Question

I dont understand why for N2, pi4(2px2py) goes first, but for O2 and F2, sigma2(2pz) goes first.

Sigma^2_g (1s) Sigma^*2_mu (1s) Sigma^2_g(2s) Sigma^*2_u (2s) (same for all diatomic molecules) pi^4_mu (2p_x 2p_y) Sigma^2_g(2p_z) pi^*4_mu (2p _x 2P _y) Sigma^*2_g(2p_z) Sigma^2_g (2p_z) pi^4_mu (2p_x 2p_y) pi^*4_mu (2p_x 2p_y) Sigma^*2_g (2p _z)

Answer 1

This above phenomenon can be explained by s-p mixing. all the elements in the second period before oxygen have the differance in energy between the 2s and 2p orbitals is small enough, so that the s-p mixing can occur ..

By moving towards the right in a period, the s-orbital gets more stabilized than the p-orbital and the differance in their energies increases making the s-p mixing for oxygen much smaller ...