Empirical rules in atomic spectra that determine the lowest energy level for a configuration of two equivalent electrons (i.e. electrons with the same n and l quantum numbers) in a many-electron atom. (1) The lowest energy state has the maximum multiplicity consistent with the Pauli exclusion principle. (2) The lowest energy state has the maximum total electron orbital angular momentum quantum number, consistent with rule (1). These rules were put forward by the German physicist Friedrich Hund (1896–1997) in 1925. Hund’s rules are explained by the quantum theory of atoms by calculations involving the repulsion between two electrons and the attraction between these electrons and nuclei.