In my atomic theory lecture notes it always says the central field approximation is 'excellent' for alkali atoms due to the spherical symmetry of the core electrons and the $s$-shell of the valence electron. It also says therefore that the perturbation due to the residual electric field is 'essentially 0'.
My question is, why are they using the words 'excellent' and 'essentially' when it looks like in this case the central field should be perfect. What effects are they assuming that break this?
