# A question on energy levels of molecular orbitals

Here is a phenomenon from MO theory for diatomic homonuclear molecules that I could not understand.

If $Z \le 7$ then $\sigma_{2pz}$'s energy is higher than $\pi_{2px}$'s energy.

I did not get the idea behind this, the difference can be seen by looking at MO diagrams of $N_2$ and $O_2$: Could you help out?

I encountered this in my CHEM101 class, so I need some intuition. Please keep it basic :)

• The determination of the energy levels of MOs is largely empirical, as exact solutions of the Schrodinger Equation for these MOs aren't available. As far as I know there is no ab initio explanation available for these energy levels. – Gert Dec 26 '15 at 4:03
• I see, but maybe there is someone who can explain. Still waiting. – frosh Dec 26 '15 at 19:19
• files.rushim.ru/books/neorganika/housecroft.pdf Download that *.pdf and look at p.34-35 and figure 1.23 for the so-called '$\sigma$-$\pi$ swap'. – Gert Dec 26 '15 at 19:37
• Nice! Write it as answer so that I can finish the discussion :) By the way, do undergrads take this inorganic chemistry class in their second year? – frosh Dec 26 '15 at 20:55
• As an answer it's likely to get down-votes because it's only quoting from a link. Yes, it's second year material (or at least it was in my course). – Gert Dec 26 '15 at 23:58