnian model turned out to have more in common with modern theory than any of its contemporaries. === Bohr atom === In 1909, Ernest Rutherford discovered that the bulk of the atomic mass was tightly condensed into a nucleus, which was also found to be positively charged. It became clear from his analysis in 1911 that the plum pudding model could not explain atomic structure. In 1913, Rutherford's post-doctoral student, Niels Bohr, proposed a new model of the atom, wherein electrons orbited the nucleus with classical periods, but were permitted to have only discrete values of angular momentum, quantized in units ħ. This constraint automatically allowed only certain electron energies. The Bohr model of the atom fixed the problem of energy loss from radiation from a ground state (by declaring that there was no state below
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"page_id": 1206,
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this), and more importantly explained the origin of spectral lines. After Bohr's use of Einstein's explanation of the photoelectric effect to relate energy levels in atoms with the wavelength of emitted light, the connection between the structure of electrons in atoms and the emission and absorption spectra of atoms became an increasingly useful tool in the understanding of electrons in atoms. The most prominent feature of emission and absorption spectra (known experimentally since the middle of the 19th century), was that these atomic spectra contained discrete lines. The significance of the Bohr model was that it related the lines in emission and absorption spectra to the energy differences between the orbits that electrons could take around an atom. This was, however, not achieved by Bohr through giving the electrons some kind of wave-like properties, since the idea that electrons could behave as matter waves was not suggested until eleven years later. Still, the Bohr model's use of quantized angular momenta and therefore quantized energy levels was a significant step toward the understanding of electrons in atoms, and also a significant step towards the development of quantum mechanics in suggesting that quantized restraints must account for all discontinuous energy levels and spectra in atoms. With de Broglie's suggestion of the existence of electron matter waves in 1924, and for a short time before the full 1926 Schrödinger equation treatment of hydrogen-like atoms, a Bohr electron "wavelength" could be seen to be a function of its momentum; so a Bohr orbiting electron was seen to orbit in a circle at a multiple of its half-wavelength. The Bohr model for a short time could be seen as a classical model with an additional constraint provided by the 'wavelength' argument. However, this period was immediately superseded by the full three-dimensional wave mechanics of 1926.
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In our current understanding of physics, the Bohr model is called a semi-classical model because of its quantization of angular momentum, not primarily because of its relationship with electron wavelength, which appeared in hindsight a dozen years after the Bohr model was proposed. The Bohr model was able to explain the emission and absorption spectra of hydrogen. The energies of electrons in the n = 1, 2, 3, etc. states in the Bohr model match those of current physics. However, this did not explain similarities between different atoms, as expressed by the periodic table, such as the fact that helium (two electrons), neon (10 electrons), and argon (18 electrons) exhibit similar chemical inertness. Modern quantum mechanics explains this in terms of electron shells and subshells which can each hold a number of electrons determined by the Pauli exclusion principle. Thus the n = 1 state can hold one or two electrons, while the n = 2 state can hold up to eight electrons in 2s and 2p subshells. In helium, all n = 1 states are fully occupied; the same is true for n = 1 and n = 2 in neon. In argon, the 3s and 3p subshells are similarly fully occupied by eight electrons; quantum mechanics also allows a 3d subshell but this is at higher energy than the 3s and 3p in argon (contrary to the situation for hydrogen) and remains empty. === Modern conceptions and connections to the Heisenberg uncertainty principle === Immediately after Heisenberg discovered his uncertainty principle, Bohr noted that the existence of any sort of wave packet implies uncertainty in the wave frequency and wavelength, since a spread of frequencies is needed to create the packet itself. In quantum mechanics, where all particle momenta are associated with waves, it is the formation of
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such a wave packet which localizes the wave, and thus the particle, in space. In states where a quantum mechanical particle is bound, it must be localized as a wave packet, and the existence of the packet and its minimum size implies a spread and minimal value in particle wavelength, and thus also momentum and energy. In quantum mechanics, as a particle is localized to a smaller region in space, the associated compressed wave packet requires a larger and larger range of momenta, and thus larger kinetic energy. Thus the binding energy to contain or trap a particle in a smaller region of space increases without bound as the region of space grows smaller. Particles cannot be restricted to a geometric point in space, since this would require infinite particle momentum. In chemistry, Erwin Schrödinger, Linus Pauling, Mulliken and others noted that the consequence of Heisenberg's relation was that the electron, as a wave packet, could not be considered to have an exact location in its orbital. Max Born suggested that the electron's position needed to be described by a probability distribution which was connected with finding the electron at some point in the wave-function which described its associated wave packet. The new quantum mechanics did not give exact results, but only the probabilities for the occurrence of a variety of possible such results. Heisenberg held that the path of a moving particle has no meaning if we cannot observe it, as we cannot with electrons in an atom. In the quantum picture of Heisenberg, Schrödinger and others, the Bohr atom number n for each orbital became known as an n-sphere in a three-dimensional atom and was pictured as the most probable energy of the probability cloud of the electron's wave packet which surrounded the atom. == Orbital names ==
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=== Orbital notation and subshells === Orbitals have been given names, which are usually given in the form: X t y p e {\displaystyle X\,\mathrm {type} \ } where X is the energy level corresponding to the principal quantum number n; type is a lower-case letter denoting the shape or subshell of the orbital, corresponding to the angular momentum quantum number ℓ. For example, the orbital 1s (pronounced as the individual numbers and letters: "'one' 'ess'") is the lowest energy level (n = 1) and has an angular quantum number of ℓ = 0, denoted as s. Orbitals with ℓ = 1, 2 and 3 are denoted as p, d and f respectively. The set of orbitals for a given n and ℓ is called a subshell, denoted X t y p e y {\displaystyle X\,\mathrm {type} ^{y}\ } . The superscript y shows the number of electrons in the subshell. For example, the notation 2p4 indicates that the 2p subshell of an atom contains 4 electrons. This subshell has 3 orbitals, each with n = 2 and ℓ = 1. === X-ray notation === There is also another, less common system still used in X-ray science known as X-ray notation, which is a continuation of the notations used before orbital theory was well understood. In this system, the principal quantum number is given a letter associated with it. For n = 1, 2, 3, 4, 5, ..., the letters associated with those numbers are K, L, M, N, O, ... respectively. == Hydrogen-like orbitals == The simplest atomic orbitals are those that are calculated for systems with a single electron, such as the hydrogen atom. An atom of any other element ionized down to a single electron (He+, Li2+, etc.) is very similar to hydrogen, and the orbitals take
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the same form. In the Schrödinger equation for this system of one negative and one positive particle, the atomic orbitals are the eigenstates of the Hamiltonian operator for the energy. They can be obtained analytically, meaning that the resulting orbitals are products of a polynomial series, and exponential and trigonometric functions. (see hydrogen atom). For atoms with two or more electrons, the governing equations can be solved only with the use of methods of iterative approximation. Orbitals of multi-electron atoms are qualitatively similar to those of hydrogen, and in the simplest models, they are taken to have the same form. For more rigorous and precise analysis, numerical approximations must be used. A given (hydrogen-like) atomic orbital is identified by unique values of three quantum numbers: n, ℓ, and mℓ. The rules restricting the values of the quantum numbers, and their energies (see below), explain the electron configuration of the atoms and the periodic table. The stationary states (quantum states) of a hydrogen-like atom are its atomic orbitals. However, in general, an electron's behavior is not fully described by a single orbital. Electron states are best represented by time-depending "mixtures" (linear combinations) of multiple orbitals. See Linear combination of atomic orbitals molecular orbital method. The quantum number n first appeared in the Bohr model where it determines the radius of each circular electron orbit. In modern quantum mechanics however, n determines the mean distance of the electron from the nucleus; all electrons with the same value of n lie at the same average distance. For this reason, orbitals with the same value of n are said to comprise a "shell". Orbitals with the same value of n and also the same value of ℓ are even more closely related, and are said to comprise a "subshell". == Quantum numbers == Because
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of the quantum mechanical nature of the electrons around a nucleus, atomic orbitals can be uniquely defined by a set of integers known as quantum numbers. These quantum numbers occur only in certain combinations of values, and their physical interpretation changes depending on whether real or complex versions of the atomic orbitals are employed. === Complex orbitals === In physics, the most common orbital descriptions are based on the solutions to the hydrogen atom, where orbitals are given by the product between a radial function and a pure spherical harmonic. The quantum numbers, together with the rules governing their possible values, are as follows: The principal quantum number n describes the energy of the electron and is always a positive integer. In fact, it can be any positive integer, but for reasons discussed below, large numbers are seldom encountered. Each atom has, in general, many orbitals associated with each value of n; these orbitals together are sometimes called electron shells. The azimuthal quantum number ℓ describes the orbital angular momentum of each electron and is a non-negative integer. Within a shell where n is some integer n0, ℓ ranges across all (integer) values satisfying the relation 0 ≤ ℓ ≤ n 0 − 1 {\displaystyle 0\leq \ell \leq n_{0}-1} . For instance, the n = 1 shell has only orbitals with ℓ = 0 {\displaystyle \ell =0} , and the n = 2 shell has only orbitals with ℓ = 0 {\displaystyle \ell =0} , and ℓ = 1 {\displaystyle \ell =1} . The set of orbitals associated with a particular value of ℓ are sometimes collectively called a subshell. The magnetic quantum number, m ℓ {\displaystyle m_{\ell }} , describes the projection of the orbital angular momentum along a chosen axis. It determines the magnitude of the current circulating
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"page_id": 1206,
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around that axis and the orbital contribution to the magnetic moment of an electron via the Ampèrian loop model. Within a subshell ℓ {\displaystyle \ell } , m ℓ {\displaystyle m_{\ell }} obtains the integer values in the range − ℓ ≤ m ℓ ≤ ℓ {\displaystyle -\ell \leq m_{\ell }\leq \ell } . The above results may be summarized in the following table. Each cell represents a subshell, and lists the values of m ℓ {\displaystyle m_{\ell }} available in that subshell. Empty cells represent subshells that do not exist. Subshells are usually identified by their n {\displaystyle n} - and ℓ {\displaystyle \ell } -values. n {\displaystyle n} is represented by its numerical value, but ℓ {\displaystyle \ell } is represented by a letter as follows: 0 is represented by 's', 1 by 'p', 2 by 'd', 3 by 'f', and 4 by 'g'. For instance, one may speak of the subshell with n = 2 {\displaystyle n=2} and ℓ = 0 {\displaystyle \ell =0} as a '2s subshell'. Each electron also has angular momentum in the form of quantum mechanical spin given by spin s = 1/2. Its projection along a specified axis is given by the spin magnetic quantum number, ms, which can be +1/2 or −1/2. These values are also called "spin up" or "spin down" respectively. The Pauli exclusion principle states that no two electrons in an atom can have the same values of all four quantum numbers. If there are two electrons in an orbital with given values for three quantum numbers, (n, ℓ, m), these two electrons must differ in their spin projection ms. The above conventions imply a preferred axis (for example, the z direction in Cartesian coordinates), and they also imply a preferred direction along this preferred axis. Otherwise there
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"page_id": 1206,
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would be no sense in distinguishing m = +1 from m = −1. As such, the model is most useful when applied to physical systems that share these symmetries. The Stern–Gerlach experiment—where an atom is exposed to a magnetic field—provides one such example. === Real orbitals === Instead of the complex orbitals described above, it is common, especially in the chemistry literature, to use real atomic orbitals. These real orbitals arise from simple linear combinations of complex orbitals. Using the Condon–Shortley phase convention, real orbitals are related to complex orbitals in the same way that the real spherical harmonics are related to complex spherical harmonics. Letting ψ n , ℓ , m {\displaystyle \psi _{n,\ell ,m}} denote a complex orbital with quantum numbers n, ℓ, and m, the real orbitals ψ n , ℓ , m real {\displaystyle \psi _{n,\ell ,m}^{\text{real}}} may be defined by ψ n , ℓ , m real = { 2 ( − 1 ) m Im { ψ n , ℓ , | m | } for m < 0 ψ n , ℓ , | m | for m = 0 2 ( − 1 ) m Re { ψ n , ℓ , | m | } for m > 0 = { i 2 ( ψ n , ℓ , − | m | − ( − 1 ) m ψ n , ℓ , | m | ) for m < 0 ψ n , ℓ , | m | for m = 0 1 2 ( ψ n , ℓ , − | m | + ( − 1 ) m ψ n , ℓ , | m | ) for m > 0 {\displaystyle {\begin{aligned}\psi _{n,\ell ,m}^{\text{real}}&={\begin{cases}{\sqrt {2}}(-1)^{m}{\text{Im}}\left\{\psi _{n,\ell ,|m|}\right\}&{\text{ for }}m<0\\[2pt]\psi _{n,\ell ,|m|}&{\text{ for }}m=0\\[2pt]{\sqrt {2}}(-1)^{m}{\text{Re}}\left\{\psi _{n,\ell ,|m|}\right\}&{\text{
|
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for }}m>0\end{cases}}\\[4pt]&={\begin{cases}{\frac {i}{\sqrt {2}}}\left(\psi _{n,\ell ,-|m|}-(-1)^{m}\psi _{n,\ell ,|m|}\right)&{\text{ for }}m<0\\[2pt]\psi _{n,\ell ,|m|}&{\text{ for }}m=0\\[4pt]{\frac {1}{\sqrt {2}}}\left(\psi _{n,\ell ,-|m|}+(-1)^{m}\psi _{n,\ell ,|m|}\right)&{\text{ for }}m>0\end{cases}}\end{aligned}}} If ψ n , ℓ , m ( r , θ , ϕ ) = R n l ( r ) Y ℓ m ( θ , ϕ ) {\displaystyle \psi _{n,\ell ,m}(r,\theta ,\phi )=R_{nl}(r)Y_{\ell }^{m}(\theta ,\phi )} , with R n l ( r ) {\displaystyle R_{nl}(r)} the radial part of the orbital, this definition is equivalent to ψ n , ℓ , m real ( r , θ , ϕ ) = R n l ( r ) Y ℓ m ( θ , ϕ ) {\displaystyle \psi _{n,\ell ,m}^{\text{real}}(r,\theta ,\phi )=R_{nl}(r)Y_{\ell m}(\theta ,\phi )} where Y ℓ m {\displaystyle Y_{\ell m}} is the real spherical harmonic related to either the real or imaginary part of the complex spherical harmonic Y ℓ m {\displaystyle Y_{\ell }^{m}} . Real spherical harmonics are physically relevant when an atom is embedded in a crystalline solid, in which case there are multiple preferred symmetry axes but no single preferred direction. Real atomic orbitals are also more frequently encountered in introductory chemistry textbooks and shown in common orbital visualizations. In real hydrogen-like orbitals, quantum numbers n and ℓ have the same interpretation and significance as their complex counterparts, but m is no longer a good quantum number (but its absolute value is). Some real orbitals are given specific names beyond the simple ψ n , ℓ , m {\displaystyle \psi _{n,\ell ,m}} designation. Orbitals with quantum number ℓ = 0, 1, 2, 3, 4, 5, 6... are called s, p, d, f, g, h, i, ... orbitals. With this one can already assign names to complex orbitals such as 2 p ± 1 = ψ 2 , 1 , ± 1
|
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{\displaystyle 2{\text{p}}_{\pm 1}=\psi _{2,1,\pm 1}} ; the first symbol is the n quantum number, the second character is the symbol for that particular ℓ quantum number and the subscript is the m quantum number. As an example of how the full orbital names are generated for real orbitals, one may calculate ψ n , 1 , ± 1 real {\displaystyle \psi _{n,1,\pm 1}^{\text{real}}} . From the table of spherical harmonics, ψ n , 1 , ± 1 = R n , 1 Y 1 ± 1 = ∓ R n , 1 3 / 8 π ⋅ ( x ± i y ) / r {\textstyle \psi _{n,1,\pm 1}=R_{n,1}Y_{1}^{\pm 1}=\mp R_{n,1}{\sqrt {3/8\pi }}\cdot (x\pm iy)/r} with r = x 2 + y 2 + z 2 {\textstyle r={\sqrt {x^{2}+y^{2}+z^{2}}}} . Then ψ n , 1 , + 1 real = R n , 1 3 4 π ⋅ x r ψ n , 1 , − 1 real = R n , 1 3 4 π ⋅ y r {\displaystyle {\begin{aligned}\psi _{n,1,+1}^{\text{real}}&=R_{n,1}{\sqrt {\frac {3}{4\pi }}}\cdot {\frac {x}{r}}\\\psi _{n,1,-1}^{\text{real}}&=R_{n,1}{\sqrt {\frac {3}{4\pi }}}\cdot {\frac {y}{r}}\end{aligned}}} Likewise ψ n , 1 , 0 = R n , 1 3 / 4 π ⋅ z / r {\textstyle \psi _{n,1,0}=R_{n,1}{\sqrt {3/4\pi }}\cdot z/r} . As a more complicated example: ψ n , 3 , + 1 real = R n , 3 1 4 21 2 π ⋅ x ⋅ ( 5 z 2 − r 2 ) r 3 {\displaystyle \psi _{n,3,+1}^{\text{real}}=R_{n,3}{\frac {1}{4}}{\sqrt {\frac {21}{2\pi }}}\cdot {\frac {x\cdot (5z^{2}-r^{2})}{r^{3}}}} In all these cases we generate a Cartesian label for the orbital by examining, and abbreviating, the polynomial in x, y, z appearing in the numerator. We ignore any terms in the z, r polynomial except for the term with the highest exponent in
|
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"page_id": 1206,
"source": null,
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z. We then use the abbreviated polynomial as a subscript label for the atomic state, using the same nomenclature as above to indicate the n {\displaystyle n} and ℓ {\displaystyle \ell } quantum numbers. ψ n , 1 , − 1 real = n p y = i 2 ( n p − 1 + n p + 1 ) ψ n , 1 , 0 real = n p z = 2 p 0 ψ n , 1 , + 1 real = n p x = 1 2 ( n p − 1 − n p + 1 ) ψ n , 3 , + 1 real = n f x z 2 = 1 2 ( n f − 1 − n f + 1 ) {\displaystyle {\begin{aligned}\psi _{n,1,-1}^{\text{real}}&=n{\text{p}}_{y}={\frac {i}{\sqrt {2}}}\left(n{\text{p}}_{-1}+n{\text{p}}_{+1}\right)\\\psi _{n,1,0}^{\text{real}}&=n{\text{p}}_{z}=2{\text{p}}_{0}\\\psi _{n,1,+1}^{\text{real}}&=n{\text{p}}_{x}={\frac {1}{\sqrt {2}}}\left(n{\text{p}}_{-1}-n{\text{p}}_{+1}\right)\\\psi _{n,3,+1}^{\text{real}}&=nf_{xz^{2}}={\frac {1}{\sqrt {2}}}\left(nf_{-1}-nf_{+1}\right)\end{aligned}}} The expression above all use the Condon–Shortley phase convention which is favored by quantum physicists. Other conventions exist for the phase of the spherical harmonics. Under these different conventions the p x {\displaystyle {\text{p}}_{x}} and p y {\displaystyle {\text{p}}_{y}} orbitals may appear, for example, as the sum and difference of p + 1 {\displaystyle {\text{p}}_{+1}} and p − 1 {\displaystyle {\text{p}}_{-1}} , contrary to what is shown above. Below is a list of these Cartesian polynomial names for the atomic orbitals. There does not seem to be reference in the literature as to how to abbreviate the long Cartesian spherical harmonic polynomials for ℓ > 3 {\displaystyle \ell >3} so there does not seem be consensus on the naming of g {\displaystyle g} orbitals or higher according to this nomenclature. == Shapes of orbitals == Simple pictures showing orbital shapes are intended to describe the angular forms of regions in space where the electrons occupying the orbital are likely
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"page_id": 1206,
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}
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to be found. The diagrams cannot show the entire region where an electron can be found, since according to quantum mechanics there is a non-zero probability of finding the electron (almost) anywhere in space. Instead the diagrams are approximate representations of boundary or contour surfaces where the probability density | ψ(r, θ, φ) |2 has a constant value, chosen so that there is a certain probability (for example 90%) of finding the electron within the contour. Although | ψ |2 as the square of an absolute value is everywhere non-negative, the sign of the wave function ψ(r, θ, φ) is often indicated in each subregion of the orbital picture. Sometimes the ψ function is graphed to show its phases, rather than | ψ(r, θ, φ) |2 which shows probability density but has no phase (which is lost when taking absolute value, since ψ(r, θ, φ) is a complex number). |ψ(r, θ, φ)|2 orbital graphs tend to have less spherical, thinner lobes than ψ(r, θ, φ) graphs, but have the same number of lobes in the same places, and otherwise are recognizable. This article, to show wave function phase, shows mostly ψ(r, θ, φ) graphs. The lobes can be seen as standing wave interference patterns between the two counter-rotating, ring-resonant traveling wave m and −m modes; the projection of the orbital onto the xy plane has a resonant m wavelength around the circumference. Although rarely shown, the traveling wave solutions can be seen as rotating banded tori; the bands represent phase information. For each m there are two standing wave solutions ⟨m⟩ + ⟨−m⟩ and ⟨m⟩ − ⟨−m⟩. If m = 0, the orbital is vertical, counter rotating information is unknown, and the orbital is z-axis symmetric. If ℓ = 0 there are no counter rotating modes. There are only radial
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
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modes and the shape is spherically symmetric. Nodal planes and nodal spheres are surfaces on which the probability density vanishes. The number of nodal surfaces is controlled by the quantum numbers n and ℓ. An orbital with azimuthal quantum number ℓ has ℓ radial nodal planes passing through the origin. For example, the s orbitals (ℓ = 0) are spherically symmetric and have no nodal planes, whereas the p orbitals (ℓ = 1) have a single nodal plane between the lobes. The number of nodal spheres equals n−ℓ−1, consistent with the restriction ℓ ≤ n−1 on the quantum numbers. The principal quantum number controls the total number of nodal surfaces which is n−1. Loosely speaking, n is energy, ℓ is analogous to eccentricity, and m is orientation. In general, n determines size and energy of the orbital for a given nucleus; as n increases, the size of the orbital increases. The higher nuclear charge Z of heavier elements causes their orbitals to contract by comparison to lighter ones, so that the size of the atom remains very roughly constant, even as the number of electrons increases. Also in general terms, ℓ determines an orbital's shape, and mℓ its orientation. However, since some orbitals are described by equations in complex numbers, the shape sometimes depends on mℓ also. Together, the whole set of orbitals for a given ℓ and n fill space as symmetrically as possible, though with increasingly complex sets of lobes and nodes. The single s orbitals ( ℓ = 0 {\displaystyle \ell =0} ) are shaped like spheres. For n = 1 it is roughly a solid ball (densest at center and fades outward exponentially), but for n ≥ 2, each single s orbital is made of spherically symmetric surfaces which are nested shells (i.e., the "wave-structure" is
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
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|
radial, following a sinusoidal radial component as well). See illustration of a cross-section of these nested shells, at right. The s orbitals for all n numbers are the only orbitals with an anti-node (a region of high wave function density) at the center of the nucleus. All other orbitals (p, d, f, etc.) have angular momentum, and thus avoid the nucleus (having a wave node at the nucleus). Recently, there has been an effort to experimentally image the 1s and 2p orbitals in a SrTiO3 crystal using scanning transmission electron microscopy with energy dispersive x-ray spectroscopy. Because the imaging was conducted using an electron beam, Coulombic beam-orbital interaction that is often termed as the impact parameter effect is included in the outcome (see the figure at right). The shapes of p, d and f orbitals are described verbally here and shown graphically in the Orbitals table below. The three p orbitals for n = 2 have the form of two ellipsoids with a point of tangency at the nucleus (the two-lobed shape is sometimes referred to as a "dumbbell"—there are two lobes pointing in opposite directions from each other). The three p orbitals in each shell are oriented at right angles to each other, as determined by their respective linear combination of values of mℓ. The overall result is a lobe pointing along each direction of the primary axes. Four of the five d orbitals for n = 3 look similar, each with four pear-shaped lobes, each lobe tangent at right angles to two others, and the centers of all four lying in one plane. Three of these planes are the xy-, xz-, and yz-planes—the lobes are between the pairs of primary axes—and the fourth has the center along the x and y axes themselves. The fifth and final d
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
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|
orbital consists of three regions of high probability density: a torus in between two pear-shaped regions placed symmetrically on its z axis. The overall total of 18 directional lobes point in every primary axis direction and between every pair. There are seven f orbitals, each with shapes more complex than those of the d orbitals. Additionally, as is the case with the s orbitals, individual p, d, f and g orbitals with n values higher than the lowest possible value, exhibit an additional radial node structure which is reminiscent of harmonic waves of the same type, as compared with the lowest (or fundamental) mode of the wave. As with s orbitals, this phenomenon provides p, d, f, and g orbitals at the next higher possible value of n (for example, 3p orbitals vs. the fundamental 2p), an additional node in each lobe. Still higher values of n further increase the number of radial nodes, for each type of orbital. The shapes of atomic orbitals in one-electron atom are related to 3-dimensional spherical harmonics. These shapes are not unique, and any linear combination is valid, like a transformation to cubic harmonics, in fact it is possible to generate sets where all the d's are the same shape, just like the px, py, and pz are the same shape. Although individual orbitals are most often shown independent of each other, the orbitals coexist around the nucleus at the same time. Also, in 1927, Albrecht Unsöld proved that if one sums the electron density of all orbitals of a particular azimuthal quantum number ℓ of the same shell n (e.g., all three 2p orbitals, or all five 3d orbitals) where each orbital is occupied by an electron or each is occupied by an electron pair, then all angular dependence disappears; that is, the
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
}
|
resulting total density of all the atomic orbitals in that subshell (those with the same ℓ) is spherical. This is known as Unsöld's theorem. === Orbitals table === This table shows the real hydrogen-like wave functions for all atomic orbitals up to 7s, and therefore covers the occupied orbitals in the ground state of all elements in the periodic table up to radium and some beyond. "ψ" graphs are shown with − and + wave function phases shown in two different colors (arbitrarily red and blue). The pz orbital is the same as the p0 orbital, but the px and py are formed by taking linear combinations of the p+1 and p−1 orbitals (which is why they are listed under the m = ±1 label). Also, the p+1 and p−1 are not the same shape as the p0, since they are pure spherical harmonics. * No elements with 6f, 7d or 7f electrons have been discovered yet. † Elements with 7p electrons have been discovered, but their electronic configurations are only predicted – save the exceptional Lr, which fills 7p1 instead of 6d1. ‡ For the elements whose highest occupied orbital is a 6d orbital, only some electronic configurations have been confirmed. (Mt, Ds, Rg and Cn are still missing). These are the real-valued orbitals commonly used in chemistry. Only the m = 0 {\displaystyle m=0} orbitals where are eigenstates of the orbital angular momentum operator, L ^ z {\displaystyle {\hat {L}}_{z}} . The columns with m = ± 1 , ± 2 , ⋯ {\displaystyle m=\pm 1,\pm 2,\cdots } are combinations of two eigenstates. See comparison in the following picture: === Qualitative understanding of shapes === The shapes of atomic orbitals can be qualitatively understood by considering the analogous case of standing waves on a circular drum. To see
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
}
|
the analogy, the mean vibrational displacement of each bit of drum membrane from the equilibrium point over many cycles (a measure of average drum membrane velocity and momentum at that point) must be considered relative to that point's distance from the center of the drum head. If this displacement is taken as being analogous to the probability of finding an electron at a given distance from the nucleus, then it will be seen that the many modes of the vibrating disk form patterns that trace the various shapes of atomic orbitals. The basic reason for this correspondence lies in the fact that the distribution of kinetic energy and momentum in a matter-wave is predictive of where the particle associated with the wave will be. That is, the probability of finding an electron at a given place is also a function of the electron's average momentum at that point, since high electron momentum at a given position tends to "localize" the electron in that position, via the properties of electron wave-packets (see the Heisenberg uncertainty principle for details of the mechanism). This relationship means that certain key features can be observed in both drum membrane modes and atomic orbitals. For example, in all of the modes analogous to s orbitals (the top row in the animated illustration below), it can be seen that the very center of the drum membrane vibrates most strongly, corresponding to the antinode in all s orbitals in an atom. This antinode means the electron is most likely to be at the physical position of the nucleus (which it passes straight through without scattering or striking it), since it is moving (on average) most rapidly at that point, giving it maximal momentum. A mental "planetary orbit" picture closest to the behavior of electrons in s orbitals, all
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
}
|
of which have no angular momentum, might perhaps be that of a Keplerian orbit with the orbital eccentricity of 1 but a finite major axis, not physically possible (because particles were to collide), but can be imagined as a limit of orbits with equal major axes but increasing eccentricity. Below, a number of drum membrane vibration modes and the respective wave functions of the hydrogen atom are shown. A correspondence can be considered where the wave functions of a vibrating drum head are for a two-coordinate system ψ(r, θ) and the wave functions for a vibrating sphere are three-coordinate ψ(r, θ, φ). s-type drum modes and wave functions None of the other sets of modes in a drum membrane have a central antinode, and in all of them the center of the drum does not move. These correspond to a node at the nucleus for all non-s orbitals in an atom. These orbitals all have some angular momentum, and in the planetary model, they correspond to particles in orbit with eccentricity less than 1.0, so that they do not pass straight through the center of the primary body, but keep somewhat away from it. In addition, the drum modes analogous to p and d modes in an atom show spatial irregularity along the different radial directions from the center of the drum, whereas all of the modes analogous to s modes are perfectly symmetrical in radial direction. The non-radial-symmetry properties of non-s orbitals are necessary to localize a particle with angular momentum and a wave nature in an orbital where it must tend to stay away from the central attraction force, since any particle localized at the point of central attraction could have no angular momentum. For these modes, waves in the drum head tend to avoid the central point.
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
}
|
Such features again emphasize that the shapes of atomic orbitals are a direct consequence of the wave nature of electrons. p-type drum modes and wave functions d-type drum modes == Orbital energy == In atoms with one electron (hydrogen-like atom), the energy of an orbital (and, consequently, any electron in the orbital) is determined mainly by n {\displaystyle n} . The n = 1 {\displaystyle n=1} orbital has the lowest possible energy in the atom. Each successively higher value of n {\displaystyle n} has a higher energy, but the difference decreases as n {\displaystyle n} increases. For high n {\displaystyle n} , the energy becomes so high that the electron can easily escape the atom. In single electron atoms, all levels with different ℓ {\displaystyle \ell } within a given n {\displaystyle n} are degenerate in the Schrödinger approximation, and have the same energy. This approximation is broken slightly in the solution to the Dirac equation (where energy depends on n and another quantum number j), and by the effect of the magnetic field of the nucleus and quantum electrodynamics effects. The latter induce tiny binding energy differences especially for s electrons that go nearer the nucleus, since these feel a very slightly different nuclear charge, even in one-electron atoms; see Lamb shift. In atoms with multiple electrons, the energy of an electron depends not only on its orbital, but also on its interactions with other electrons. These interactions depend on the detail of its spatial probability distribution, and so the energy levels of orbitals depend not only on n {\displaystyle n} but also on ℓ {\displaystyle \ell } . Higher values of ℓ {\displaystyle \ell } are associated with higher values of energy; for instance, the 2p state is higher than the 2s state. When ℓ = 2 {\displaystyle
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
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|
\ell =2} , the increase in energy of the orbital becomes so large as to push the energy of orbital above the energy of the s orbital in the next higher shell; when ℓ = 3 {\displaystyle \ell =3} the energy is pushed into the shell two steps higher. The filling of the 3d orbitals does not occur until the 4s orbitals have been filled. The increase in energy for subshells of increasing angular momentum in larger atoms is due to electron–electron interaction effects, and it is specifically related to the ability of low angular momentum electrons to penetrate more effectively toward the nucleus, where they are subject to less screening from the charge of intervening electrons. Thus, in atoms with higher atomic number, the ℓ {\displaystyle \ell } of electrons becomes more and more of a determining factor in their energy, and the principal quantum numbers n {\displaystyle n} of electrons becomes less and less important in their energy placement. The energy sequence of the first 35 subshells (e.g., 1s, 2p, 3d, etc.) is given in the following table. Each cell represents a subshell with n {\displaystyle n} and ℓ {\displaystyle \ell } given by its row and column indices, respectively. The number in the cell is the subshell's position in the sequence. For a linear listing of the subshells in terms of increasing energies in multielectron atoms, see the section below. Note: empty cells indicate non-existent sublevels, while numbers in italics indicate sublevels that could (potentially) exist, but which do not hold electrons in any element currently known. == Electron placement and the periodic table == Several rules govern the placement of electrons in orbitals (electron configuration). The first dictates that no two electrons in an atom may have the same set of values of quantum numbers (this
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
}
|
is the Pauli exclusion principle). These quantum numbers include the three that define orbitals, as well as the spin magnetic quantum number ms. Thus, two electrons may occupy a single orbital, so long as they have different values of ms. Because ms takes one of only two values (1/2 or −1/2), at most two electrons can occupy each orbital. Additionally, an electron always tends to fall to the lowest possible energy state. It is possible for it to occupy any orbital so long as it does not violate the Pauli exclusion principle, but if lower-energy orbitals are available, this condition is unstable. The electron will eventually lose energy (by releasing a photon) and drop into the lower orbital. Thus, electrons fill orbitals in the order specified by the energy sequence given above. This behavior is responsible for the structure of the periodic table. The table may be divided into several rows (called 'periods'), numbered starting with 1 at the top. The presently known elements occupy seven periods. If a certain period has number i, it consists of elements whose outermost electrons fall in the ith shell. Niels Bohr was the first to propose (1923) that the periodicity in the properties of the elements might be explained by the periodic filling of the electron energy levels, resulting in the electronic structure of the atom. The periodic table may also be divided into several numbered rectangular 'blocks'. The elements belonging to a given block have this common feature: their highest-energy electrons all belong to the same ℓ-state (but the n associated with that ℓ-state depends upon the period). For instance, the leftmost two columns constitute the 's-block'. The outermost electrons of Li and Be respectively belong to the 2s subshell, and those of Na and Mg to the 3s subshell. The following
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
}
|
is the order for filling the "subshell" orbitals, which also gives the order of the "blocks" in the periodic table: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p The "periodic" nature of the filling of orbitals, as well as emergence of the s, p, d, and f "blocks", is more obvious if this order of filling is given in matrix form, with increasing principal quantum numbers starting the new rows ("periods") in the matrix. Then, each subshell (composed of the first two quantum numbers) is repeated as many times as required for each pair of electrons it may contain. The result is a compressed periodic table, with each entry representing two successive elements: Although this is the general order of orbital filling according to the Madelung rule, there are exceptions, and the actual electronic energies of each element are also dependent upon additional details of the atoms (see Electron configuration § Atoms: Aufbau principle and Madelung rule). The number of electrons in an electrically neutral atom increases with the atomic number. The electrons in the outermost shell, or valence electrons, tend to be responsible for an element's chemical behavior. Elements that contain the same number of valence electrons can be grouped together and display similar chemical properties. === Relativistic effects === For elements with high atomic number Z, the effects of relativity become more pronounced, and especially so for s electrons, which move at relativistic velocities as they penetrate the screening electrons near the core of high-Z atoms. This relativistic increase in momentum for high speed electrons causes a corresponding decrease in wavelength and contraction of 6s orbitals relative to 5d orbitals (by comparison to corresponding s and d electrons in lighter elements in the same column of the
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
}
|
periodic table); this results in 6s valence electrons becoming lowered in energy. Examples of significant physical outcomes of this effect include the lowered melting temperature of mercury (which results from 6s electrons not being available for metal bonding) and the golden color of gold and caesium. In the Bohr model, an n = 1 electron has a velocity given by v = Z α c {\displaystyle v=Z\alpha c} , where Z is the atomic number, α {\displaystyle \alpha } is the fine-structure constant, and c is the speed of light. In non-relativistic quantum mechanics, therefore, any atom with an atomic number greater than 137 would require its 1s electrons to be traveling faster than the speed of light. Even in the Dirac equation, which accounts for relativistic effects, the wave function of the electron for atoms with Z > 137 {\displaystyle Z>137} is oscillatory and unbounded. The significance of element 137, also known as untriseptium, was first pointed out by the physicist Richard Feynman. Element 137 is sometimes informally called feynmanium (symbol Fy). However, Feynman's approximation fails to predict the exact critical value of Z due to the non-point-charge nature of the nucleus and very small orbital radius of inner electrons, resulting in a potential seen by inner electrons which is effectively less than Z. The critical Z value, which makes the atom unstable with regard to high-field breakdown of the vacuum and production of electron-positron pairs, does not occur until Z is about 173. These conditions are not seen except transiently in collisions of very heavy nuclei such as lead or uranium in accelerators, where such electron-positron production from these effects has been claimed to be observed. There are no nodes in relativistic orbital densities, although individual components of the wave function will have nodes. === pp hybridization (conjectured)
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
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|
=== In late period 8 elements, a hybrid of 8p3/2 and 9p1/2 is expected to exist, where "3/2" and "1/2" refer to the total angular momentum quantum number. This "pp" hybrid may be responsible for the p-block of the period due to properties similar to p subshells in ordinary valence shells. Energy levels of 8p3/2 and 9p1/2 come close due to relativistic spin–orbit effects; the 9s subshell should also participate, as these elements are expected to be analogous to the respective 5p elements indium through xenon. == Transitions between orbitals == Bound quantum states have discrete energy levels. When applied to atomic orbitals, this means that the energy differences between states are also discrete. A transition between these states (i.e., an electron absorbing or emitting a photon) can thus happen only if the photon has an energy corresponding with the exact energy difference between said states. Consider two states of the hydrogen atom: State n = 1, ℓ = 0, mℓ = 0 and ms = +1/2 State n = 2, ℓ = 0, mℓ = 0 and ms = −1/2 By quantum theory, state 1 has a fixed energy of E1, and state 2 has a fixed energy of E2. Now, what would happen if an electron in state 1 were to move to state 2? For this to happen, the electron would need to gain an energy of exactly E2 − E1. If the electron receives energy that is less than or greater than this value, it cannot jump from state 1 to state 2. Now, suppose we irradiate the atom with a broad-spectrum of light. Photons that reach the atom that have an energy of exactly E2 − E1 will be absorbed by the electron in state 1, and that electron will jump to state 2. However,
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
}
|
photons that are greater or lower in energy cannot be absorbed by the electron, because the electron can jump only to one of the orbitals, it cannot jump to a state between orbitals. The result is that only photons of a specific frequency will be absorbed by the atom. This creates a line in the spectrum, known as an absorption line, which corresponds to the energy difference between states 1 and 2. The atomic orbital model thus predicts line spectra, which are observed experimentally. This is one of the main validations of the atomic orbital model. The atomic orbital model is nevertheless an approximation to the full quantum theory, which only recognizes many electron states. The predictions of line spectra are qualitatively useful but are not quantitatively accurate for atoms and ions other than those containing only one electron. == See also == == References == McCaw, Charles S. (2015). Orbitals: With Applications in Atomic Spectra. Singapore: World Scientific Publishing Company. ISBN 9781783264162. Tipler, Paul; Llewellyn, Ralph (2003). Modern Physics (4 ed.). New York: W. H. Freeman and Company. ISBN 978-0-7167-4345-3. Scerri, Eric (2007). The Periodic Table, Its Story and Its Significance. New York: Oxford University Press. ISBN 978-0-19-530573-9. Levine, Ira (2014). Quantum Chemistry (7th ed.). Pearson Education. ISBN 978-0-321-80345-0. Griffiths, David (2000). Introduction to Quantum Mechanics (2 ed.). Benjamin Cummings. ISBN 978-0-13-111892-8. Cohen, Irwin; Bustard, Thomas (1966). "Atomic Orbitals: Limitations and Variations". J. Chem. Educ. 43 (4): 187. Bibcode:1966JChEd..43..187C. doi:10.1021/ed043p187. == External links == 3D representation of hydrogenic orbitals The Orbitron, a visualization of all common and uncommon atomic orbitals, from 1s to 7g Grand table Still images of many orbitals
|
{
"page_id": 1206,
"source": null,
"title": "Atomic orbital"
}
|
The Eschweiler–Clarke reaction (also called the Eschweiler–Clarke methylation) is a chemical reaction whereby a primary (or secondary) amine is methylated using excess formic acid and formaldehyde. Reductive amination reactions such as this one will not produce quaternary ammonium salts, but instead will stop at the tertiary amine stage. It is named for the German chemist Wilhelm Eschweiler (1860–1936) and the British chemist Hans Thacher Clarke (1887–1972). == Mechanism == The reaction is generally performed in an aqueous solution at close to boiling. The first methylation of the amine begins with imine formation with formaldehyde. The formic acid acts as a source of hydride and reduces the imine to a secondary amine. Loss of carbon dioxide gas renders the reaction irreversible. Despite being more hindered, the formation of the tertiary amine is more favorable, as the intermediate in iminium ion is formed without needing to protonate. Hence the treatment of a primary amine with less than 2 equivalents of formaldehyde will give more tertiary than secondary amine, along with unreacted starting material. From this mechanism it is clear that a quaternary ammonium salt will never form, because it is impossible for a tertiary amine to form another imine or iminium ion. Chiral amines typically do not racemize under these conditions. Altered versions of this reaction replace formic acid with sodium cyanoborohydride. == See also == Leuckart–Wallach reaction Pictet–Spengler reaction == References ==
|
{
"page_id": 2032822,
"source": null,
"title": "Eschweiler–Clarke reaction"
}
|
The molecular formula C15H22N2O2 may refer to: Alprenoxime, a beta blocker and prodrug to alprenolol Mepindolol, a non-selective beta blocker used to treat glaucoma
|
{
"page_id": 26608825,
"source": null,
"title": "C15H22N2O2"
}
|
A Brief History of Time is a 1991 biographical documentary film about the physicist Stephen Hawking, directed by Errol Morris. The title derives from Hawking's bestselling 1988 book A Brief History of Time, but, whereas the book is solely an explanation of cosmology, the film is also a biography of Hawking, featuring interviews with some of his family members and colleagues. The film is scored by frequent Morris collaborator Philip Glass. == Production == This project originated with executive producer Gordon Freedman, who brought it to Anglia Television. After acquiring the property, Freedman met with director Steven Spielberg for advice on how to make the project into a documentary film. Spielberg suggested Errol Morris as director. Morris had studied the history and philosophy of science at Princeton and later Berkeley, so was familiar with many of the topics in Hawking's book. Freedman's production company partnered with Anglia Television and Tokyo Broadcasting. David Hickman, of Anglia, became the film's producer. Morris only had a few days of access to film and interview Hawking. Because of Hawking's ALS, a disease that progressively affects nerve cells within the spine and brain, Morris filmed various static shots of Hawking, his wheelchair, and the tools he used to communicate, such as his battery-powered computer-based communication system with an electronic voicebox (which was sponsored and provided by Intel Corporation), to later edit together for the video component of Hawking's interview segments in the film. Although Hawking had an aversion to featuring his personal life in the film, Morris saw A Brief History of Time as being as much a biography as a science text, and much of his directing and editing work was dedicated to finding ways to depict ideas from theoretical physics and cosmology and then connect those ideas with details from the life of
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{
"page_id": 1443000,
"source": null,
"title": "A Brief History of Time (film)"
}
|
Hawking. He employed stylized interview sequences, graphic illustrations, and music written by Glass. Morris also included clips from Disney's The Black Hole (1979). Instead of Morris traveling around and filming the various interview subjects in their native surroundings, all of the interviews for this film were shot on specially built sets on a sound stage in England. Morris said he was "very moved by Hawking as a man", calling him "immensely likable, perverse, funny...and yes, he's a genius." He remembers that Hawking had posters of Marilyn Monroe in his office, and one of them fell down while they were filming. "A fallen woman", Hawking's speech synthesizer intoned. Hawking's mother, Isobel, is the first person we hear from in the movie, and near the end she describes her son as "a seeker" for truth. After the movie premiered, Hawking told Morris, "Thank you for making my mother a star." == List of interviewees == (in order of appearance) Isobel Hawking, Hawking's mother Janet Humphrey, Hawking's aunt Mary Hawking, Hawking's sister Basil King, neighbor of the Hawkings Derek Powney, classmate of Hawking at Oxford Norman Dix, classmate of Hawking at Oxford Robert Berman, tutor of Hawking at Oxford Gordon Berry, classmate of Hawking at Oxford Roger Penrose, mathematical physicist who worked with Hawking on Penrose-Hawking singularity theorems Dennis Sciama, cosmologist and PhD supervisor for Hawking John Wheeler, theoretical physicist who coined the phrase "black hole" Brandon Carter, physicist John Taylor, physicist Kip Thorne, astrophysicist and friend of Hawking Don Page, theoretical physicist, doctoral student of Hawking Christopher Isham, physicist Brian Whitt, physicist and editor of Hawking's A Brief History of Time Raymond Laflamme, theoretical physicist, doctoral student of Hawking == Music == The soundtrack for A Brief History of Time was composed by Philip Glass. Morris says he had Glass compose the
|
{
"page_id": 1443000,
"source": null,
"title": "A Brief History of Time (film)"
}
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