Electronic configuration for ions

[Log]

When we try to predict electronic configurations by the building up principle we add electrons to the d-orbitals before the p-orbitals for principal quantum numbers n ≥ 4.

What I don't understand is why, according to my textbook, we're supposed to remove electrons from np-orbitals first, then ns-orbitals and then (n-1)d-orbitals when predicting the configuration for cations.

Why do we add electrons to (n-1)d-orbitals after ns-orbitals but remove electrons from ns-orbitals before (n-1)d-orbitals? - makes no sense.

Furthermore, if someone could explain or point me somewhere to an explanation as to how the energy levels are calculated (why E((n-1)d) > E(ns) for n ≥ 4) that would be great.

 

[DrDu]

Because it makes a difference whether you are comparing neutral atoms or an atom and an ion.
On ionization the ordering of the orbitals becomes more hydrogen like, i.e. the splitting of the s, p, and d orbitals decreases.

 

[Log]

I see, thanks - for the answer to my other question as well!

I'm guessing this is beyond undergraduate courses?