An Etymological Dictionary of Astronomy and Astrophysics
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A coupling scheme of electronic → spin angular momenta and → orbital angular momenta for heavy atoms (generally Z > 30), where the spin and orbital angular momenta of individual electrons couple strongly, and therefore the → LS coupling scheme does not apply. The coupling between spin and orbital angular momentum of each electron is much stronger than the coupling between different electrons. Therefore, the total angular momentum, j_i, for the i-th electron is obtained by combining l_i and s_i and then coupling these j's together to give the total angular momentum J = Σ_i j_i. In the jj coupling scheme the total orbital angular momentum quantum number, L, and the total spin angular momentum number, S, are not specified.

j referring to the symbol of the total angular momentum for individual electrons; → coupling.