What is the difference between the MO diagrams for Nitrogen and Oxygen?

Dear student,

Dinitrogen

Dinitrogen, N2, is well known, it has a bond order of three, and it has HOMO + LUMO FMOs:

Dioxygen

Before we can consider two oxygen atoms reacting to give dioxygen, O2, it should be noted that there is a slight reordering of the MO energy levels.

The σ 2px bonding MO is lower in energy than the π 2py and π 2pz MOs. And, correspondingly, the σ* 2px MO rises in energy above the π* 2py and π* 2pz MOs.

The result is that dioxygen, O2, is a diradical.

Oxygen, O2, is a well known, stable, diradical, bond order 2.