CHEMISTRY COMMUNITY

The question states: Ionization energies usually increase on going from left to right across the periodic table. The ionization energy for oxygen, however, is lower than that of either nitrogen or fluorine. Explain this anomaly.

I understand why oxygen has a lower ionization energy than fluorine, but why does oxygen have a lower ionization energy than nitrogen?
I'd really appreciate the help.

The reason for this anomaly is because oxygen is the first element in its sub-shell to have a paired electron in one of its orbitals. Thus, there is added repulsion between those two electrons. Therefore, Oxygen wants to lose that electron so it can be even in all of the orbitals which is lower energy. That is why its ionization energy is lower. It wants to be "even" while nitrogen is already "even" (aka has one electron in each orbital).

Nitrogen also has a half-filled sub shell, which means it has lower energy and is more stable. Since it would be more difficult to take away an electron from a more stable subshell (aka higher ionization energy), Oxygen has the lower ionization energy.