[edwinksl]

Let's say we have a heteronuclear diatomic molecule (A-B),and let's talk about the σ and σ* MOs formed from the 2s orbitals of A and B. Assume A is more electronegative than B, so the energy of A's 2s AO is lower than B's 2s AO. We then have a certain set of σ and σ* MOs formed. If I change A to an even more electronegative atom, C (so A's 2s Ao is replaced C's 2s AO which has a lower energy), we will have a new set of MOs formed.

I am wondering if both the energies of the new σ and σ* MOs are lower as compared to the initial σ and σ* MOs respectively. In other words, when we reduce the energy of a AO of an atom by simply using a more electronegative atom (while keeping the other atom unchanged), do BOTH the bonding and antibonding orbitals reduce in energy? I am quite sure the antibonding orbital does reduce in energy, but I am not unsure about the bonding orbital though.

Thanks.

[Yggdrasil]

I definitely agree with you that the energy of the antibonding orbital will decrease, but I too am unsure about the bonding orbital.  Since you are increasing the difference in energy between the atomic orbitals, you are getting less stabilization from bonding, so the energy of the bonding orbital will be closer to the energy of the 2s orbital of C.  But, the energy of the 2s orbital of C is lower than the 2s orbital of A.  So, it depends on the magnitudes of each type of change.