Explain why the first ionization energy of oxygen is slightly less than that of
Ionization energy generally increases across a period so oxygen should have a higher ionization energy than nitrogen but it doesn’t. If you look at the orbital diagrams for nitrogen and oxygen you will see that oxygen has one p orbital with two electrons and two other p orbitals with one electron each. The added repulsion from the two electrons in one orbital and the fact that there is some stability gained by having a half-filled sub-level, explains why it is easier to take an electron from oxygen than nitrogen. Nitrogen already has a half-filled sub-level.