text stringlengths 11 1.65k | source stringlengths 38 44 |
|---|---|
Chemistry is considered to have become an established science with the work of Antoine Lavoisier, who developed a law of conservation of mass that demanded careful measurement and quantitative observations of chemical phenomena. The history of chemistry is intertwined with the history of thermodynamics, especially thro... | https://en.wikipedia.org/wiki?curid=5180 |
Chemistry More recently, in 1998, Professor Raymond Chang broadened the definition of "chemistry" to mean the study of matter and the changes it undergoes. Early civilizations, such as the Egyptians Babylonians, Indians amassed practical knowledge concerning the arts of metallurgy, pottery and dyes, but didn't develop ... | https://en.wikipedia.org/wiki?curid=5180 |
Chemistry In the Hellenistic world the art of alchemy first proliferated, mingling magic and occultism into the study of natural substances with the ultimate goal of transmuting elements into gold and discovering the elixir of eternal life. Work, particularly the development of distillation, continued in the early Byza... | https://en.wikipedia.org/wiki?curid=5180 |
Chemistry In the Middle Ages, Jabir's treatises on alchemy were translated into Latin and became standard texts for European alchemists. These include the "Kitab al-Kimya" (titled "Book of the Composition of Alchemy" in Europe), translated by Robert of Chester (1144); and the "Kitab al-Sab'een" ("Book of Seventy") by G... | https://en.wikipedia.org/wiki?curid=5180 |
Chemistry The theory of phlogiston (a substance at the root of all combustion) was propounded by the German Georg Ernst Stahl in the early 18th century and was only overturned by the end of the century by the French chemist Antoine Lavoisier, the chemical analogue of Newton in physics; who did more than any other to es... | https://en.wikipedia.org/wiki?curid=5180 |
Chemistry Davy discovered nine new elements including the alkali metals by extracting them from their oxides with electric current. British William Prout first proposed ordering all the elements by their atomic weight as all atoms had a weight that was an exact multiple of the atomic weight of hydrogen. J.A.R. Newlands... | https://en.wikipedia.org/wiki?curid=5180 |
Chemistry His work on atomic structure was improved on by his students, the Danish physicist Niels Bohr and Henry Moseley. The electronic theory of chemical bonds and molecular orbitals was developed by the American scientists Linus Pauling and Gilbert N. Lewis. The year 2011 was declared by the United Nations as the I... | https://en.wikipedia.org/wiki?curid=5180 |
Chemistry These include inorganic chemistry, the study of inorganic matter; organic chemistry, the study of organic (carbon-based) matter; biochemistry, the study of substances found in biological organisms; physical chemistry, the study of chemical processes using physical concepts such as thermodynamics and quantum m... | https://en.wikipedia.org/wiki?curid=5180 |
Chemistry The global top 50 chemical producers in 2013 had sales of US$980.5 billion with a profit margin of 10.3%. | https://en.wikipedia.org/wiki?curid=5180 |
Constellation A constellation is an area on the celestial sphere in which a group of stars forms an imaginary outline or pattern, typically representing an animal, mythological person or creature, or an inanimate object. The origins of the earliest constellations likely go back to prehistory. People used them to relate... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation In 1922, the International Astronomical Union (IAU) formally accepted the modern list of 88 constellations, and in 1928 adopted official constellation boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations. Some... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation Colloquial usage does not draw a sharp distinction between "constellations" and smaller "asterisms" (pattern of stars), yet the modern accepted astronomical constellations employ such a distinction. E.g., the Pleiades and the Hyades are both asterisms, and each lies within the boundaries of the constellat... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring individual stars' common proper motions or cpm by accurate astrometry and their radial velocities by astronomical spectroscopy. Both th... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation It seems that the bulk of the Mesopotamian constellations were created within a relatively short interval from around 1300 to 1000 BC. Mesopotamian constellations appeared later in many of the classical Greek constellations. The oldest Babylonian catalogues of stars and constellations date back to the beg... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation The term "Mazzaroth" , translated as "a garland of crowns", is a "hapax legomenon" in Job 38:32, and it might refer to the zodiacal constellations. There is only limited information on ancient Greek constellations, with some fragmentary evidence being found in the "Works and Days" of the Greek poet Hesiod... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation Ptolemy's "Almagest" remained the standard definition of constellations in the medieval period both in Europe and in Islamic astronomy. Ancient China had a long tradition of observing celestial phenomena. Nonspecific Chinese star names, later categorized in the twenty-eight mansions, have been found on or... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation A well known map from the Song period is the Suzhou Astronomical Chart, which was prepared with carvings of stars on the planisphere of the Chinese sky on a stone plate; it is done accurately based on observations, and it shows the supernova of the year of 1054 in Taurus. Influenced by European astronomy ... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation Southern constellations are more modern inventions, sometimes as substitutes for ancient constellations (e.g. Argo Navis). Some southern constellations had long names that were shortened to more usable forms; e.g. Musca Australis became simply Musca. Some of the early constellations were never universally... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation Knowledge that northern and southern star patterns differed goes back to Classical writers, who describe, for example, the African circumnavigation expedition commissioned by Egyptian Pharaoh Necho II in c. 600 BC and those of Hanno the Navigator in c. 500 BC. However, much of this history was lost with t... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation The northern constellation Quadrans Muralis survived into the 19th century (when its name was attached to the Quadrantid meteor shower), but is now divided between Boötes and Draco. A general list of 88 constellations was produced for the International Astronomical Union in 1922. It is roughly based on th... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation Where possible, these modern constellations usually share the names of their Graeco-Roman predecessors, such as Orion, Leo or Scorpius. The aim of this system is area-mapping, i.e. the division of the celestial sphere into contiguous fields. Out of the 88 modern constellations, 36 lie predominantly in the... | https://en.wikipedia.org/wiki?curid=5267 |
Constellation Australian Aboriginal astronomy also describes dark cloud constellations, the most famous being the "emu in the sky" whose head is formed by the Coalsack, a dark nebula, instead of the stars. "General & Nonspecialized – Entire Celestial Heavens": "Northern Celestial Hemisphere & North Circumpolar Region":... | https://en.wikipedia.org/wiki?curid=5267 |
Chemical element In chemistry an element is a species of atom having the same number of protons in its atomic nuclei (that is, the same atomic number, or "Z"). For example, the atomic number of oxygen is 8, so the element oxygen describes all atoms which have 8 protons. In total, 118 elements have been identified. The ... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Elements with greater than 26 protons are formed by nucleosynthesis in supernovae, which, when they explode, blast these elements as supernova remnants far into space, where they may become incorporated into planets when they are formed. The term "element" is used for atoms with a given number of proto... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element For example, atmospheric air is primarily a mixture of nitrogen, oxygen, and argon, and native solid elements occur in alloys, such as that of iron and nickel. The history of the discovery and use of the elements began with primitive human societies that found native elements like carbon, sulfur, coppe... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element On Earth, small amounts of new atoms are naturally produced in nucleogenic reactions, or in cosmogenic processes, such as cosmic ray spallation. New atoms are also naturally produced on Earth as radiogenic daughter isotopes of ongoing radioactive decay processes such as alpha decay, beta decay, spontan... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element The very heaviest elements (those beyond plutonium, element 94) undergo radioactive decay with half-lives so short that they are not found in nature and must be synthesized. As of 2010, there are 118 known elements (in this context, "known" means observed well enough, even from just a few decay product... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Technetium was the first purportedly non-naturally occurring element synthesized, in 1937, although trace amounts of technetium have since been found in nature (and also the element may have been discovered naturally in 1925). This pattern of artificial production and later natural discovery has been r... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element The electrons are placed into atomic orbitals that determine the atom's various chemical properties. The number of neutrons in a nucleus usually has very little effect on an element's chemical properties (except in the case of hydrogen and deuterium). Thus, all carbon isotopes have nearly identical che... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element All of the elements have some isotopes that are radioactive (radioisotopes), although not all of these radioisotopes occur naturally. The radioisotopes typically decay into other elements upon radiating an alpha or beta particle. If an element has isotopes that are not radioactive, these are termed "st... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Whereas the mass number simply counts the total number of neutrons and protons and is thus a natural (or whole) number, the atomic mass of a single atom is a real number giving the mass of a particular isotope (or "nuclide") of the element, expressed in atomic mass units (symbol: u). In general, the ma... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Whenever a relative atomic mass value differs by more than 1% from a whole number, it is due to this averaging effect, as significant amounts of more than one isotope are naturally present in a sample of that element. Chemists and nuclear scientists have different definitions of a "pure element". In ch... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element For example, carbon can be found as diamond, which has a tetrahedral structure around each carbon atom; graphite, which has layers of carbon atoms with a hexagonal structure stacked on top of each other; graphene, which is a single layer of graphite that is very strong; fullerenes, which have nearly sp... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element A first distinction is between metals, which readily conduct electricity, nonmetals, which do not, and a small group, (the "metalloids"), having intermediate properties and often behaving as semiconductors. A more refined classification is often shown in colored presentations of the periodic table. Thi... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Only bromine and mercury are liquids at 0 degrees Celsius (32 degrees Fahrenheit) and normal atmospheric pressure; caesium and gallium are solids at that temperature, but melt at 28.4 °C (83.2 °F) and 29.8 °C (85.6 °F), respectively. Melting and boiling points, typically expressed in degrees Celsius at... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element The elements studied to date as solid samples have eight kinds of crystal structures: cubic, body-centered cubic, face-centered cubic, hexagonal, monoclinic, orthorhombic, rhombohedral, and tetragonal. For some of the synthetically produced transuranic elements, available samples have been too small to... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Observationally stable isotopes of some elements (such as tungsten and lead), however, are predicted to be slightly radioactive with very long half-lives: for example, the half-lives predicted for the observationally stable lead isotopes range from 10 to 10 years. Elements with atomic numbers 43, 61, a... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Although earlier precursors to this presentation exist, its invention is generally credited to the Russian chemist Dmitri Mendeleev in 1869, who intended the table to illustrate recurring trends in the properties of the elements. The layout of the table has been refined and extended over time as new el... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element The terms "light" and "heavy" are sometimes also used informally to indicate relative atomic numbers (not densities), as in "lighter than carbon" or "heavier than lead", although technically the weight or mass of atoms of an element (their atomic weights or atomic masses) do not always increase monoton... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element For purposes of international communication and trade, the official names of the chemical elements both ancient and more recently recognized are decided by the International Union of Pure and Applied Chemistry (IUPAC), which has decided on a sort of international English language, drawing on traditiona... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element These are also named by IUPAC, which generally adopts the name chosen by the discoverer. This practice can lead to the controversial question of which research group actually discovered an element, a question that delayed the naming of elements with atomic number of 104 and higher for a considerable am... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element The first of these symbols were intended to be fully universal. Since Latin was the common language of science at that time, they were abbreviations based on the Latin names of metals. Cu comes from cuprum, Fe comes from ferrum, Ag from argentum. The symbols were not followed by a period (full stop) as... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element These are often a single capital letter, and the letters are reserved and not used for names of specific elements. For example, an "X" indicates a variable group (usually a halogen) in a class of compounds, while "R" is a radical, meaning a compound structure such as a hydrocarbon chain. The letter "Q"... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element As a special case, the three naturally occurring isotopes of the element hydrogen are often specified as H for H (protium), D for H (deuterium), and T for H (tritium). This convention is easier to use in chemical equations, replacing the need to write out the mass number for each atom. For example, the... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Elements heavier in atomic number than iron, as heavy as uranium and plutonium, are produced by explosive nucleosynthesis in supernovas and other cataclysmic cosmic events. Cosmic ray spallation (fragmentation) of carbon, nitrogen, and oxygen is important to the production of lithium, beryllium and bor... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element These include some produced by cosmic rays or other nuclear reactions (see cosmogenic and nucleogenic nuclides), and others produced as decay products of long-lived primordial nuclides. For example, trace (but detectable) amounts of carbon-14 (C) are continually produced in the atmosphere by cosmic ray... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element The more distant galaxies are being viewed as they appeared in the past, so their abundances of elements appear closer to the primordial mixture. As physical laws and processes appear common throughout the visible universe, however, scientist expect that these galaxies evolved elements in similar abund... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element The abundance of the chemical elements on Earth varies from air to crust to ocean, and in various types of life. The abundance of elements in Earth's crust differs from that in the Solar System (as seen in the Sun and heavy planets like Jupiter) mainly in selective loss of the very lightest elements (h... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Certain kinds of organisms require particular additional elements, for example the magnesium in chlorophyll in green plants, the calcium in mollusc shells, or the iron in the hemoglobin in vertebrate animals' red blood cells. The concept of an "element" as an undivisible substance has developed through... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Aristotle defined an element as: In 1661, Robert Boyle proposed his theory of corpuscularism which favoured the analysis of matter as constituted by irreducible units of matter (atoms) and, choosing to side with neither Aristotle's view of the four elements nor Paracelsus' view of three fundamental ele... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element The use of atomic numbers, rather than atomic weights, to distinguish elements has greater predictive value (since these numbers are integers), and also resolves some ambiguities in the chemistry-based view due to varying properties of isotopes and allotropes within the same element. Currently, IUPAC d... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical element Most of the remaining naturally occurring chemical elements were identified and characterized by 1900, including: Elements isolated or produced since 1900 include: The first transuranium element (element with atomic number greater than 92) discovered was neptunium in 1940. Since 1999 claims for the dis... | https://en.wikipedia.org/wiki?curid=5659 |
Chemical reaction A chemical reaction is a process that leads to the chemical transformation of one set of chemical substances to another. Classically, chemical reactions encompass changes that only involve the positions of electrons in the forming and breaking of chemical bonds between atoms, with no change to the nuc... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction Reactions may proceed in the forward or reverse direction until they go to completion or reach equilibrium. Reactions that proceed in the forward direction to approach equilibrium are often described as spontaneous, requiring no input of free energy to go forward. Non-spontaneous reactions require inp... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction Initial theories of transformation of materials were developed by Greek philosophers, such as the Four-Element Theory of Empedocles stating that any substance is composed of the four basic elements – fire, water, air and earth. In the Middle Ages, chemical transformations were studied by Alchemists. T... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction It postulated the existence of a fire-like element called "phlogiston", which was contained within combustible bodies and released during combustion. This proved to be false in 1785 by Antoine Lavoisier who found the correct explanation of the combustion as reaction with oxygen from the air. Joseph Lo... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction They are separated by an arrow (→) which indicates the direction and type of the reaction; the arrow is read as the word "yields". The tip of the arrow points in the direction in which the reaction proceeds. A double arrow () pointing in opposite directions is used for equilibrium reactions. Equations... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction Most experimentally observed reactions are built up from many elementary reactions that occur in parallel or sequentially. The actual sequence of the individual elementary reactions is known as reaction mechanism. An elementary reaction involves a few molecules, usually one or two, because of the low ... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction Another possibility is that only a portion of one molecule is transferred to the other molecule. This type of reaction occurs, for example, in redox and acid-base reactions. In redox reactions, the transferred particle is an electron, whereas in acid-base reactions it is a proton. This type of reactio... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction Chemical reactions are determined by the laws of thermodynamics. Reactions can proceed by themselves if they are exergonic, that is if they release energy. The associated free energy of the reaction is composed of two different thermodynamic quantities, enthalpy and entropy: Reactions can be exothermi... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction The shift in reaction direction tendency occurs at . Reactions can also be characterized by the internal energy which takes into account changes in the entropy, volume and chemical potential. The latter depends, among other things, on the activities of the involved substances. The speed at which react... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction In a synthesis reaction, two or more simple substances combine to form a more complex substance. These reactions are in the general form: Two or more reactants yielding one product is another way to identify a synthesis reaction. One example of a synthesis reaction is the combination of iron and sulfu... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction Another example of a double displacement reaction is the reaction of lead(II) nitrate with potassium iodide to form lead(II) iodide and potassium nitrate: In a combustion reaction, an element or compound reacts with oxygen, often producing energy in the form of heat or light. Combustion reactions alwa... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction On the other hand, the chlorine gas goes from an oxidation of 0 (it is also a pure element) to −1: the chlorine gains one electron and is said to have been reduced. Because the chlorine is the one reduced, it is considered the electron acceptor, or in other words, induces oxidation in the sodium – thu... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction The reverse process in which electrons are released in redox reactions and can be used as electrical energy is possible and used in batteries. In complexation reactions, several ligands react with a metal atom to form a coordination complex. This is achieved by providing lone pairs of the ligand into ... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction In other words, acids act as proton donors and bases act as proton acceptors according to the following equation: The reverse reaction is possible, and thus the acid/base and conjugated base/acid are always in equilibrium. The equilibrium is determined by the acid and base dissociation constants ("K" ... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction Reaction can take place at the solid|gas interface, surfaces at very low pressure such as ultra-high vacuum. Via scanning tunneling microscopy, it is possible to observe reactions at the solid|gas interface in real space, if the time scale of the reaction is in the correct range. Reactions at the soli... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction However, it can be inhibited, deactivated or destroyed by secondary processes. Catalysts can be used in a different phase (heterogeneous) or in the same phase (homogeneous) as the reactants. In heterogeneous catalysis, typical secondary processes include coking where the catalyst becomes covered by po... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction Therefore, heterogeneous catalysts are preferred in many industrial processes. In organic chemistry, in addition to oxidation, reduction or acid-base reactions, a number of other reactions can take place which involve covalent bonds between carbon atoms or carbon and heteroatoms (such as oxygen, nitro... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction In their names, S stands for substitution, N for nucleophilic, and the number represents the kinetic order of the reaction, unimolecular or bimolecular. The S1 reaction proceeds in two steps. First, the leaving group is eliminated creating a carbocation. This is followed by a rapid reaction with the n... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction It is similar to the nucleophilic aliphatic substitution and also has two major types, S1 and S2 In the third type of substitution reaction, radical substitution, the attacking particle is a radical. This process usually takes the form of a chain reaction, for example in the reaction of alkanes with h... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction In contrast to the E1 eliminations, different stereochemical configurations are possible for the reaction product in the E2 mechanism, because the attack of the base preferentially occurs in the anti-position with respect to the leaving group. Because of the similar conditions and reagents, the E2 eli... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction In this case, one has to use the hydroboration–oxidation reaction, where in the first step, the boron atom acts as electrophile and adds to the less substituted carbon atom. At the second step, the nucleophilic hydroperoxide or halogen anion attacks the boron atom. While the addition to the electron-r... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction As with the free-radical substitution, the radical addition proceeds as a chain reaction, and such reactions are the basis of the free-radical polymerization. In a rearrangement reaction, the carbon skeleton of a molecule is rearranged to give a structural isomer of the original molecule. These includ... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction For example, the [4+2] Diels-Alder reactions can be assisted by heat whereas the [2+2] cycloaddition is selectively induced by light. Because of the orbital character, the potential for developing stereoisomeric products upon cycloaddition is limited, as described by the Woodward–Hoffmann rules. Bioch... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction All organisms use this energy to produce adenosine triphosphate (ATP), which can then be used to energize other reactions. Chemical reactions are central to chemical engineering where they are used for the synthesis of new compounds from natural raw materials such as petroleum and mineral ores. It is ... | https://en.wikipedia.org/wiki?curid=6271 |
Chemical reaction Faster reactions are usually studied with ultrafast laser spectroscopy where utilization of femtosecond lasers allows short-lived transition states to be monitored at time scaled down to a few femtoseconds. | https://en.wikipedia.org/wiki?curid=6271 |
Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids (see crystal structure). The word "crystallography" is derived from the Greek words "crystallon" "cold drop, frozen drop", with its meaning extending to all solids with some degree of transparency, and "graphein" "... | https://en.wikipedia.org/wiki?curid=7794 |
Crystallography These three types of radiation interact with the specimen in different ways. Because of these different forms of interaction, the three types of radiation are suitable for different crystallographic studies. With conventional imaging technique such as optical microscopy, obtaining an image of a small ob... | https://en.wikipedia.org/wiki?curid=7794 |
Crystallography Because of their highly ordered and repetitive atomic structure (Bravais lattice), crystals diffracts x-rays in a coherent manner, also referred as Bragg's reflection. Some materials that have been analyzed crystallographically, such as proteins, do not occur naturally as crystals. Typically, such molec... | https://en.wikipedia.org/wiki?curid=7794 |
Crystallography Producing an image from a diffraction pattern requires sophisticated mathematics and often an iterative process of modelling and refinement. In this process, the mathematically predicted diffraction patterns of an hypothesized or "model" structure are compared to the actual pattern generated by the crys... | https://en.wikipedia.org/wiki?curid=7794 |
Crystallography In single crystals, the effects of the crystalline arrangement of atoms is often easy to see macroscopically, because the natural shapes of crystals reflect the atomic structure. In addition, physical properties are often controlled by crystalline defects. The understanding of crystal structures is an i... | https://en.wikipedia.org/wiki?curid=7794 |
Crystallography When manufacturing or using a material, it is generally desirable to know what compounds and what phases are present in the material, as their composition, structure and proportions will influence the material's properties. Each phase has a characteristic arrangement of atoms. X-ray or neutron diffracti... | https://en.wikipedia.org/wiki?curid=7794 |
Crystallography Neutron crystallography is often used to help refine structures obtained by X-ray methods or to solve a specific bond; the methods are often viewed as complementary, as X-rays are sensitive to electron positions and scatter most strongly off heavy atoms, while neutrons are sensitive to nucleus positions... | https://en.wikipedia.org/wiki?curid=7794 |
DNA Deoxyribonucleic acid (; DNA) is a molecule composed of two polynucleotide chains that coil around each other to form a double helix carrying genetic instructions for the development, functioning, growth and reproduction of all known organisms and many viruses. and ribonucleic acid (RNA) are nucleic acids. Alongsid... | https://en.wikipedia.org/wiki?curid=7955 |
DNA A large part of (more than 98% for humans) is non-coding, meaning that these sections do not serve as patterns for protein sequences. The two strands of run in opposite directions to each other and are thus antiparallel. Attached to each sugar is one of four types of nucleobases (informally, "bases"). It is the seq... | https://en.wikipedia.org/wiki?curid=7955 |
DNA was first isolated by Friedrich Miescher in 1869. Its molecular structure was first identified by Francis Crick and James Watson at the Cavendish Laboratory within the University of Cambridge in 1953, whose model-building efforts were guided by X-ray diffraction data acquired by Raymond Gosling, who was a post-grad... | https://en.wikipedia.org/wiki?curid=7955 |
DNA Although each individual nucleotide is very small, a polymer can be very large and contain hundreds of millions, such as in chromosome 1. Chromosome 1 is the largest human chromosome with approximately 220 million base pairs, and would be 85 mm long if straightened. does not usually exist as a single strand, but in... | https://en.wikipedia.org/wiki?curid=7955 |
DNA Therefore, any strand normally has one end at which there is a phosphate group attached to the 5′ carbon of a ribose (the 5′ phosphoryl) and another end at which there is a free hydroxyl group attached to the 3′ carbon of a ribose (the 3′ hydroxyl). The orientation of the 3′ and 5′ carbons along the sugar-phosphate... | https://en.wikipedia.org/wiki?curid=7955 |
DNA A fifth pyrimidine nucleobase, uracil (U), usually takes the place of thymine in RNA and differs from thymine by lacking a methyl group on its ring. In addition to RNA and DNA, many artificial nucleic acid analogues have been created to study the properties of nucleic acids, or for use in biotechnology. Modified ba... | https://en.wikipedia.org/wiki?curid=7955 |
DNA The width of the major groove means that the edges of the bases are more accessible in the major groove than in the minor groove. As a result, proteins such as transcription factors that can bind to specific sequences in double-stranded usually make contact with the sides of the bases exposed in the major groove. T... | https://en.wikipedia.org/wiki?curid=7955 |
DNA This reversible and specific interaction between complementary base pairs is critical for all the functions of in organisms. As noted above, most molecules are actually two polymer strands, bound together in a helical fashion by noncovalent bonds; this double-stranded (dsDNA) structure is maintained largely by the ... | https://en.wikipedia.org/wiki?curid=7955 |
DNA In biology, parts of the double helix that need to separate easily, such as the TATAAT Pribnow box in some promoters, tend to have a high AT content, making the strands easier to pull apart. In the laboratory, the strength of this interaction can be measured by finding the temperature necessary to break half of the... | https://en.wikipedia.org/wiki?curid=7955 |
DNA In these cases, some sequences do double duty, encoding one protein when read along one strand, and a second protein when read in the opposite direction along the other strand. In bacteria, this overlap may be involved in the regulation of gene transcription, while in viruses, overlapping genes increase the amount ... | https://en.wikipedia.org/wiki?curid=7955 |
DNA The first published reports of A-X-ray diffraction patterns—and also B-DNA—used analyses based on Patterson transforms that provided only a limited amount of structural information for oriented fibers of DNA. An alternative analysis was then proposed by Wilkins "et al.", in 1953, for the "in vivo" B-X-ray diffracti... | https://en.wikipedia.org/wiki?curid=7955 |
DNA These unusual structures can be recognized by specific Z-binding proteins and may be involved in the regulation of transcription. For many years, exobiologists have proposed the existence of a shadow biosphere, a postulated microbial biosphere of Earth that uses radically different biochemical and molecular process... | https://en.wikipedia.org/wiki?curid=7955 |
DNA These flat four-base units then stack on top of each other to form a stable G-quadruplex structure. These structures are stabilized by hydrogen bonding between the edges of the bases and chelation of a metal ion in the centre of each four-base unit. Other structures can also be formed, with the central set of four ... | https://en.wikipedia.org/wiki?curid=7955 |
DNA Several artificial nucleobases have been synthesized, and successfully incorporated in the eight-base analogue named Hachimoji DNA. Dubbed S, B, P, and Z, these artificial bases are capable of bonding with each other in a predictable way (S–B and P–Z), maintain the double helix structure of DNA, and be transcribed ... | https://en.wikipedia.org/wiki?curid=7955 |
DNA Despite the importance of 5-methylcytosine, it can deaminate to leave a thymine base, so methylated cytosines are particularly prone to mutations. Other base modifications include adenine methylation in bacteria, the presence of 5-hydroxymethylcytosine in the brain, and the glycosylation of uracil to produce the "J... | https://en.wikipedia.org/wiki?curid=7955 |
DNA , also occur frequently. Although most of these damages are repaired, in any cell some damage may remain despite the action of repair processes. These remaining damages accumulate with age in mammalian postmitotic tissues. This accumulation appears to be an important underlying cause of aging. Many mutagens fit int... | https://en.wikipedia.org/wiki?curid=7955 |
Subsets and Splits
No community queries yet
The top public SQL queries from the community will appear here once available.