A rule for building up the electronic configuration
of atoms and molecules. It states that a maximum of two electrons are put into orbitals in the order of increasing orbital energy
: the lowest-energy orbitals are filled before electrons are placed in higher-energy orbitals.
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