A rule, complementary to the aufbau principle
, of building up the electronic configuration
of atoms and molecules: a maximum of two electrons can occupy an orbital and then only providing that the spins of the electrons are paired, i.e. opposed. The principle demands that the wavefunction for a many-electron system must be asymmetric
with respect to the permutation of the space-spin coordinates for every pair of electrons.
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