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.
Background
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.
The rule was first stated in 1923 by German-American physicist Alfred Landé.
References
Landé interval rule Wikipedia(Text) CC BY-SA