Editing Electron counting (section)

{{Short description|Formalism used for classifying compounds}}
In [[chemistry]], '''electron counting''' is a formalism for assigning a number of [[valence electrons]] to individual atoms in a molecule.  It is used for classifying compounds and for explaining or predicting their electronic structure and [[chemical bond|bonding]].<ref>{{Cite journal
| issn = 0021-9584
| volume = 83
| pages = 791
| last = Parkin
| first = Gerard
| title = Valence, Oxidation Number, and Formal Charge: Three Related but Fundamentally Different Concepts
| journal = Journal of Chemical Education
| access-date = 2009-11-10
| year = 2006
| issue = 5
| url = http://jchemed.chem.wisc.edu/Journal/Issues/2006/May/abs791.html
|bibcode = 2006JChEd..83..791P |doi = 10.1021/ed083p791 | url-access = subscription
}}</ref>  Many rules in chemistry rely on electron-counting:

*[[Octet rule]] is used with [[Lewis structure]]s for [[main group element]]s, especially the lighter ones such as [[carbon]], [[nitrogen]], and [[oxygen]],
*[[18-electron rule]]<ref>{{Cite journal |last=Rasmussen |first=Seth C. |date=March 2015 |title=The 18-electron rule and electron counting in transition metal compounds: theory and application |url=http://link.springer.com/10.1007/s40828-015-0010-4 |journal=ChemTexts |language=en |volume=1 |issue=1 |doi=10.1007/s40828-015-0010-4 |issn=2199-3793|url-access=subscription }}</ref> in [[inorganic chemistry]] and [[organometallic chemistry]] of [[transition metal]]s,
*[[Hückel's rule]] for the [[pi electron|π-electron]]s of [[aromaticity|aromatic compounds]],
*[[Polyhedral skeletal electron pair theory]] for [[Polyhedron|polyhedral]] [[cluster compound]]s, including transition metals and main group elements and mixtures thereof, such as [[boranes]].

Atoms are called "[[electron deficiency|electron-deficient]]" when they have too few electrons as compared to their respective rules, or "[[hypervalent molecule|hypervalent]]" when they have too many electrons.  Since these compounds tend to be more reactive than compounds that obey their rule, electron counting is an important tool for identifying the reactivity of molecules.  While the counting formalism considers each atom separately, these individual atoms (with their hypothetical assigned charge) do not generally exist as free [[chemical species|species]].