Why is nucleus coupling neglected to explain electronic states but LS coupling only rises because of?

Here, with LS coupling I mean coupling between total orbital momenta and S total spin momenta and treating eletrons in light atoms with Russell-Saunders approximation. I found a bit paradoxical that LS coupling happens because of a positive nucleus charge Ze - and its dependence follows Z^4 - but the coupling between the spin momentum of the nucleus and the eletrons' is alltogether neglected in at least this approximation. My reference is Hollas, Modern Spectroscopy, 4th Edition, pg. 208, cap. 7 (Electronic Spectroscopy). That is, why is nucleus spin momentum weak?


Also, what are the evidences that support the efficiency in Russell-Saunders approximation for lighter atoms (ll coupling strong, ss appreciable, j weak) and jj- coupling for "a few states of heavy atoms" (j strong, jj appreciable, ll and ss weak)?
It is curious that the extent of the coupling j apparently inverts from light to heavy atoms, given these 2 approximations.


