Molecular orbitals?

Question

Is sigma 2p filled first or pi 2p?

Answer 1

It depends.

Up to N2, pi 2p first. From O2 on, sigma 2p first.

Why? Because sigma 2p is raised by interaction with sigma 2s, but separation of s and p increases with atomic number, reducing this interaction.

Einstein is supposed to have said that every scientific problem should be made as simple as possible, and no simpler.