aufbau principle
A rule for building up the @E01995@ of atoms and molecules. It states that a maximum of two electrons are put into orbitals in the order of increasing @O04318@: the lowest-energy orbitals are filled before electrons are placed in higher-energy orbitals.
PAC, 1999, 71, 1919. (Glossary of terms used in theoretical organic chemistry) on page 1925 [Terms] [Paper]