https://doi.org/10.1351/goldbook.LT07065
A classical rule for describing the @C01248@ of atoms in molecules: the maximum number of electron pairs that can be accommodated in the @V06588@ shell of a first-row element is four. For the second and subsequent row elements there are many exceptions to this rule.
Source:
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1949 [Terms] [Paper]