Octet Rule Definition in Chemistry

Chemistry Glossary Definition of Octet Rule

This is an example of the octet rule, using carbon dioxide.
This is the Lewis structure of carbon dioxide, illustrating the octet rule. Ben Mills

The octet rule in chemistry is the principle that bonded atoms share their eight outer electrons. This gives the atom a valence shell resembling that of a noble gas. The octet rule is a "rule" that is sometimes broken. However, it applies to carbon, nitrogen, oxygen, the halogens, and most metals, especially the alkali metals and alkaline earths.

A Lewis electron dot diagram may be drawn to illustrate the octet rule. In such a structure, electrons shared in a covalent bond between two atoms are counted twice (once for each atom). Other electrons are counted once.


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  • Lewis, Gilbert N. (1916). "The Atom and the Molecule". Journal of the American Chemical Society. 38 (4): 762–785. doi:10.1021/ja02261a002
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Helmenstine, Anne Marie, Ph.D. "Octet Rule Definition in Chemistry." ThoughtCo, Aug. 27, 2020, thoughtco.com/definition-of-octet-rule-604588. Helmenstine, Anne Marie, Ph.D. (2020, August 27). Octet Rule Definition in Chemistry. Retrieved from https://www.thoughtco.com/definition-of-octet-rule-604588 Helmenstine, Anne Marie, Ph.D. "Octet Rule Definition in Chemistry." ThoughtCo. https://www.thoughtco.com/definition-of-octet-rule-604588 (accessed January 27, 2021).