How Does Rubidium's Electron Spin Affect Its Energy Levels?

[Dunphy]

Hello everybody,

while I tried to understand the Rubidium level scheme, I realized that I have some issues concerning the energy levels in general... If in addition to the orbital Momentum the spin momentum of an electron is considered for example the p level splits to the ²P_1/2-level and the ²P_3/2-level. So there are the cases that the total angular momentum of the electron shell can be J=L-S=1-1/2 or J=L+S=1+1/2.
Same thing as hyperfine splitting: The ²P_1/2-level splits up into F=I+J=3 and F=I-J=2.
Why does it split into two levels and not more? Is there a way to describe this with the orientation of the spin momentum vector with regard to the angular momentum vector?

Many thanks in advance!
Dunphy

 

[Astronuc]

One electron (one angular momentum), two spin states.