CH 2 6TH EDITION 2.81

[Jeremiah Hutauruk]

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.

[Matthew D 3F]

This is because of the amount of electrons in the p-orbitals of oxygen. There are 4 of them, and it is the first element where the p electrons have to be paired. This causes some repulsion energy between the electrons, so the ionization energy becomes lower because it takes less work to remove an electron from oxygen.

[Mona Lee 4L]

When it comes to these types of questions, always write out the electron configuration then look where the electrons are and if they're any different than the ones that follow the expected behavior.