Ionization Energy 2.81

[Semi Yoon]

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. How do you explain this anomaly?

[Albert Duong 4C]

Oxygen has 4 electrons in the 2p-orbital which is essentially an extra electron added to an already half full orbital, which results in electron electron repulsion (lowers the ionization energy). Nitrogen on the other hand, has a more stable orbital a half full 2p-orbital, so its ionization energy is greater. I believe that fluorine has a higher ionization energy than nitrogen because at that number of valence electrons, the effective nuclear charge ("pull" or attraction on electrons by the protons in the nucleus) felt by the valence electrons is greater than nitrogen. Hopes this helps!