Hund's rules are two empirical rules for atomic spectra, formulated by the German physicist Friedrich Hund in 1925, which determine the lowest energy level for two electrons having the same n and l quantum numbers in a many-electron atom. Hund's rules are as follows.
1. Subshells in an atom fill so that the number of unpaired spins (due to orbitals with lone electrons) is maximized. In other words, the lowest energy state has the maximum multiplicity consistent with the Pauli exclusion principle. Also known as the rule of maximum multiplicity
2. The lowest energy state has the maximum total electron orbital angular momentum quantum number, consistent with rule (1).
Hund's rules are explained by the quantum theory of atoms by calculations involving the repulsion between two electrons.