Editing Valence bond theory (section)

==History==
In 1916, [[G. N. Lewis]] proposed that a chemical bond forms by the interaction of two shared bonding electrons, with the representation of molecules as [[Lewis structure]]s. The chemist [[Charles Rugeley Bury]] suggested in 1921 that eight and eighteen electrons in a shell form stable configurations. Bury proposed that the electron configurations in transitional elements depended upon the valence electrons in their outer shell.<ref>{{Cite journal|last=Bury|first=Charles R.|author-link=Charles Rugeley Bury|date=July 1921|title=Langmuir's Theory of the Arrangement of Electrons in Atoms and Molecules|journal=[[Journal of the American Chemical Society]]|language=en|volume=43|issue=7|pages=1602–1609|doi=10.1021/ja01440a023|bibcode=1921JAChS..43.1602B |issn=0002-7863|url=https://zenodo.org/record/1428812 }}</ref> In 1916, Kossel put forth his theory of the [[ionic bond|ionic chemical bond]] ([[octet rule]]), also independently advanced in the same year by [[Gilbert N. Lewis]].<ref>[http://www.ucc.ie/academic/chem/dolchem/html/dict/000c1.html University College Cork], [http://www.origin-life.gr.jp/2904/2904174/2904174.html University City Tübingen], and (Pauling, 1960, p. 5).</ref><ref>Walther Kossel, “Uber Molkulbildung als Frage der Atombau”, Ann. Phys., 1916, 49:229–362.</ref> [[Walther Kossel]] put forward a theory similar to Lewis' only his model assumed complete transfers of electrons between atoms, and was thus a model of [[ionic bond]]ing. Both Lewis and Kossel structured their bonding models on that of [[Abegg's rule]] (1904).

Although there is no mathematical formula either in chemistry or quantum mechanics for the arrangement of electrons in the atom, the hydrogen atom can be described by the [[Schrödinger equation]] and the [[Matrix Mechanics]] equation both derived in 1925. However, for hydrogen alone, in 1927 the Heitler–London theory was formulated which for the first time enabled the calculation of bonding properties of the [[hydrogen]] molecule H<sub>2</sub> based on quantum mechanical considerations. Specifically, [[Walter Heitler]] determined how to use [[Schrödinger equation|Schrödinger's wave equation]] (1926) to show how two hydrogen atom [[wavefunction]]s join together, with plus, minus, and exchange terms, to form a [[covalent bond]]. He then called up his associate [[Fritz London]] and they worked out the details of the theory over the course of the night.<ref>[http://osulibrary.oregonstate.edu/specialcollections/coll/pauling/bond/people/heitler.html Walter Heitler] – Key participants in the development of Linus Pauling's ''The Nature of the Chemical Bond''.</ref> Later, [[Linus Pauling]] used the pair bonding ideas of Lewis together with Heitler–London theory to develop two other key concepts in VB theory: [[resonance (chemistry)|resonance]] (1928) and [[orbital hybridization]] (1930). According to [[Charles Coulson]], author of the noted 1952 book ''Valence'', this period marks the start of "modern valence bond theory", as contrasted with older valence bond theories, which are essentially electronic theories of [[valence (chemistry)|valence]] couched in pre-wave-mechanical terms.<ref>{{Cite book |last=Coulson |first=Charles Alfred |url=https://books.google.com/books?id=Hv3BAAAAIAAJ |title=Valence |date=1952 |publisher=Clarendon Press |language=en}}</ref>

Linus Pauling published in 1931 his landmark paper on valence bond theory: "On the Nature of the Chemical Bond". Building on this article, Pauling's 1939 textbook: ''On the Nature of the Chemical Bond'' would become what some have called the bible of modern chemistry. This book helped experimental chemists to understand the impact of quantum theory on chemistry. However, the later edition in 1959 failed to adequately address the problems that appeared to be better understood by molecular orbital theory. The impact of valence theory declined during the 1960s and 1970s as molecular orbital theory grew in usefulness as it was implemented in large [[digital computer]] programs. Since the 1980s, the more difficult problems, of implementing valence bond theory into computer programs, have been solved largely, and valence bond theory has seen a resurgence.<ref>{{Cite journal |last1=Shaik |first1=Sason |last2=Danovich |first2=David |last3=Hiberty |first3=Philippe C. |date=2021-03-15 |title=Valence Bond Theory—Its Birth, Struggles with Molecular Orbital Theory, Its Present State and Future Prospects |journal=Molecules |language=en |volume=26 |issue=6 |pages=1624 |doi=10.3390/molecules26061624 |doi-access=free |pmid=33804038 |pmc=8001733 |issn=1420-3049}}</ref>