Editing Covalent bond (section)

== History ==
[[File:covalent.svg|thumb|200px|Early concepts in covalent bonding arose from this kind of image of the molecule of [[methane]]. Covalent bonding is implied in the [[Lewis structure]] by indicating electrons shared between atoms.]]

The term ''covalence'' in regard to bonding was first used in 1919 by [[Irving Langmuir]] in a ''[[Journal of the American Chemical Society]]'' article entitled "The Arrangement of Electrons in Atoms and Molecules". Langmuir wrote that "we shall denote by the term ''covalence'' the number of pairs of electrons that a given atom shares with its neighbors."<ref>{{cite journal|doi=10.1021/ja02227a002|volume=41|issue=6|pages=868–934|last=Langmuir|first=Irving|title=The Arrangement of Electrons in Atoms and Molecules|journal=Journal of the American Chemical Society|date=1919-06-01|url=https://zenodo.org/record/1429026}}</ref>

The idea of covalent bonding can be traced several years before 1919 to [[Gilbert N. Lewis]], who in 1916 described the sharing of electron pairs between atoms<ref>{{cite journal|doi=10.1021/ja02261a002|volume=38|issue=4|pages=762–785|last=Lewis|first=Gilbert N.|title=The atom and the molecule|journal=Journal of the American Chemical Society|date=1916-04-01|s2cid=95865413 |url=https://zenodo.org/record/1429068}}</ref> (and in 1926 he also coined the term "[[photon]]" for the smallest unit of radiant energy). He introduced the ''[[Lewis Structure|Lewis notation]]'' or ''electron dot notation'' or ''Lewis dot structure'', in which valence electrons (those in the outer shell) are represented as dots around the atomic symbols. Pairs of electrons located between atoms represent covalent bonds. Multiple pairs represent multiple bonds, such as [[double bond]]s and [[triple bond]]s. An alternative form of representation, not shown here, has bond-forming electron pairs represented as solid lines.<ref name=":0">{{Cite book|last=McMurry|first=John|title=Chemistry|publisher=Pearson|year=2016|isbn=978-0-321-94317-0|edition=7}}</ref>

Lewis proposed that an atom forms enough covalent bonds to form a full (or closed) outer electron shell. In the diagram of methane shown here, the carbon atom has a valence of four and is, therefore, surrounded by eight electrons (the [[octet rule]]), four from the carbon itself and four from the hydrogens bonded to it. Each hydrogen has a valence of one and is surrounded by two electrons (a duet rule) – its own one electron plus one from the carbon. The numbers of electrons correspond to full shells in the quantum theory of the atom; the outer shell of a carbon atom is the ''n''&nbsp;=&nbsp;2 shell, which can hold eight electrons, whereas the outer (and only) shell of a hydrogen atom is the ''n''&nbsp;=&nbsp;1 shell, which can hold only two.<ref name=":1">{{Cite book|last=Bruice|first=Paula|title=Organic Chemistry|publisher=Pearson|year=2016|isbn=978-0-13-404228-2|edition=8}}</ref>

While the idea of shared electron pairs provides an effective qualitative picture of covalent bonding, [[quantum mechanics]] is needed to understand the nature of these bonds and predict the structures and properties of simple molecules. [[Walter Heitler]] and [[Fritz London]] are credited with the first successful quantum mechanical explanation of a chemical bond ([[molecular hydrogen]]) in 1927.<ref name= London>{{cite journal|first1=W.|last1=Heitler|first2=F.|last2=London|title=Wechselwirkung neutraler Atome und homöopolare Bindung nach der Quantenmechanik|trans-title=Interaction of neutral atoms and homeopolar bonds according to quantum mechanics|journal=Zeitschrift für Physik|volume=44|issue=6–7|pages=455–472|date=1927|doi=10.1007/bf01397394|bibcode=1927ZPhy...44..455H|s2cid=119739102}} English translation in {{cite book| last = Hettema| first = H.| title = Quantum Chemistry: Classic Scientific Papers| url = https://books.google.com/books?id=qsidHRJmUoIC| access-date = 2012-02-05| year = 2000| publisher = World Scientific| isbn = 978-981-02-2771-5| pages = 140}}</ref> Their work was based on the valence bond model, which assumes that a chemical bond is formed when there is good overlap between the [[atomic orbitals]] of participating atoms.