Question about the bond MO's for diatomic early 2nd period elements

[Hammad Shahid]

So this question is about the bond molecular orbital diagrams for elements in the 2nd period, before O.

When the diatomic bond forms, the two pi 2p* orbitals are shown lower in energy than the sigma 2p*, uptil we get to O2.
I want to understand why this is?
From what I know, a sigma bond resulting resulting from constructive interference of an end-to-end overlap of a p-orbital forms a stronger bond than the side-by-side overlaps of the pi bond.

So why is it that for certain elements, the pi bonds are formed before the sigma bond? Shouldn't the sigma bond always be lower in energy?

Quick responses would be appreciated. I have a final tomorrow.

Thanks.

 

[DrClaude]

The energy of the π vs the σ orbital depends on the overlap between the corresponding atomic orbitals, as well as the energy of the s and p atomic orbitals making up the MO.

If you look at homonuclear diatomic molecules from atoms in the second period, it is actually more common to have $E(\pi) < E(\sigma)$ and it is only when you reach O that there is a reversal. This is consistent with the fact that the gap between the 2s and 2p orbitals increases as you go up in atomic number in the period.