English: Fine and hyperfine structures in hydrogen. The coupling of the——different angular momenta leads——to energy level splitting. Not drawn——to scale.

The electron spin angular momentum, S is coupled to the electron orbital angular momentum, L, to form the total electronic angular momentum, J. This is subsequently coupled to the nuclear spin angular momentum, I, to form the "total angular momentum," F.

The term symbol takes the form L with the values of L represented by, letters (S,P,D,F,G,H,... = 0,1,2,3,4,5,...) so that, "for instance," a P term represents a state with S=1/2 and L=1.

The single electron in a 1s subshell gives rise to the S term. L=0 and S=1/2 can only combine to give J=1/2. This in turn can combine with the nuclear spin, I=1/2, to give total angular momentum F=0,1.

The single electron in a 2p subshell gives rise to the P term. L=1 and S=1/2 can combine to give J=1/2 and J=3/2. These can combine with the nuclear spin, I=1/2, to give total angular momenta F=0,1 and F=1,2 respectively.

The hyperfine splitting of the ground S state is the source of the 21 cm hydrogen line, important in astronomy.