Molecular Orbital Treatment model

Question

how can we tell while writing electronic configurations according to MOT model that p-s mixing has occured or not.?

Answer 1

It has to do with the relative energies of the s and p orbitals.

moving across the periodic table causes p-orbitals to drop in energy faster than s-orbitals... in other words.. the energy difference between O's p and s orbitals is smaller than the difference between N's p and s energies.  the smaller gap in O's energies causes the p's and s's to mix.  this causes the sigma orbitals to mix which lowers their energy.  so in O2 the sigma orbital created by the pz orbitals is lower than the two pi-orbitals made by the px's and py's.. so in N2 the pi's are lower in energy than the sigmas and the opposite is true in O2.