Consider an electron in a hydrogen atom. As we have already seen, the electron’s motion through space is parameterized by the three quantum numbers , , and . (See Section [s10.4].) To these we must now add the two quantum numbers and that parameterize the electron’s internal motion. (See the previous chapter.) Now, the quantum numbers and specify the electron’s orbital angular momentum vector, , (as much as it can be specified) whereas the quantum numbers and specify its spin angular momentum vector, . But, if the electron possesses both orbital and spin angular momentum then what is its total angular momentum?
