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The largest group of students during the first seven decades of the school were from Martha's Vineyard, and they brought MVSL with them. There were also 44 students from around Henniker, New Hampshire, and 27 from the Sandy River valley in Maine, each of which had their own village sign language. Other students brought knowledge of their own home signs. Laurent Clerc, the first teacher at ASD, taught using French Sign Language (LSF), which itself had developed in the Parisian school for the deaf established in 1755. From that situation of language contact, a new language emerged, now known as ASL. More schools for the deaf were founded after ASD, and knowledge of ASL spread to those schools. In addition, the rise of Deaf community organizations bolstered the continued use of ASL. Societies such as the National Association of the Deaf and the National Fraternal Society of the Deaf held national conventions that attracted signers from across the country. All of that contributed to ASL's wide use over a large geographical area, atypical of a sign language.
While oralism, an approach to educating deaf students focusing on oral language, had previously been used in American schools, the Milan Congress made it dominant and effectively banned the use of sign languages at schools in the United States and Europe. However, the efforts of Deaf advocates and educators, more lenient enforcement of the Congress's mandate, and the use of ASL in religious education and proselytism ensured greater use and documentation compared to European sign languages, albeit more influenced by fingerspelled loanwords and borrowed idioms from English as students were societally pressured to achieve fluency in spoken language. Nevertheless, oralism remained the predominant method of deaf education up to the 1950s. Linguists did not consider sign language to be true "language" but as something inferior. Recognition of the legitimacy of ASL was achieved by William Stokoe, a linguist who arrived at Gallaudet University in 1955 when that was still the dominant assumption. Aided by the Civil Rights Movement of the 1960s, Stokoe argued for manualism, the use of sign language in deaf education. Stokoe noted that sign language shares the important features that oral languages have as a means of communication, and even devised a transcription system for ASL. In doing so, Stokoe revolutionized both deaf education and linguistics. In the 1960s, ASL was sometimes referred to as "Ameslan", but that term is now considered obsolete.
Population. Counting the number of ASL signers is difficult because ASL users have never been counted by the American census. The ultimate source for current estimates of the number of ASL users in the United States is a report for the National Census of the Deaf Population (NCDP) by Schein and Delk (1974). Based on a 1972 survey of the NCDP, Schein and Delk provided estimates consistent with a signing population between 250,000 and 500,000. The survey did not distinguish between ASL and other forms of signing; in fact, the name "ASL" was not yet in widespread use. Incorrect figures are sometimes cited for the population of ASL users in the United States based on misunderstandings of known statistics. Demographics of the deaf population have been confused with those of ASL use since adults who become deaf late in life rarely use ASL in the home. That accounts for currently-cited estimations that are greater than 500,000; such mistaken estimations can reach as high as 15,000,000. A 100,000-person lower bound has been cited for ASL users; the source of that figure is unclear, but it may be an estimate of prelingual deafness, which is correlated with but not equivalent to signing.
ASL is sometimes incorrectly cited as the third- or fourth-most-spoken language in the United States. Those figures misquote Schein and Delk (1974), who actually concluded that ASL speakers constituted the third-largest population "requiring an interpreter in court". Although that would make ASL the third-most used language among monolinguals other than English, it does not imply that it is the fourth-most-spoken language in the United States since speakers of other languages may also speak English. Geographic distribution. ASL is used throughout Anglo-America. That contrasts with Europe, where a variety of sign languages are used within the same continent. The unique situation of ASL seems to have been caused by the proliferation of ASL through schools influenced by the American School for the Deaf, wherein ASL originated, and the rise of community organizations for the Deaf. Throughout West Africa, ASL-based sign languages are signed by educated Deaf adults. Such languages, imported by boarding schools, are often considered by associations to be the official sign languages of their countries and are named accordingly, such as Nigerian Sign Language and Ghanaian Sign Language. Such signing systems are found in Benin, Burkina Faso, Ivory Coast, Ghana, Liberia, Mauritania, Mali, Nigeria, and Togo. Due to lack of data, it is still an open question how similar those sign languages are to the variety of ASL used in America.
In addition to the aforementioned West African countries, ASL is reported to be used as a first language in Barbados, Bolivia, Cambodia (alongside Cambodian Sign Language), the Central African Republic, Chad, China (Hong Kong), the Democratic Republic of the Congo, Gabon, Jamaica, Kenya, Madagascar, the Philippines, Singapore, and Zimbabwe. ASL is also used as a lingua franca throughout the deaf world, widely learned as a second language. Regional variation. Sign production. Sign production can often vary according to location. Signers from the South tend to sign with more flow and ease. Native signers from New York have been reported as signing comparatively quicker and sharper. Sign production of native Californian signers has also been reported as being fast. Research on that phenomenon often concludes that the fast-paced production for signers from the coasts could be due to the fast-paced nature of living in large metropolitan areas. That conclusion also supports how the ease with which Southerners sign could be caused by the easygoing environment of the South in comparison to that of the coasts.
Sign production can also vary depending on age and native language. For example, sign production of letters may vary in older signers. Slight differences in finger spelling production can be a signal of age. Additionally, signers who learned American Sign Language as a second language vary in production. For Deaf signers who learned a different sign language before learning American Sign Language, qualities of their native language may show in their ASL production. Some examples of that varied production include fingerspelling towards the body, instead of away from it, and signing certain movement from bottom to top, instead of top to bottom. Hearing people who learn American Sign Language also have noticeable differences in signing production. The most notable production difference of hearing people learning American Sign Language is their rhythm and arm posture. Sign variants. Most popularly, there are variants of the signs for English words such as "birthday", "pizza", "Halloween", "early", and "soon", just a sample of the most commonly recognized signs with variants based on regional change. The sign for "school" is commonly varied between black and white signers; the variants used by black signers are sometimes called Black American Sign Language. Social variation is also found between citation forms and forms used by Deaf gay men for words such as "pain" and "protest".
History and implications. The prevalence of residential Deaf schools can account for much of the regional variance of signs and sign productions across the United States. Deaf schools often serve students of the state in which the school resides. That limited access to signers from other regions, combined with the residential quality of Deaf Schools promoted specific use of certain sign variants. Native signers did not have much access to signers from other regions during the beginning years of their education. It is hypothesized that because of that seclusion, certain variants of a sign prevailed over others due to the choice of variant used by the student of the school/signers in the community. However, American Sign Language does not appear to be vastly varied in comparison to other signed languages. That is because when Deaf education was beginning in the United States, many educators flocked to the American School for the Deaf in Hartford, Connecticut, whose central location for the first generation of educators in Deaf education to learn American Sign Language allows ASL to be more standardized than its variant.
Varieties. Varieties of ASL are found throughout the world. There is little difficulty in comprehension among the varieties of the United States and Canada. Mutual intelligibility among those ASL varieties is high, and the variation is primarily lexical. For example, there are three different words for English "about" in Canadian ASL; the standard way, and two regional variations (Atlantic and Ontario). Variation may also be phonological, meaning that the same sign may be signed in a different way depending on the region. For example, an extremely common type of variation is between the handshapes /1/, /L/, and /5/ in signs with one handshape. There is also a distinct variety of ASL used by the Black Deaf community. Black ASL evolved as a result of racially segregated schools in some states, which included the residential schools for the deaf. Black ASL differs from standard ASL in vocabulary, phonology, and some grammatical structure. While African American English (AAE) is generally viewed as more innovating than standard English, Black ASL is more conservative than standard ASL, preserving older forms of many signs. Black sign language speakers use more two-handed signs than in mainstream ASL, are less likely to show assimilatory lowering of signs produced on the forehead (e.g. KNOW) and use a wider signing space. Modern Black ASL borrows a number of idioms from AAE; for instance, the AAE idiom "I feel you" is calqued into Black ASL.
ASL is used internationally as a lingua franca, and a number of closely related sign languages derived from ASL are used in many different countries. Even so, there have been varying degrees of divergence from standard ASL in those imported ASL varieties. Bolivian Sign Language is reported to be a dialect of ASL, no more divergent than other acknowledged dialects. On the other hand, it is also known that some imported ASL varieties have diverged to the extent of being separate languages. For example, Malaysian Sign Language, which has ASL origins, is no longer mutually comprehensible with ASL and must be considered its own language. For some imported ASL varieties, such as those used in West Africa, it is still an open question how similar they are to American ASL. When communicating with hearing English speakers, ASL-speakers often use what is commonly called Pidgin Signed English (PSE) or 'contact signing', a blend of English structure with ASL vocabulary. Various types of PSE exist, ranging from highly English-influenced PSE (practically relexified English) to PSE which is quite close to ASL lexically and grammatically, but may alter some subtle features of ASL grammar. Fingerspelling may be used more often in PSE than it is normally used in ASL. There have been some constructed sign languages, known as Manually Coded English (MCE), which match English grammar exactly and simply replace spoken words with signs; those systems are not considered to be varieties of ASL.
Tactile ASL (TASL) is a variety of ASL used throughout the United States by and with the deaf-blind. It is particularly common among those with Usher's syndrome. It results in deafness from birth followed by loss of vision later in life; consequently, those with Usher's syndrome often grow up in the Deaf community using ASL, and later transition to TASL. TASL differs from ASL in that signs are produced by touching the palms, and there are some grammatical differences from standard ASL in order to compensate for the lack of nonmanual signing. ASL changes over time and from generation to generation. The sign for telephone has changed as the shape of phones and the manner of holding them have changed. The development of telephones with screens has also changed ASL, encouraging the use of signs that can be seen on small screens. Stigma. In 2013, the White House published a response to a petition that gained over 37,000 signatures to "officially recognize American Sign Language as a community language and a language of instruction in schools". The response is titled "there shouldn't be any stigma about American Sign Language" and addressed that ASL is a vital language for the Deaf and hard of hearing. Stigmas associated with sign languages and the use of sign for educating children often lead to the absence of sign during periods in children's lives when they can access languages most effectively. Scholars such as Beth S. Benedict advocate not only for bilingualism (using ASL and English training) but also for early childhood intervention for children who are deaf. York University psychologist Ellen Bialystok has also campaigned for bilingualism, arguing that those who are bilingual acquire cognitive skills that may help to prevent dementia later in life.
Most children born to deaf parents are hearing. Known as CODAs ("Children of Deaf Adults"), they are often more culturally Deaf than deaf children, most of whom are born to hearing parents. Unlike many deaf children, CODAs acquire ASL as well as Deaf cultural values and behaviors from birth. Such bilingual hearing children may be mistakenly labeled as being "slow learners" or as having "language difficulties" because of preferential attitudes towards spoken language. Writing systems. Although there is no well-established writing system for ASL, written sign language dates back almost two centuries. The first systematic writing system for a sign language seems to be that of Roch-Ambroise Auguste Bébian, developed in 1825. However, written sign language remained marginal among the public. In the 1960s, linguist William Stokoe created Stokoe notation specifically for ASL. It is alphabetic, with a letter or diacritic for every phonemic (distinctive) hand shape, orientation, motion, and position, though it lacks any representation of facial expression, and is better suited for individual words than for extended passages of text. Stokoe used that system for his 1965 "A Dictionary of American Sign Language on Linguistic Principles".
SignWriting, proposed in 1974 by Valerie Sutton, is the first writing system to gain use among the public and the first writing system for sign languages to be included in the Unicode Standard. SignWriting consists of more than 5000 distinct iconic graphs/glyphs. Currently, it is in use in many schools for the Deaf, particularly in Brazil, and has been used in International Sign forums with speakers and researchers in more than 40 countries, including Brazil, Ethiopia, France, Germany, Italy, Portugal, Saudi Arabia, Slovenia, Tunisia, and the United States. Sutton SignWriting has both a printed and an electronically produced form so that persons can use the system anywhere that oral languages are written (personal letters, newspapers, and media, academic research). The systematic examination of the International Sign Writing Alphabet (ISWA) as an equivalent usage structure to the International Phonetic Alphabet for spoken languages has been proposed. According to some researchers, SignWriting is not a phonemic orthography and does not have a one-to-one map from phonological forms to written forms. That assertion has been disputed, and the process for each country to look at the ISWA and create a phonemic/morphemic assignment of features of each sign language was proposed by researchers Msc. Roberto Cesar Reis da Costa and Madson Barreto in a thesis forum on June 23, 2014. The SignWriting community has an open project on Wikimedia Labs to support the various Wikimedia projects on Wikimedia Incubator and elsewhere involving SignWriting. The ASL Wikipedia request was marked as eligible in 2008 and the test ASL Wikipedia has 50 articles written in ASL using SignWriting.
The most widely used transcription system among academics is HamNoSys, developed at the University of Hamburg. Based on Stokoe Notation, HamNoSys was expanded to about 200 graphs in order to allow transcription of any sign language. Phonological features are usually indicated with single symbols, though the group of features that make up a handshape is indicated collectively with a symbol. Several additional candidates for written ASL have appeared over the years, including SignFont, ASL-phabet, and Si5s. For English-speaking audiences, ASL is often glossed using English words. Such glosses are typically all-capitalized and are arranged in ASL order. For example, the ASL sentence "DOG NOW CHASE>IX=3 CAT", meaning "the dog is chasing the cat", uses NOW to mark ASL progressive aspect and shows ASL verbal inflection for the third person (">IX=3"). However, glossing is not used to write the language for speakers of ASL. Phonology. Each sign in ASL is composed of a number of distinctive components, generally referred to as parameters. A sign may use one hand or both. All signs can be described using the five parameters involved in signed languages, which are handshape, movement, palm orientation, location and nonmanual markers. Just as phonemes of sound distinguish meaning in spoken languages, those parameters are the phonemes that distinguish meaning in signed languages like ASL. Changing any one of them may change the meaning of a sign, as illustrated by the ASL signs THINK and DISAPPOINTED:
There are also meaningful nonmanual signals in ASL, which may include movement of the eyebrows, the cheeks, the nose, the head, the torso, and the eyes. William Stokoe proposed that such components are analogous to the phonemes of spoken languages. There has also been a proposal that they are analogous to classes like place and manner of articulation. As in spoken languages, those phonological units can be split into distinctive features. For instance, the handshapes /2/ and /3/ are distinguished by the presence or absence of the feature [± closed thumb], as illustrated to the right. ASL has processes of allophony and phonotactic restrictions. There is ongoing research into whether ASL has an analog of syllables in spoken language. Grammar. Morphology. ASL has a rich system of verbal inflection, which involves both grammatical aspect: how the action of verbs flows in time—and agreement marking. Aspect can be marked by changing the manner of movement of the verb; for example, continuous aspect is marked by incorporating rhythmic, circular movement, while punctual aspect is achieved by modifying the sign so that it has a stationary hand position. Verbs may agree with both the subject and the object, and are marked for number and reciprocity. Reciprocity is indicated by using two one-handed signs; for example, the sign SHOOT, made with an L-shaped handshape with inward movement of the thumb, inflects to SHOOT[reciprocal], articulated by having two L-shaped hands "shooting" at each other.
ASL has a productive system of classifiers, which are used to classify objects and their movement in space. For example, a rabbit running downhill would use a classifier consisting of a bent V classifier handshape with a downhill-directed path; if the rabbit is hopping, the path is executed with a bouncy manner. In general, classifiers are composed of a "classifier handshape" bound to a "movement root". The classifier handshape represents the object as a whole, incorporating such attributes as surface, depth, and shape, and is usually very iconic. The movement root consists of a path, a direction and a manner. In linguistics, there are two primary ways of changing the form of a word: derivation and inflection. Derivation involves creating new words by adding something to an existing word, while inflection involves changing the form of a word to convey grammatical information without altering its fundamental meaning or category. For example, adding the suffix "-ship" to the noun "friend" creates the new word "friendship", which has a different meaning than the original word. Inflection, on the other hand, involves modifying a word's form to indicate grammatical features such as tense, number, gender, person, case, and degree of comparison.
In American Sign Language (ASL), inflection is conveyed through facial expressions, body movements, and other non-manual markers. For instance, to indicate past tense in ASL, one might sign the present tense of a verb (such as "walk"), and then add a facial expression and head tilt to signify that the action occurred in the past (i.e., "walked"). According to the book Linguistics of American Sign Language, ASL signs have two main components: hold segments and movement segments. Hold segments consist of hand-shape, location, orientation, and non-manual features, while movement segments possess similar features. Morphology is the study of how languages form words by using smaller units to construct larger units. The smallest meaningful unit in a language is known as a "morpheme", with some morphemes able to stand alone as independent units (free morphemes), while others must occur with other morphemes (bound morphemes). For example, the plural "-s" and third person "-s" in English are bound morphemes. In ASL, the 3 handshape in signs like THREE-WEEKS and THREE-MONTHS are also bound morphemes.
Affixes, which are morphemes added to words to create new words or modify their meanings, are part of the derivational process. For example, in English, prefixes like "re-" and suffixes like "-able" are affixes. In ASL, affixation can be used to modify the sign for CHAIR to indicate different types of chairs. The inflectional process, on the other hand, adds grammatical information to existing units. By studying morphemes and how they can be combined or modified, linguists gain insight into the underlying structure of language and the creative ways in which it can be used to express meaning. Understanding morphology is essential to understanding how languages are built and how new signs or words can be formed. Furthermore, understanding morphology has practical applications in language learning and teaching. For example, teaching students the basic morphological structures of a language can help them to better understand the language's grammar and syntax, and can also aid in their acquisition of new vocabulary.
In summary, morphology is an essential component of language and provides valuable insights into the structure and function of languages. By understanding the morphological processes involved in language formation, we can gain a deeper understanding of how languages work and how they can be effectively taught and learned. Fingerspelling. American Sign Language possesses a set of 26 signs known as the American manual alphabet, which can be used to spell out words from the English language. It is rather a representation of the English alphabet, and not a unique alphabet of ASL, although commonly labeled as the "ASL alphabet". It is borrowed from French Sign Language (LSF), as much of ASL is derived from LSF. Such signs make use of the 19 handshapes of ASL. For example, the signs for 'p' and 'k' use the same handshape but different orientations. A common misconception is that ASL consists only of fingerspelling; although such a method (Rochester Method) has been used, it is not ASL. Fingerspelling is a form of borrowing, a linguistic process wherein words from one language are incorporated into another. In ASL, fingerspelling is used for proper nouns and for technical terms with no native ASL equivalent. There are also some other loan words which are fingerspelled, either very short English words or abbreviations of longer English words, e.g. "O-N" from English 'on', and "A-P-T" from English 'apartment'. Fingerspelling may also be used to emphasize a word that would normally be signed otherwise.
Syntax. ASL is a subject–verb–object (SVO) language, but various phenomena affect that basic word order. Basic SVO sentences are signed without any pauses: However, other word orders may also occur since ASL allows the topic of a sentence to be moved to sentence-initial position, a phenomenon known as topicalization. In object–subject–verb (OSV) sentences, the object is topicalized, marked by a forward head-tilt and a pause: Besides, word orders can be obtained through the phenomenon of subject copy in which the subject is repeated at the end of the sentence, accompanied by head nodding for clarification or emphasis: ASL also allows null subject sentences whose subject is implied, rather than stated explicitly. Subjects can be copied even in a null subject sentence, and the subject is then omitted from its original position, yielding a verb–object–subject (VOS) construction: Topicalization, accompanied with a null subject and a subject copy, can produce yet another word order, object–verb–subject (OVS). Those properties of ASL allow it a variety of word orders, leading many to question which is the true, underlying, "basic" order. There are several other proposals that attempt to account for the flexibility of word order in ASL. One proposal is that languages like ASL are best described with a topic–comment structure whose words are ordered by their importance in the sentence, rather than by their syntactic properties. Another hypothesis is that ASL exhibits free word order, in which syntax is not encoded in word order but can be encoded by other means such as head nods, eyebrow movement, and body position.
Iconicity. Common misconceptions are that signs are iconically self-explanatory, that they are a transparent imitation of what they mean, or even that they are pantomime. In fact, many signs bear no resemblance to their referent because they were originally arbitrary symbols, or their iconicity has been obscured over time. Even so, in ASL iconicity plays a significant role; a high percentage of signs resemble their referents in some way. That may be because the medium of sign, three-dimensional space, naturally allows more iconicity than oral language. In the era of the influential linguist Ferdinand de Saussure, it was assumed that the mapping between form and meaning in language must be completely arbitrary. Although onomatopoeia is a clear exception, since words like "choo-choo" bear clear resemblance to the sounds that they mimic, the Saussurean approach was to treat them as marginal exceptions. ASL, with its significant inventory of iconic signs, directly challenges that theory. Research on acquisition of pronouns in ASL has shown that children do not always take advantage of the iconic properties of signs when they interpret their meaning. It has been found that when children acquire the pronoun "you", the iconicity of the point (at the child) is often confused, being treated more like a name. That is a similar finding to research in oral languages on pronoun acquisition. It has also been found that iconicity of signs does not affect immediate memory and recall; less iconic signs are remembered just as well as highly-iconic signs.
Applet In computing, an applet is any small application that performs one specific task that runs within the scope of a dedicated widget engine or a larger program, often as a plug-in. The term is frequently used to refer to a Java applet, a program written in the Java programming language that is designed to be placed on a web page. Applets are typical examples of transient and auxiliary applications that do not monopolize the user's attention. Applets are not full-featured application programs, and are intended to be easily accessible. History. The word "applet" was first used in 1990 in "PC Magazine". However, the concept of an applet, or more broadly a small interpreted program downloaded and executed by the user, dates at least to RFC 5 (1969) by Jeff Rulifson, which described the Decode-Encode Language, which was designed to allow remote use of the oN-Line System over ARPANET, by downloading small programs to enhance the interaction. This has been specifically credited as a forerunner of Java's downloadable programs in RFC 2555.
Applet as an extension of other software. In some cases, an applet does not run independently. These applets must run either in a container provided by a host program, through a plugin, or a variety of other applications including mobile devices that support the applet programming model. Web-based applets. Applets were used to provide interactive features to web applications that historically could not be provided by HTML alone. They could capture mouse input and also had controls like buttons or check boxes. In response to the user action, an applet could change the provided graphic content. This made applets well suited for demonstration, visualization, and teaching. There were online applet collections for studying various subjects, from physics to heart physiology. Applets were also used to create online game collections that allowed players to compete against live opponents in real-time. An applet could also be a text area only, providing, for instance, a cross-platform command-line interface to some remote system. If needed, an applet could leave the dedicated area and run as a separate window. However, applets had very little control over web page content outside the applet dedicated area, so they were less useful for improving the site appearance in general (while applets like news tickers or WYSIWYG editors are also known). Applets could also play media in formats that are not natively supported by the browser.
HTML pages could embed parameters that were passed to the applet. Hence, the same applet could appear differently depending on the parameters that were passed. Examples of Web-based applets include: Applet Vs. Subroutine. A larger application distinguishes its applets through several features: Java applets. A Java applet is a Java program that is launched from HTML and run in a web browser. It takes code from server and run in a web browser. It can provide web applications with interactive features that cannot be provided by HTML. Since Java's bytecode is platform-independent, Java applets can be executed by browsers running under many platforms, including Windows, Unix, macOS, and Linux. When a Java technology-enabled web browser processes a page that contains an applet, the applet's code is transferred to the client's system and executed by the browser's Java virtual machine. An HTML page references an applet either via the deprecated tag or via its replacement, the tag. Security. Recent developments in the coding of applications, including mobile and embedded systems, have led to the awareness of the security of applets.
Open platform applets. Applets in an open platform environment should provide secure interactions between different applications. A compositional approach can be used to provide security for open platform applets. Advanced compositional verification methods have been developed for secure applet interactions. Java applets. A Java applet contains different security models: unsigned Java applet security, signed Java applet security, and self-signed Java applet security. Web-based applets. In an applet-enabled web browser, many methods can be used to provide applet security for malicious applets. A malicious applet can infect a computer system in many ways, including denial of service, invasion of privacy, and annoyance. A typical solution for malicious applets is to make the web browser to monitor applets' activities. This will result in a web browser that will enable the manual or automatic stopping of malicious applets.
Alternate history Alternate history (also referred to as alternative history, allohistory, althist, or simply AH) is a subgenre of speculative fiction in which one or more historical events have occurred but are resolved differently than in actual history. As conjecture based upon historical fact, alternate history stories propose "What if?" scenarios about crucial events in human history, and present outcomes very different from the historical record. Some alternate histories are considered a subgenre of science fiction, or historical fiction. Since the 1950s, as a subgenre of science fiction, some alternative history stories have featured the tropes of time travel between histories, the psychic awareness of the existence of an alternative universe by the inhabitants of a given universe, and time travel that divides history into various timestreams. Definition. Often described as a subgenre of science fiction, alternative history is a genre of fiction wherein the author speculates upon how the course of history might have been altered if a particular historical event had an outcome different from the real life outcome. An alternate history requires three conditions: (i) A point of divergence from the historical record, before the time in which the author is writing; (ii) A change that would alter known history; and (iii) An examination of the ramifications of that alteration to history. Occasionally, some types of genre fiction are misidentified as "alternative history", specifically science fiction stories set in a time that was the future for the writer, but now is the past for the reader, such as the novels "" (1968) by Arthur C. Clarke, "1984" (1949) by George Orwell and the movie "2012" (2009) because the authors did not alter the real history of the past when they wrote the stories.
Similar to the genre of alternative history, there is also the genre of secret history - which can be either fictional or non-fictional - which documents events that might have occurred in history, but which had no effect upon the recorded historical outcome. Alternative history also is thematically related to, but distinct from, counterfactual history, which is a form of historiography that explores historical events in an extrapolated timeline in which key historical events either did not occur or had an outcome different from the historical record, in order to understand what did happen. History of literature. Antiquity and medieval. The earliest example of alternate (or counterfactual) history is found in Livy's "Ab Urbe Condita Libri" (book IX, sections 17–19). Livy contemplated an alternative 4th century BC in which Alexander the Great had survived to attack Europe as he had planned; asking, "What would have been the results for Rome if she had been engaged in a war with Alexander?" Livy concluded that the Romans would likely have defeated Alexander. An even earlier possibility is Herodotus's "Histories", which contains speculative material.
Another example of counterfactual history was posited by cardinal and Doctor of the Church Peter Damian in the 11th century. In his famous work "De Divina Omnipotentia", a long letter in which he discusses God's omnipotence, he treats questions related to the limits of divine power, including the question of whether God can change the past, for example, bringing about that Rome was never founded:I see I must respond finally to what many people, on the basis of your holiness's [own] judgment, raise as an objection on the topic of this dispute. For they say: If, as you assert, God is omnipotent in all things, can he manage this, that things that have been made were not made? He can certainly destroy all things that have been made, so that they do not exist now. But it cannot be seen how he can bring it about that things that have been made were not made. To be sure, it can come about that from now on and hereafter Rome does not exist; for it can be destroyed. But no opinion can grasp how it can come about that it was not founded long ago...One early work of fiction detailing an alternate history is Joanot Martorell's 1490 epic romance "Tirant lo Blanch", which was written when the fall of Constantinople to the Turks was still a recent and traumatic memory for Christian Europe. It tells the story of the knight Tirant the White from Brittany who travels to the embattled remnants of the Byzantine Empire. He becomes a Megaduke and commander of its armies and manages to fight off the invading Ottoman armies of . He saves the city from Islamic conquest, and even chases the Turks deeper into lands they had previously conquered. 19th century.
One of the earliest works of alternate history published in large quantities for the reception of a large audience may be Louis Geoffroy's "Histoire de la Monarchie universelle : Napoléon et la conquête du monde (1812–1832)" (History of the Universal Monarchy: Napoleon and the Conquest of the World) (1836), which imagines Napoleon's First French Empire emerging victorious in the French invasion of Russia in 1812 and in an invasion of England in 1814, later unifying the world under Bonaparte's rule. "The Book of Mormon" (published 1830) is described as an "alternative history" by Richard Lyman Bushman, a biographer of Joseph Smith. Smith claimed to have translated the document from golden plates, which told the story of a Jewish group who migrated from Israel to the Americas and inhabited the region from about 600 B.C. to 400 A.D., becoming the ancestors of Native Americans. In the 2005 biography "", Bushman wrote that the "Book of Mormon" "turned American history upside down [and] works on the premise that a history—a book—can reconstitute a nation. It assumes that by giving a nation an alternative history, alternative values can be made to grow."
In the English language, the first known complete alternate history may be Nathaniel Hawthorne's short story "P.'s Correspondence", published in 1845. It recounts the tale of a man who is considered "a madman" due to his perceptions of a different 1845, a reality in which long-dead famous people, such as the poets Robert Burns, Lord Byron, Percy Bysshe Shelley, and John Keats, the actor Edmund Kean, the British politician George Canning, and Napoleon Bonaparte, are still alive. The first novel-length alternate history in English would seem to be Castello Holford's "Aristopia" (1895). While not as nationalistic as Geoffroy's "Napoléon et la conquête du monde, 1812–1823", "Aristopia" is another attempt to portray a Utopian society. In "Aristopia", the earliest settlers in Virginia discover a reef made of solid gold and are able to build a Utopian society in North America. Early 20th century and the era of the pulps. In 1905, H. G. Wells published "A Modern Utopia". As explicitly noted in the book itself, Wells's main aim in writing it was to set out his social and political ideas, the plot serving mainly as a vehicle to expound them. This book introduced the idea of a person being transported from a point in our familiar world to the precise geographical equivalent point in an alternate world in which history had gone differently. The protagonists undergo various adventures in the alternate world, and then are finally transported back to our world, again to the precise geographical equivalent point. Since then, that has become a staple of the alternate history genre.
A number of alternate history stories and novels appeared in the late 19th and early 20th centuries (see, for example, Joseph Edgar Chamberlin's "" [1907] and Charles Petrie's "If: A Jacobite Fantasy" [1926]). In 1931, British historian Sir John Squire collected a series of essays from some of the leading historians of the period for his anthology "If It Had Happened Otherwise". In that work, scholars from major universities, as well as important non-academic authors, turned their attention to such questions as "If the Moors in Spain Had Won" and "If Louis XVI Had Had an Atom of Firmness". The essays range from serious scholarly efforts to Hendrik Willem van Loon's fanciful and satiric portrayal of an independent 20th-century New Amsterdam, a Dutch city-state on the island of Manhattan. Among the authors included were Hilaire Belloc, André Maurois, and Winston Churchill.
The American humorist author James Thurber parodied alternate history stories about the American Civil War in his 1930 story "If Grant Had Been Drinking at Appomattox", which he accompanied with this very brief introduction: ""Scribner's" magazine is publishing a series of three articles: 'If Booth Had Missed Lincoln', 'If Lee Had Won the Battle of Gettysburg', and 'If Napoleon Had Escaped to America'. This is the fourth". Another example of alternate history from this period (and arguably the first that explicitly posited cross-time travel from one universe to another as anything more than a visionary experience) is H.G. Wells' "Men Like Gods" (1923) in which the London-based journalist Mr. Barnstable, along with two cars and their passengers, is mysteriously teleported into "another world", which the "Earthlings" call Utopia. Being far more advanced than Earth, Utopia is some 3000 years ahead of humanity in its development. Wells describes a multiverse of alternative worlds, complete with the paratime travel machines that would later become popular with American pulp writers. However, since his hero experiences only a single alternate world, the story is not very different from conventional alternate history.
In the 1930s, alternate history moved into a new arena. The December 1933 issue of "Astounding" published Nat Schachner's "Ancestral Voices", which was quickly followed by Murray Leinster's "Sidewise in Time" (1934). While earlier alternate histories examined reasonably-straightforward divergences, Leinster attempted something completely different. In his "World gone mad", pieces of Earth traded places with their analogs from different timelines. The story follows Professor Minott and his students from a fictitious Robinson College as they wander through analogues of worlds that followed a different history. "Sidewise in Time" has been described as "the point at which the alternate history narrative first enters science fiction as a plot device" and is the story for which the Sidewise Award for Alternate History is named. A somewhat similar approach was taken by Robert A. Heinlein in his 1941 novelette "Elsewhen" in which a professor trains his mind to move his body across timelines. He then hypnotizes his students so that they can explore more of them. Eventually, each settles into the reality that is most suitable for him or her. Some of the worlds they visit are mundane, some are very odd, and others follow science fiction or fantasy conventions.
World War II produced alternate history for propaganda: both British and American authors wrote works depicting Nazi invasions of their respective countries as cautionary tales. Time travel to create historical divergences. The period around World War II also saw the publication of the time travel novel "Lest Darkness Fall" by L. Sprague de Camp in which an American academic travels to Italy at the time of the Byzantine invasion of the Ostrogoths. De Camp's time traveler, Martin Padway, is depicted as making permanent historical changes and implicitly forming a new time branch, thereby making the work an alternate history. In William Tenn's short story "Brooklyn Project" (1948), a tyrannical US Government brushes aside the warnings of scientists about the dangers of time travel and goes on with a planned experiment - with the result that minor changes to the prehistoric past cause Humanity to never have existed, its place taken by tentacled underwater intelligent creatures - who also have a tyrannical government which also insists on experimenting with time-travel.
In Ray Bradbury's classic short story "A Sound of Thunder" (1952) a group of hunters travel to the Late Cretaceous to hunt dinosaurs whose death would not be considered consequential as they are about to die a natural death within two minutes of the encounter. To minimize risking changes history they are told to stay on a levitating antigravity path that touches nothing. However one of the hunters stumbles off the path, inadvertently crushing a butterfly. When the group returns they find that history became significantly harsher and a fascist is now President. Time travel as the cause of a point of divergence (POD), which can denote either the bifurcation of a historical timeline or a simple replacement of the future that existed before the time-travelling event, has continued to be a popular theme. In Ward Moore's "Bring the Jubilee" (1953), the protagonist lives in an alternate history in which the Confederacy has won the American Civil War. He travels backward through time and brings about a Union victory at the Battle of Gettysburg.
When a story's assumptions about the nature of time travel lead to the complete replacement of the visited time's future, rather than just the creation of an additional time line, the device of a "time patrol" is often used where guardians move through time to preserve the "correct" history. A more recent example is "Making History" by Stephen Fry in which a time machine is used to alter history so that Adolf Hitler was never born. That ironically results in a more competent leader of Nazi Germany and results in the country's ascendancy and longevity in the altered timeline. Quantum theory of many worlds. While many justifications for alternate histories involve a multiverse, the "many world" theory would naturally involve many worlds, in fact a continually exploding array of universes. In quantum theory, new worlds would proliferate with every quantum event, and even if the writer uses human decisions, every decision that could be made differently would result in a different timeline. A writer's fictional multiverse may, in fact, preclude some decisions as humanly impossible, as when, in "Night Watch", Terry Pratchett depicts a character informing Vimes that while anything that can happen, has happened, nevertheless there is no history whatsoever in which Vimes has ever murdered his wife. When the writer explicitly maintains that "all" possible decisions are made in all possible ways, one possible conclusion is that the characters were neither brave, nor clever, nor skilled, but simply lucky enough to happen on the universe in which they did not choose the cowardly route, take the stupid action, fumble the crucial activity, etc.; few writers focus on this idea, although it has been explored in stories such as Larry Niven's story "All the Myriad Ways", where the reality of all possible universes leads to an epidemic of suicide and crime because people conclude their choices have no moral import.
In any case, even if it is true that every possible outcome occurs in some world, it can still be argued that traits such as bravery and intelligence might still affect the relative frequency of worlds in which better or worse outcomes occurred (even if the total number of worlds with each type of outcome is infinite, it is still possible to assign a different measure to different infinite sets). The physicist David Deutsch, a strong advocate of the many-worlds interpretation of quantum mechanics, has argued along these lines, saying that "By making good choices, doing the right thing, we thicken the stack of universes in which versions of us live reasonable lives. When you succeed, all the copies of you who made the same decision succeed too. What you do for the better increases the portion of the multiverse where good things happen." This view is perhaps somewhat too abstract to be explored directly in science fiction stories, but a few writers have tried, such as Greg Egan in his short story "The Infinite Assassin", where an agent is trying to contain reality-scrambling "whirlpools" that form around users of a certain drug, and the agent is constantly trying to maximize the consistency of behavior among his alternate selves, attempting to compensate for events and thoughts he experiences, he guesses are of low measure relative to those experienced by most of his other selves.
Many writers—perhaps the majority—avoid the discussion entirely. In one novel of this type, H. Beam Piper's "Lord Kalvan of Otherwhen", a Pennsylvania State Police officer, who knows how to make gunpowder, is transported from our world to an alternate universe where the recipe for gunpowder is a tightly held secret and saves a country that is about to be conquered by its neighbors. The paratime patrol members are warned against going into the timelines immediately surrounding it, where the country "will" be overrun, but the book never depicts the slaughter of the innocent thus entailed, remaining solely in the timeline where the country is saved. The cross-time theme was further developed in the 1960s by Keith Laumer in the first three volumes of his "Imperium" sequence, which would be completed in "Zone Yellow" (1990). Piper's politically more sophisticated variant was adopted and adapted by Michael Kurland and Jack Chalker in the 1980s; Chalker's "G.O.D. Inc" trilogy (1987–89), featuring paratime detectives Sam and Brandy Horowitz, marks the first attempt at merging the paratime thriller with the police procedural. Kurland's "Perchance" (1988), the first volume of the never-completed "Chronicles of Elsewhen", presents a multiverse of secretive cross-time societies that utilize a variety of means for cross-time travel, ranging from high-tech capsules to mutant powers. Crosstime Traffic is a 6-book series written by Harry aimed at teenagers featuring a variant of H. Beam Piper's paratime trading empire. While the home timeline appears to be the same in each of the books there is no overlap in characters or repetition of the alternative worlds.
Rival paratime worlds. The concept of a cross-time version of a world war, involving rival paratime empires, was developed in Fritz Leiber's Change War series, starting with the Hugo Award winning "The Big Time" (1958); followed by Richard C. Meredith's "Timeliner" trilogy in the 1970s, Michael McCollum's "A Greater Infinity" (1982) and John Barnes' "Timeline Wars" trilogy in the 1990s. Such "paratime" stories may include speculation that the laws of nature can vary from one universe to the next, providing a science fictional explanation—or veneer—for what is normally fantasy. Aaron Allston's "Doc Sidhe" and "Sidhe Devil" take place between our world, the "grim world" and an alternate "fair world" where the Sidhe retreated to. Although technology is clearly present in both worlds, and the "fair world" parallels our history, about fifty years out of step, there is functional magic in the fair world. Even with such explanation, the more explicitly the alternate world resembles a normal fantasy world, the more likely the story is to be labelled fantasy, as in Poul Anderson's "House Rule" and "Loser's Night". In both science fiction and fantasy, whether a given parallel universe is an alternate history may not be clear. The writer might allude to a POD only to explain the existence and make no use of the concept, or may present the universe without explanation of its existence.
Major writers explore alternate histories. Isaac Asimov's short story "What If—" (1952) is about a couple who can explore alternate realities by means of a television-like device. This idea can also be found in Asimov's novel "The End of Eternity" (1955), in which the "Eternals" can change the realities of the world, without people being aware of it. Poul Anderson's "Time Patrol" stories feature conflicts between forces intent on changing history and the Patrol who work to preserve it. One story, Delenda Est, describes a world in which Carthage triumphed over the Roman Republic. "The Big Time", by Fritz Leiber, describes a Change War ranging across all of history. Keith Laumer's "Worlds of the Imperium" is one of the earliest alternate history novels; it was published by "Fantastic Stories of the Imagination" in 1961, in magazine form, and reprinted by Ace Books in 1962 as one half of an Ace Double. Besides our world, Laumer describes a world ruled by an Imperial aristocracy formed by the merger of European empires, in which the American Revolution never happened, and a third world in post-war chaos ruled by the protagonist's doppelganger.
Philip K. Dick's novel, "The Man in the High Castle" (1962), is an alternate history in which Nazi Germany and Imperial Japan won World War II. This book contains an example of "alternate-alternate" history, in that one of its characters authored a book depicting a reality in which the Allies won the war, itself divergent from real-world history in several aspects. The several characters live within a divided United States, in which the Empire of Japan takes the Pacific states, governing them as a puppet, Nazi Germany takes the East Coast of the United States and parts of the Midwest, with the remnants of the old United States' government as the Neutral Zone, a buffer state between the two superpowers. The book has inspired an Amazon series of the same name. Vladimir Nabokov's novel, "" (1969), is a story of incest that takes place within an alternate North America settled in part by Czarist Russia and that borrows from Dick's idea of "alternate-alternate" history (the world of Nabokov's hero is wracked by rumors of a "counter-earth" that apparently is ours). Some critics believe that the references to a counter-earth suggest that the world portrayed in "Ada" is a delusion in the mind of the hero (another favorite theme of Dick's novels). Strikingly, the characters in "Ada" seem to acknowledge their own world as the copy or negative version, calling it "Anti-Terra", while its mythical twin is the real "Terra". Like history, science has followed a divergent path on Anti-Terra: it boasts all the same technology as our world, but all based on water instead of electricity; e.g., when a character in "Ada" makes a long-distance call, all the toilets in the house flush at once to provide hydraulic power.
Guido Morselli described the defeat of Italy (and subsequently France) in World War I in his novel, "Past Conditional" (1975; ), wherein the static Alpine front line which divided Italy from Austria during that war collapses when the Germans and the Austrians forsake trench warfare and adopt blitzkrieg twenty years in advance. Kingsley Amis set his novel, "The Alteration" (1976), in the 20th century, but major events in the Reformation did not take place, and Protestantism is limited to the breakaway Republic of New England. Martin Luther was reconciled to the Roman Catholic Church and later became Pope Germanian I. In Nick Hancock and Chris England's 1997 book "What Didn't Happen Next: An Alternative History of Football" it is suggested that, had Gordon Banks been fit to play in the 1970 FIFA World Cup quarter-final, there would have been no Thatcherism and the post-war consensus would have continued indefinitely. Kim Stanley Robinson's novel, "The Years of Rice and Salt" (2002), starts at the point of divergence with Timur turning his army away from Europe, and the Black Death has killed 99% of Europe's population, instead of only a third. Robinson explores world history from that point in AD 1405 (807 AH) to about AD 2045 (1467 AH). Rather than following the great man theory of history, focusing on leaders, wars, and major events, Robinson writes more about social history, similar to the Annales School of history theory and Marxist historiography, focusing on the lives of ordinary people living in their time and place.
Philip Roth's novel, "The Plot Against America" (2004), looks at an America where Franklin D. Roosevelt is defeated in 1940 in his bid for a third term as President of the United States, and Charles Lindbergh is elected, leading to a US that features increasing fascism and anti-Semitism. Michael Chabon, occasionally an author of speculative fiction, contributed to the genre with his novel "The Yiddish Policemen's Union" (2007), which explores a world in which the State of Israel was destroyed in its infancy and many of the world's Jews instead live in a small strip of Alaska set aside by the US government for Jewish settlement. The story follows a Jewish detective solving a murder case in the Yiddish-speaking semi-autonomous city state of Sitka. Stylistically, Chabon borrows heavily from the noir and detective fiction genres, while exploring social issues related to Jewish history and culture. Apart from the alternate history of the Jews and Israel, Chabon also plays with other common tropes of alternate history fiction; in the book, Germany actually loses the war even "harder" than they did in reality, getting hit with a nuclear bomb instead of just simply losing a ground war (subverting the common "what if Germany won WWII?" trope).
Contemporary alternate history in popular literature. The late 1980s and the 1990s saw a boom in popular-fiction versions of alternate history, fueled by the emergence of the prolific alternate history author Harry Turtledove, as well as the development of the steampunk genre and two series of anthologies—the "What Might Have Been" series edited by Gregory Benford and the "Alternate ..." series edited by Mike Resnick. This period also saw alternate history works by S. M. Stirling, Kim Stanley Robinson, Harry Harrison, Howard Waldrop, Peter Tieryas, and others. In 1986, a sixteen-part epic comic book series called "Captain Confederacy" began examining a world where the Confederate States of America won the American Civil War. In the series, the Captain and others heroes are staged government propaganda events featuring the feats of these superheroes.
Perhaps the most incessantly explored theme in popular alternate history focuses on the aftermath of an Axis victory in World War II. In some versions, the Nazis and/or Axis Powers win; or in others, they conquer most of the world but a "Fortress America" exists under siege; while in others, there is a Nazi/Japanese Cold War comparable to the US/Soviet equivalent in 'our' timeline. "Fatherland" (1992), by Robert Harris, is set in Europe following the Nazi victory. The novel "Dominion" by C.J. Sansom (2012) is similar in concept but is set in England, with Churchill the leader of an anti-German Resistance and other historic persons in various fictional roles. In the Mecha Samurai Empire series (2016), Peter Tieryas focuses on the Asian-American side of the alternate history, exploring an America ruled by the Japanese Empire while integrating elements of Asian pop culture like mechas and videogames. Several writers have posited points of departure for such a world but then have injected time splitters from the future. For instance James P. Hogan's "The Proteus Operation". Norman Spinrad wrote "The Iron Dream" in 1972, which is intended to be a science fiction novel written by Adolf Hitler after fleeing from Europe to North America in the 1920s.
In Jo Walton's "Small Change" series, the United Kingdom made peace with Hitler before the involvement of the United States in World War II, and slowly collapses due to severe economic depression. Former House Speaker Newt Gingrich and William R. Forstchen have written a novel, "1945", in which the US defeated Japan but not Germany in World War II, resulting in a Cold War with Germany rather than the Soviet Union. Gingrich and Forstchen neglected to write the promised sequel; instead, they wrote a trilogy about the American Civil War, starting with "", in which the Confederates win a victory at the Battle of Gettysburg - however, after Lincoln responds by bringing Grant and his forces to the eastern theater, the Army of Northern Virginia is soon trapped and destroyed in Maryland, and the war ends within weeks. While World War II has been a common point of divergence in alternate history literature, several works have been based on other points of divergence. For example, Martin Cruz Smith, in his first novel, posited an independent American Indian nation following the defeat of Custer in "The Indians Won" (1970). Beginning with "The Probability Broach" in 1980, L. Neil Smith wrote several novels that postulated the disintegration of the US Federal Government after Albert Gallatin joins the Whiskey Rebellion in 1794 and eventually leads to the creation of a libertarian utopia. In the 2022 novel "Poutine and Gin" by Steve Rhinelander, the point of divergence is the Battle of the Plains of Abraham of the French and Indian War. That novel is a mystery set in 1940 of that time line.
A recent time traveling splitter variant involves entire communities being shifted elsewhere to become the unwitting creators of new time branches. These communities are transported from the present (or the near-future) to the past or to another timeline via a natural disaster, the action of technologically advanced aliens, or a human experiment gone wrong. S. M. Stirling wrote the "Island in the Sea of Time" trilogy, in which Nantucket Island and all its modern inhabitants are transported to Bronze Age times to become the world's first superpower. In Eric Flint's "1632" series, a small town in West Virginia is transported to 17th century central Europe and drastically changes the course of the Thirty Years' War, which was then underway. John Birmingham's "Axis of Time" trilogy deals with the culture shock when a United Nations naval task force from 2021 finds itself back in 1942 helping the Allies against the Empire of Japan and the Germans (and doing almost as much harm as good in spite of its advanced weapons). The series also explores the cultural impacts of people with 2021 ideals interacting with 1940s culture. Similarly, Robert Charles Wilson's "Mysterium" depicts a failed US government experiment which transports a small American town into an alternative version of the US run by Gnostics, who are engaged in a bitter war with the "Spanish" in Mexico (the chief scientist at the laboratory where the experiment occurred is described as a Gnostic, and references to Christian Gnosticism appear repeatedly in the book).
Although not dealing in physical time travel, in his alt-history novel "Marx Returns", Jason Barker introduces anachronisms into the life and times of Karl Marx, such as when his wife Jenny sings a verse from the Sex Pistols's song "Anarchy in the U.K.", or in the games of chess she plays with the Marxes' housekeeper Helene Demuth, which on one occasion involves a Caro–Kann Defence. In her review of the novel, Nina Power writes of "Jenny's 'utopian' desire for an end to time", an attitude which, according to Power, is inspired by her husband's co-authored book "The German Ideology". However, in keeping with the novel's anachronisms, the latter was not published until 1932. By contrast, the novel's timeline ends in 1871. In the 2022 novel "Hydrogen Wars: Atomic Sunrise" by R.M. Christianson, a small change in post-war Japanese history leads to the election of General Douglas MacArthur as President of the United States. This minor change ultimately leads to all-out atomic war between the major Cold War powers.
Through crowdfunding on Kickstarter, Alan Jenkins and Gan Golan produced a graphic novel series called "1/6" depicting a dystopian alternate reality in which the January 6 United States Capitol attack was successful. What follows is the burning down of the Capitol building and the hanging of Vice President Mike Pence. Under Donald Trump's second term as president, a solid gold statue of him is erected and armed thugs patrol the streets of Washington DC suppressing civilian resistance with brutal violence under the banner of the Confederate flag. In fantasy genre. Many works of straight fantasy and science fantasy take place in historical settings, though with the addition of, for example, magic or mythological beasts. Some present a secret history in which the modern day world no longer believes that these elements ever existed. Many ambiguous alternate/secret histories are set in Renaissance or pre-Renaissance times, and may explicitly include a "retreat" from the world, which would explain the current absence of such phenomena. Other stories make plan a divergence of some kind.
In Poul Anderson's "Three Hearts and Three Lions" in which the Matter of France is history and the fairy folk are real and powerful. The same author's "A Midsummer Tempest" occurs in a world in which the plays of William Shakespeare (called here "the Great Historian"), presented the literal truth in every instance. The novel itself takes place in the era of Oliver Cromwell and Charles I. Here, the English Civil War had a different outcome, and the Industrial Revolution has occurred early. Randall Garrett's "Lord Darcy" series presents a point of divergence: a monk systemizes magic rather than science, so the use of foxglove to treat heart disease is regarded as superstition. Another point of divergence occurs in 1199, when Richard the Lionheart survives the Siege of Chaluz and returns to England and makes the Angevin Empire so strong that it survives into the 20th century. "Jonathan Strange & Mr Norrell" by Susanna Clarke takes place in an England where a separate Kingdom ruled by the Raven King and founded on magic existed in Northumbria for over 300 years. In Patricia Wrede's Regency fantasies, Great Britain has a Royal Society of Wizards.
"The Tales of Alvin Maker" series by Orson Scott Card (a parallel to the life of Joseph Smith, founder of the Latter Day Saint movement) takes place in an alternate America, beginning in the early 19th century. Prior to that time, a POD occurred: England, under the rule of Oliver Cromwell, had banished "makers", or anyone else demonstrating "knacks" (an ability to perform seemingly supernatural feats) to the North American continent. Thus the early American colonists embraced these gifts as perfectly ordinary, and counted on them as a part of their daily lives. The political division of the continent is considerably altered, with two large English colonies bookending a smaller "American" nation, one aligned with England, and the other governed by exiled Cavaliers. Actual historical figures are seen in a much different light: Ben Franklin is revered as the continent's finest "maker", George Washington was executed after being captured, and "Tom" Jefferson is the first president of "Appalachia", the result of a compromise between the Continentals and the British Crown.
On the other hand, when the "Old Ones" (fairies) still manifest themselves in England in Keith Roberts's "Pavane", which takes place in a technologically backward world after a Spanish assassination of Elizabeth I allowed the Spanish Armada to conquer England, the possibility that the fairies were real but retreated from modern advances makes the POD possible: the fairies really were present all along, in a secret history. Again, in the English Renaissance fantasy "Armor of Light" by Melissa Scott and Lisa A. Barnett, the magic used in the book, by Dr. John Dee and others, actually was practiced in the Renaissance; positing a secret history of effective magic makes this an alternate history with a point of departure. Sir Philip Sidney survives the Battle of Zutphen in 1586, and shortly thereafter saving the life of Christopher Marlowe. When the magical version of our world's history is set in contemporary times, the distinction becomes clear between alternate history on the one hand and contemporary fantasy, using in effect a form of secret history (as when Josepha Sherman's "Son of Darkness" has an elf living in New York City, in disguise) on the other. In works such as Robert A. Heinlein's "Magic, Incorporated" where a construction company can use magic to rig up stands at a sporting event and Poul Anderson's "Operation Chaos" and its sequel "Operation Luna", where djinns are serious weapons of war—with atomic bombs—the use of magic throughout the United States and other modern countries makes it clear that this is not secret history—although references in "Operation Chaos" to degaussing the effects of cold iron make it possible that it is the result of a POD. The sequel clarifies this as the result of a collaboration of Einstein and Planck in 1901, resulting in the theory of "rhea tics". Henry Moseley applies this theory to "degauss the effects of cold iron and release the goetic forces." This results in the suppression of ferromagnetism and the re-emergence of magic and magical creatures.
Alternate history shades off into other fantasy subgenres when the use of actual, though altered, history and geography decreases, although a culture may still be clearly the original source; Barry Hughart's "Bridge of Birds" and its sequels take place in a fantasy world, albeit one clearly based on China, and with allusions to actual Chinese history, such as the Empress Wu. Richard Garfinkle's "Celestial Matters" incorporates ancient Chinese physics and Greek Aristotelian physics, using them as if factual. Alternate history has long been a staple of Japanese speculative fiction with such authors as Futaro Yamada and Ryō Hanmura writing novels set in recognizable historical settings with added supernatural or science fiction elements. Ryō Hanmura's 1973 "Musubi no Yama Hiroku" which recreated 400 years of Japan's history from the perspective of a secret magical family with psychic abilities. The novel has since come to be recognized as a masterpiece of Japanese speculative fiction. Twelve years later, author Hiroshi Aramata wrote the groundbreaking "Teito Monogatari" which reimagined the history of Tokyo across the 20th century in a world heavily influenced by the supernatural.
Disney's "Pirates of the Caribbean" series takes place in an alternate history. The filmmakers of ' made no secret about taking liberties with the time period in which their story takes place. Producer Jerry Bruckheimer explained that the film is a fantasy, but did want to be true to the overall feel of the era, paying particular attention to the years between 1720 and 1750 "in an effort to find an approximation." Director Gore Verbinski asserted that it takes place "roughly at the tail end of the Golden Age of Piracy, when the Morgans lived. Maybe the late 1720s." The crew went to great lengths to maintain authenticity, such as Jack Sparrow's sword being an original that dates from the 1750s. Ann C. Crispin knew about the Pirates universe being an alternate history writing the prequel novel ', with Disney's instructions for Crispin being to "stick to historical fact, unless it conflicts with established Pirates of the Caribbean continuity." Crispin made a faithful effort to do this, having done plenty of research, with "Under the Black Flag" by David Cordingly being one of the four pirate-related books she found herself using the most consistently. According to production designer John Myhre, the filmmakers of the fourth film, ', picked the date of 1750, or in the range of the mid-1700s. The film also featured Blackbeard, based on the historical figure and an element retained from the novel "On Stranger Tides" by Tim Powers. The history prior to "On Stranger Tides" is also slightly different from real-world history, with Blackbeard's death at Ocracoke Inlet in 1718 was considered a legend in the film, with Jack Sparrow saying he was beheaded, and that his headless body swam three times around his ship before climbing back on board. The fifth film, ', also took place in the 1750s, with an early draft taking place sometime the Seven Years' War.
Television. "1983" is set on a world where the Iron Curtain never fell and the Cold War continues until the present (2003). "An Englishman's Castle" tells the story of the writer of a soap opera in a 1970s England which lost World War II. England is run by a collaborator government which strains to maintain a normal appearance of British life. Slowly, however, the writer begins to uncover the truth. In the "Community" episode "Remedial Chaos Theory," each of the six members of the study group rolls a die to decide who has to go downstairs to accept a pizza delivery for the group, creating 6 different alternative worlds. Characters from the worst universe, "darkest timeline," would later appear in the "prime universe". "Confederate" was a planned HBO series set on a world where the south won the US Civil War. Social media backlash during pre-production led to the series being cancelled with no episodes produced. "Counterpart" tells of a United Nations agency that is responsible for monitoring passage between alternative worlds. Two of the worlds, Alpha and Prime, are locked in a cold war.
"The Court-Martial of George Armstrong Custer" is a 1977 telemovie where George Custer survives the Battle of Little Bighorn and faces a court martial hearing over his incompetence. "" presents itself as a British TV documentary uncovering some of the dark secrets of the Confederacy on a world where the south won the US Civil War. "Dark Skies" tells that much of history having been shaped since the 1940s by a government conspiracy with aliens. One race of aliens can take over humans, while those immune to the alien's control fight back. "Doctor Who"'s main character has visited two alternative worlds in the TV show and several in its spin off media. The Third Doctor visits a world with a fascist Great Britain on the brink of destruction in "Inferno", while the Tenth Doctor visits a Britain that has a President and blimps are a common form of transportation beset by Cybermen in "Rise of the Cybermen" / "The Age of Steel". The Seventh Doctor faces a threat from an alternative world in "Battlefield", where magic is real and the alternative version of The Doctor is hinted to be that reality's Merlin.
"Fallout" shows a 1950s retro-future world that suffers a global nuclear war on the Amazon streaming service. "Fatherland" is a TV movie set in a 1960 alternative world where US President Joseph Kennedy and Adolf Hitler have agreed to meet to discuss an end to their country's Cold War 15 years after the Axis victory in World War II. However, an American reporter has discovered proof of the long denied Final Solution threatens the meeting. The anime "" featured an alternate 18th century. "For All Mankind" depicts an alternate timeline in which the Soviet crewed lunar program successfully lands on the Moon before the US Apollo program, resulting in a continued and intensified Space Race. "Fringe" has the father of one of the main characters cross into another reality to steal that world's version of his son after his son dies. The second world has a slightly different history, with a few different states in the United States, such as only one Carolina and Upper Michigan as a state. In addition, the 9/11 attack didn't take down the Twin Towers but the White House. Also, several major DC Comics events are different, such as Superman not Supergirl dying during "Crisis on Infinite Earths". The incursion to steal the son has many negative effects on that world, and while the realities start out as antagonist, they eventually work together to repair the damage.
"The Man in the High Castle," an adaptation of the novel of the same name, showed a world where the Axis Powers won World War II. "" explores a female-dominated world in which witchcraft is real. Its world diverged from our timeline when the Salem witch trials are resolved by an agreement between witches and ungifted humans. "Noughts + Crosses" is a British TV show set on a world where a powerful West African empire colonizes Europe 700 years before the start of the series. "Parallels" was a planned TV show whose pilot was later released as a Netflix movie. The plot concerns a building which can shift realities every 36 hours and those who use the building to travel to other realities. "The Plot Against America" is an HBO miniseries where Charles Lindbergh wins the 1940 US presidential election as an anti-war candidate who moves the country toward fascism. Primal features Spear and Fang from the Prehistoric encountering with Ancient Egypt and Vikings era, and in episode The Primal Theory where Charles Darwin is alive in 1890 instead of 1882.
The TV show "Sliders" explores different possible alternate realities by having the protagonist "slide" into different parallel dimensions of the same planet Earth. "The Great Martian War 1913-1917" An alternate history documentary where giant martians with machines invaded the Earth during WW1, causing huge technological upgrades and the entente and central powers fighting alongside each other. "SS-GB" shows a world where the Axis Powers quickly win World War II, killing Churchill and installing a puppet government. However, British resistance fights back. In the various "Star Trek" TV shows and spin off media a Mirror Universe has been encountered where Earth has an empire that subjugates other planets. Doppelgängers of the main cast of many the TV shows appear in that reality. The "Watchmen" series is set on a world where costumed heroes were initially welcomed but later outlawed. It is set 34 years after the events of the comic book on which the series shares a name. The Marvel Cinematic Universe series, "Loki" (2021 & 2023), on Disney+, shows an agency which prevents alterations to the timeline. Alternate versions of Loki from various universes appear.
The Marvel Cinematic Universe series, "What If...?" (2021–2024), on Disney+, shows alternate universes that depict alternate events from the MCU films. Online. Fans of alternate history have made use of the internet from a very early point to showcase their own works and provide useful tools for those fans searching for anything alternate history, first in mailing lists and usenet groups, later in web databases and forums. The "Usenet Alternate History List" was first posted on 11 April 1991, to the Usenet newsgroup rec.arts.sf-lovers. In May 1995, the dedicated newsgroup "soc.history.what-if" was created for showcasing and discussing alternate histories. Its prominence declined with the general migration from unmoderated usenet to moderated web forums, most prominently AlternateHistory.com, the self-described "largest gathering of alternate history fans on the internet" with over 10,000 active members. In addition to these discussion forums, in 1997 was created as an online repository, now containing over 2,900 alternate history novels, stories, essays, and other printed materials in several different languages. Uchronia was selected as the Sci Fi Channel's "Sci Fi Site of the Week" twice.
Uchronia. In Spanish, French, German, Portuguese, Italian, Catalan, and Galician, the words ', ', and "" are native versions of "alternate history", from which comes the English loanword "uchronia". The English term "uchronia" is a neologism that is sometimes used in its original meaning as a straightforward synonym for "alternate history." However, it may also now refer to other concepts, namely an umbrella genre of fiction that encompasses alternate history, parallel universes in fiction, and fiction based in futuristic or non-temporal settings.
Atomic orbital 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 , , and , 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 and orbitals, and are often labeled using associated harmonic polynomials (e.g., "xy", ) which describe their angular structure. An orbital can be occupied by a maximum of two electrons, each with its own projection of spin formula_1. The simple names s orbital, p orbital, d orbital, and f orbital refer to orbitals with angular momentum quantum number and 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 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 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 , particularly when the atom bears a positive charge, energies of certain sub-shells become very similar and so, 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: Particle-like properties: 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 (2, 1, 0) + formula_2 (2, 1, 1), or even the mixed state (2, 1, 0) + formula_2 (2, 1, 1). For each eigenstate, a property has an eigenvalue. So, for the three states just mentioned, the value of formula_4 is 2, and the value of formula_5 is 1. For the second and third states, the value for formula_6 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 . A superposition of eigenstates (2, 1, 1) and (3, 2, 1) would have an ambiguous formula_4 and formula_5, but formula_6 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 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 in atoms and Cartesian 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: . The angular factors of atomic orbitals generate s, p, d, etc. functions as real combinations of spherical harmonics (where and are quantum numbers). There are typically three mathematical forms for the radial functions which can be chosen as a starting point for the calculation of the properties of atoms and molecules with many electrons:
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 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 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. 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 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.
Orbital notation and subshells. Orbitals have been given names, which are usually given in the form: where "X" is the energy level corresponding to the principal quantum number ; type is a lower-case letter denoting the shape or subshell of the orbital, corresponding to the angular momentum quantum number . For example, the orbital 1s (pronounced as the individual numbers and letters: "'one' 'ess'") is the lowest energy level () and has an angular quantum number of , denoted as s. Orbitals with are denoted as p, d and f respectively. The set of orbitals for a given n and is called a "subshell", denoted 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 = 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 , 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 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: , , and . 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 first appeared in the Bohr model where it determines the radius of each circular electron orbit. In modern quantum mechanics however, 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 are even more closely related, and are said to comprise a "subshell". Quantum numbers. Because 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 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 describes the orbital angular momentum of each electron and is a non-negative integer. Within a shell where is some integer , ranges across all (integer) values satisfying the relation formula_14. For instance, the shell has only orbitals with formula_15, and the shell has only orbitals with formula_15, and formula_17. The set of orbitals associated with a particular value of are sometimes collectively called a "subshell".
The magnetic quantum number, formula_18, describes the projection of the orbital angular momentum along a chosen axis. It determines the magnitude of the current circulating around that axis and the orbital contribution to the magnetic moment of an electron via the Ampèrian loop model. Within a subshell formula_19, formula_18 obtains the integer values in the range formula_21. The above results may be summarized in the following table. Each cell represents a subshell, and lists the values of formula_18 available in that subshell. Empty cells represent subshells that do not exist. Subshells are usually identified by their formula_4- and formula_19-values. formula_4 is represented by its numerical value, but formula_19 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 formula_27 and formula_15 as a '2s subshell'. Each electron also has angular momentum in the form of quantum mechanical spin given by spin "s" = . Its projection along a specified axis is given by the spin magnetic quantum number, "ms", which can be + or −. 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, (, , ), 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 would be no sense in distinguishing from . As such, the model is most useful when applied to physical systems that share these symmetries. The Stern–Gerlach experimentwhere an atom is exposed to a magnetic fieldprovides 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 formula_29 denote a complex orbital with quantum numbers , , and , the real orbitals formula_30 may be defined by
formula_31 If formula_32, with formula_33 the radial part of the orbital, this definition is equivalent to formula_34 where formula_35 is the real spherical harmonic related to either the real or imaginary part of the complex spherical harmonic formula_36. 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 and have the same interpretation and significance as their complex counterparts, but is no longer a good quantum number (but its absolute value is). Some real orbitals are given specific names beyond the simple formula_37 designation. Orbitals with quantum number are called orbitals. With this one can already assign names to complex orbitals such as formula_38; the first symbol is the quantum number, the second character is the symbol for that particular quantum number and the subscript is the quantum number.
As an example of how the full orbital names are generated for real orbitals, one may calculate formula_39. From the table of spherical harmonics, formula_40 with formula_41. Then formula_42 Likewise formula_43. As a more complicated example: formula_44 In all these cases we generate a Cartesian label for the orbital by examining, and abbreviating, the polynomial in appearing in the numerator. We ignore any terms in the polynomial except for the term with the highest exponent in . We then use the abbreviated polynomial as a subscript label for the atomic state, using the same nomenclature as above to indicate the formula_4 and formula_19 quantum numbers. formula_47 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 formula_48 and formula_49 orbitals may appear, for example, as the sum and difference of formula_50 and formula_51, 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 formula_52 so there does not seem be consensus on the naming of formula_53 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 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 has a constant value, chosen so that there is a certain probability (for example 90%) of finding the electron within the contour. Although as the square of an absolute value is everywhere non-negative, the sign of the wave function is often indicated in each subregion of the orbital picture.
Sometimes the function is graphed to show its phases, rather than which shows probability density but has no phase (which is lost when taking absolute value, since is a complex number). orbital graphs tend to have less spherical, thinner lobes than 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 graphs. The lobes can be seen as standing wave interference patterns between the two counter-rotating, ring-resonant traveling wave and modes; the projection of the orbital onto the xy plane has a resonant 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 there are two standing wave solutions and . If , the orbital is vertical, counter rotating information is unknown, and the orbital is "z"-axis symmetric. If there are no counter rotating modes. There are only radial 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 and . An orbital with azimuthal quantum number has radial nodal planes passing through the origin. For example, the s orbitals () are spherically symmetric and have no nodal planes, whereas the p orbitals () have a single nodal plane between the lobes. The number of nodal spheres equals , consistent with the restriction on the quantum numbers. The principal quantum number controls the total number of nodal surfaces which is . Loosely speaking, is energy, is analogous to eccentricity, and is orientation. In general, determines size and energy of the orbital for a given nucleus; as increases, the size of the orbital increases. The higher nuclear charge 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, determines an orbital's shape, and its orientation. However, since some orbitals are described by equations in complex numbers, the shape sometimes depends on also. Together, the whole set of orbitals for a given and fill space as symmetrically as possible, though with increasingly complex sets of lobes and nodes.
The single s orbitals (formula_15) are shaped like spheres. For it is roughly a solid ball (densest at center and fades outward exponentially), but for , each single s orbital is made of spherically symmetric surfaces which are nested shells (i.e., the "wave-structure" is 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 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 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 . The overall result is a lobe pointing along each direction of the primary axes. Four of the five d orbitals for 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 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 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 (for example, 3p orbitals vs. the fundamental 2p), an additional node in each lobe. Still higher values of 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 and 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 of the same shell (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 resulting total density of all the atomic orbitals in that subshell (those with the same ) 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 orbital is the same as the orbital, but the and are formed by taking linear combinations of the and orbitals (which is why they are listed under the label). Also, the and are not the same shape as the , since they are pure spherical harmonics. † "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 formula_55 orbitals where are eigenstates of the orbital angular momentum operator, formula_56. The columns with formula_57 are combinations of two eigenstates. See :
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 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 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 and the wave functions for a vibrating sphere are three-coordinate .
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. Such features again emphasize that the shapes of atomic orbitals are a direct consequence of the wave nature of electrons.
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 formula_4. The formula_59 orbital has the lowest possible energy in the atom. Each successively higher value of formula_4 has a higher energy, but the difference decreases as formula_4 increases. For high formula_4, the energy becomes so high that the electron can easily escape the atom. In single electron atoms, all levels with different formula_19 within a given formula_4 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 and another quantum number ), 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 formula_4 but also on formula_19. Higher values of formula_19 are associated with higher values of energy; for instance, the 2p state is higher than the 2s state. When formula_68, 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 formula_69 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 formula_19 of electrons becomes more and more of a determining factor in their energy, and the principal quantum numbers formula_4 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 formula_4 and formula_19 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 is the Pauli exclusion principle). These quantum numbers include the three that define orbitals, as well as the spin magnetic quantum number . Thus, two electrons may occupy a single orbital, so long as they have different values of . Because takes one of only two values ( or −), 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 "i"th 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 -state (but the associated with that -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 is the order for filling the "subshell" orbitals, which also gives the order of the "blocks" in the periodic table: 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 ). 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 , 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- 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 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.
There are no nodes in relativistic orbital densities, although individual components of the wave function will have nodes. pp hybridization (conjectured). 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: By quantum theory, state 1 has a fixed energy of , and state 2 has a fixed energy of . 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 . 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 will be absorbed by the electron in state 1, and that electron will jump to state 2. However, 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.
Amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 appear in the genetic code of life. Amino acids can be classified according to the locations of the core structural functional groups (alpha- (α-), beta- (β-), gamma- (γ-) amino acids, etc.); other categories relate to polarity, ionization, and side-chain group type (aliphatic, acyclic, aromatic, polar, etc.). In the form of proteins, amino-acid "residues" form the second-largest component (water being the largest) of human muscles and other tissues. Beyond their role as residues in proteins, amino acids participate in a number of processes such as neurotransmitter transport and biosynthesis. It is thought that they played a key role in enabling life on Earth and its emergence. Amino acids are formally named by the IUPAC-IUBMB Joint Commission on Biochemical Nomenclature in terms of the fictitious "neutral" structure shown in the illustration. For example, the systematic name of alanine is 2-aminopropanoic acid, based on the formula . The Commission justified this approach as follows:
The systematic names and formulas given refer to hypothetical forms in which amino groups are unprotonated and carboxyl groups are undissociated. This convention is useful to avoid various nomenclatural problems but should not be taken to imply that these structures represent an appreciable fraction of the amino-acid molecules. History. The first few amino acids were discovered in the early 1800s. In 1806, French chemists Louis-Nicolas Vauquelin and Pierre Jean Robiquet isolated a compound from asparagus that was subsequently named asparagine, the first amino acid to be discovered. Cystine was discovered in 1810, although its monomer, cysteine, remained undiscovered until 1884. Glycine and leucine were discovered in 1820. The last of the 20 common amino acids to be discovered was threonine in 1935 by William Cumming Rose, who also determined the essential amino acids and established the minimum daily requirements of all amino acids for optimal growth. The unity of the chemical category was recognized by Wurtz in 1865, but he gave no particular name to it. The first use of the term "amino acid" in the English language dates from 1898, while the German term, , was used earlier. Proteins were found to yield amino acids after enzymatic digestion or acid hydrolysis. In 1902, Emil Fischer and Franz Hofmeister independently proposed that proteins are formed from many amino acids, whereby bonds are formed between the amino group of one amino acid with the carboxyl group of another, resulting in a linear structure that Fischer termed "peptide".

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