The rule stating that many atoms tend to form molecules such that that the outermost shell of the atom has eight electrons, thus giving the atom the same electron configuration as the noble gases. This rule was postulated by Gilbert Newton Lewis in 1916 and was the first electron counting rule in chemistry. In quantum mechanics the rule derives from the octet corresponding to an s²p⁶. There are many exceptions to the octet rule. See also 18-electron rule.