Spin orbital coupling and total angular momentum
J is the total angular momentum. L is the orbital angular momentum and S is the intrinsic total electron spin angular momentum. J = L + S
L = $\sqrt{L\left(L+1\right)}\frac{h}{2\pi }$
--&gt; L can be n-1 where n is the Principal quantum number
S = $\sqrt{S\left(S+1\right)}\frac{h}{2\pi }$
--&gt; S is an integer or half an odd integer, depending on whether the number of electrons is even or odd
J = $\sqrt{J\left(J+1\right)}\frac{h}{2\pi }$
J can be L+S ... L-S
For the  Na-D lines the J = 3/2 is caused by L=1 and S=1/2 for the 3/2 value and L=1 and S=-1/2
The values for $J,L,S$ have a range of values e.g. $-X, -X+1,\cdots X-1, X $, e.g. if $L=5/2$ the range is $-5/2,-3/2,3/2,5/2$