Energy diagram for molecular orbitals are switched in the case of number of electrons greater than 14 with respect to the case of number of electrons less than and equal to 14.
For number of electrons greater than 14 order of energy for molecular orbitals :
For number of electrons less than or equal to 14 order of energy for molecular orbitals :
As we can see clearly that in both above cases everything is same except the position of Sigma 2pz orbitals. Energy of Sigma 2pz orbital less than pie 2px and pie 2py for O2 and F2 and energy of Sigma 2pz orbital is greater than pie 2px and pie 2py in case of N2, C2, B2 ( total electrons less than 14 )
This switching in the order of molecular orbitals is because of S- P mixing.
S- P mixing is present in case of molecules having total electron less than or equal to 14 e.g. N2, B2, C2 etc
because of S-P mixing stability of Sigma 2s orbitals increases and that of Sigma 2pz decrease and hence energy of Sigma 2pz increases
s-p mixing is not present in molecule like O2 & F2 so energy of Sigma 2pz orbitals is less than pie 2px and pie 2py
What is S- P mixing ?
In S-P mixing Sigma S and Sigma Pz orbitals interact with each other when they have similar energy and similar symmetry.
Because of this interaction or you can say the wave form of Sigma 2s and 2pz combine mathematically which changes the energies of both Sigma s and Sigma Pz orbitals. By this combination no more new molecular orbitals are produced rather energy of Sigma 2pz orbitals increases and that of Sigma 2s decrease.
This s-p mixing is not present in the case of O2, F2 & Ne2 because their p orbitals have one or more paired orbital, this pairing of electrons in orbitals increases the energy of p orbitals which makes the energies of Sigma s and Sigma p orbitals dissimilar so s-p mixing is not possible but in case of molecules like N2, C2 , B2 there is no paired p orbital is present so energies of orbitals are similar and s-p mixing occurs.
In above diagram position of energies of different orbitals of Li2, Be2, B2, C2, N2, O2 & F2 are given.
We can clearly see in the diagram that energies of orbitals are decreasing on moving towards right, this is happening because of increasing effective nuclear charge which is attracting orbitals towards nucleus. This decrease in energy is more for Sigma 2s orbitals than Sigma 2pz orbitals because s orbitals is more close to nucleus.
This decrease in energy increase the gap between s and p orbitals which is making s-p mixing ineffective for O2, F2 and Ne2
So reason of switch in electronic configuration of molecules having total electron less than or equal 14 and greater 14 is S-P mixing.
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