English: Fine and hyperfine levels of the hydrogen atom with the resulting radiation frequencies. English version. Own work based on: Hydrogen-Fine-Hyperfine-Levels.svg.

Description from Fine hyperfine levels.png: "Fine and hyperfine structure 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."