In atomic physics, the Landé interval rule states that if the spin-orbit interactions of an electron are weak, the energy levels of each (i.e. the spin and orbit) are split. Subsequently, each have a different angular momentum. The rule states that as a result of this, the interval between successive energy levels is proportional to the larger of their total angular momentum values.[1][2]
The rule assumes the Russell-Saunders coupling and that interactions between spin magnetic moments can be ignored. The latter is an incorrect assumption for light atoms. As a result of this, the rule is optimally followed by atoms with medium atomic numbers.[1]
The rule was first stated in 1923 by German-American physicist Alfred Landé.[1]