Statistical weight of an atomic level ($g_J=2J+1$)

The statistical weight of an atomic level counts its degenerate quantum states. With no external field resolving magnetic substates, a specified total angular momentum $J$ gives $2J+1$ states. An unresolved term can have a larger summed weight. These weights enter the Boltzmann distribution, Saha ionization equation and detailed-balance relations; term and resolved-level weights must not be interchanged.