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 2S+1L 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 2P term represents a state with S=1/2 and L=1.
The single electron in a 1s subshell gives rise to the 2S 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 2P 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
2S state is the source of the 21 cm
hydrogen line, important in astronomy.