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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 $$n$$, $$l$$, and $$m$$. (See Section [s10.4].) To these we must now add the two quantum numbers $$s$$ and $$m_s$$ that parameterize the electron’s internal motion. (See the previous chapter.) Now, the quantum numbers $$l$$ and $$m$$ specify the electron’s orbital angular momentum vector, $${\bf L}$$, (as much as it can be specified) whereas the quantum numbers $$s$$ and $$m_s$$ specify its spin angular momentum vector, $${\bf S}$$. But, if the electron possesses both orbital and spin angular momentum then what is its total angular momentum?