Question #44238

It took a bit of reading, but I think I see the source of our misunderstandings.

The article says, "Electrons in an `s` orbital have a greater shielding power than electrons in a `p` or `d` orbital of that same shell … For example, consider an atom of carbon whose electron configuration is `1s^2 2s^2 2p^2`. The two electrons in the `1s` orbital of C will do a better job of shielding the two electrons in the `2p`orbitals than they will of shielding the two electrons in the `2s` orbital."

The article is really saying, "Electrons in an `s` orbital have a greater shielding power than electrons in a `p` or `d` orbital of that same shell for electrons in the next higher shell".

It isn't saying that a `2s` electron is more effective at shielding another `2p` electron than a `2s` electron.

It is saying that a `1s` electron is more effective at shielding a `2p` than a `2s` electron.

This explains the dip in ionization energy on going from Be to B. When you go from a `2s` orbital in Be to a `2p` orbital in B, the `1s` electrons are more effective at shielding the `2p` electron of B.