Lewis octet rule

A classical rule for describing the electronic configuration of atoms in molecules: the maximum number of electron pairs that can be accommodated in the valence shell of a first-row element is four. For the second and subsequent row elements there are many exceptions to this rule.
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1949 (https://doi.org/10.1351/pac199971101919)