Energy Level differences depending on Z

[Swati_Sharma_2D]

Can someone explain why the pi 2p energy level has less energy than the sigma 2p orbital when Z <8 and why the pi 2p energy level has more energy when Z>8? I understand that that is the case, but I don't understand why.

[Caitlin Dillon 3G]

the course reader explains this on page 110.
Because the nuclear charge is less for atoms z<8, the energy difference between filled s-orbitals and p-orbitals is less.
The electron density from filled sigma(2s) and sigma*(2s) orbitals is concentrated along the bond axis.
Since the sigma(2Pz) orbitals is also directed along bond axis there is electron-electron repulsion resulting in a higher energy sigma(2Pz) orbital.
The sigma(2Pz) orbital is therefore above the pi(2Px) and pi(2Py) orbitals. (Remember electron density in a pi-orbital is concentrated away from the bond axis)