Hund's Multiplicity Rule Revisited

Abstract

The helium atom 1s12s1 excited state gives rise to singlet and triplet terms. The fact that the triplet state has a lower energy is frequently attributed to greater electron–electron repulsion in the singlet state. However, high-level quantum mechanical calculations show that the reason the triplet state is lower than the singlet is due to greater electron–nucleus attraction in the triplet state. The purpose of this article is to present a relatively simple quantum mechanical model for Hund's multiplicity rule that is accessible to undergraduate audiences and consistent with conclusions based on high-level quantum mechanical calculations.