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.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook