MO Z<8 vs Z>/=8

[Gabrielle Schwab 1G]

Could someone please explain why diatomic molecules with Z greater than or equal to 8 have p sigma bonds that are lower in energy than their p pi bonds but it is the opposite for molecules with Z less than 8.

[Chem_Mod]

I will cover this in my review session today (Friday).

[Shaye Busse 3B]

The reason for this has to due with the electron repulsion between the s sigma * orbital and the next highest orbital. In molecules with an atom that is Z<8, the molecular orbital structure that accounts for the lowest possible energy state has the p pi orbitals in a lower position than the p sigma orbital. The opposite can be said about molecules that are strictly z>/=8 as the molecular orbital structure that accounts for the lowest possible energy state places the p sigma orbital lower than the p pi orbitals. The electron repulsion in molecules with an atom Z<8 between the s sigma * orbital and the p sigma orbital is great enough to where the p sigma and p pi orbitals swap positions along the molecular orbital structure. The resulting molecular orbital structure with p pi orbitals in a lower energy state than the p sigma energy state creates a molecule in an overall lower energy state.