Friedrich Hund

Friedrich Hund organized complicated atomic spectral lines through relationships between the angular momenta and energy levels of many-electron configurations. In modern notation, a term symbol ${}^{2S+1}{L}_J$ records total electron spin $S$, total orbital angular momentum $L$ and coupled total angular momentum $J$. For example, $S=1$ permits three spin projections, whereas $S=0$ permits one. The classification reduces a large pattern of lines to relationships between fewer quantum states. This modern spin notation should be distinguished from the historical language of his original spectral analysis.^[ref]