Postulate: Every elementary particle is either a fermion or a boson. A state of many identical particles is totally antisymmetric with respect to the interchange of any two particles if they are fermions, and it is totally symmetric if they are bosons. No two identical fermions can have exactly the same set of quantum numbers. This is called the Pauli exclusion principle.
We assume that the non-central part of the electrostatic interaction is much bigger than the spin-orbit interaction. (This is usually true for light multi-electron atoms.) The electrostatic interaction leads to a splitting of the level corresponding to a given electron configuration into a number of sublevels characterized by different values of the total orbital angular momentum of the electrons, L, and their total spin, S. The operator for the electrostatic interaction commutes with L = l1+l2+l2+... and S = s1+s2+s3+... . To each term LS belong (2L+1)(2S+1) states, differing by the values of ML and MS. The spin-orbit interaction leads to a splitting of the term LS into a number of components corresponding to different values of the total angular momentum J. But it does not completely remove the degeneracy. Each J component is degenerate with a multiplicity of 2J+1.
∑J(2J+1) = (2L+1)(2S+1).
In the LS coupling scheme, a term is designated by 2S+1LJ. 2S+1 is called the multiplicity of the term.
Which terms corresponding to a given configuration have the lowest energy?
Hund's Rule (established empirically)