# The affect of effective nuclear charge on energy gap between subshells

A few days ago my teacher taught me about $\mathrm{d}$ orbital contraction. He said that in $\ce{SF6}$ the hybridization of sulphur is $\mathrm{sp^3d^2}$. He said that although the $\mathrm{d}$ orbital is higher in energy than $\mathrm{s}$ and $\mathrm{p}$ orbitals due to increase in positive charge on sulphur, due to high electronegativity of flourine the $\mathrm{d}$ orbitals contract more than $\mathrm{s}$ and $\mathrm{p}$ orbitals, hence the difference in energy decreases and $\mathrm{d}$ orbital can participate in hybridization.

There was no problem with the explanation as it seemed perfectly plausible. But today, while teaching MOT, he told that the gap between $\mathrm{2s}$ and $\mathrm{2p}$ orbitals increases as we move from lithium to flourine due to increase in effective nuclear charge and interelectronic repulsion, which is quite opposite to what he told some days ago.

What am I missing? Which of the following interpretation is correct? A qualitative explanation involving less of quantum mechanics is that I want and involving MOT and VBT only because I know only those theories. According to advanced theories d orbitals will not participate in bonding in SF6 but according to VBT they will. As I already mentioned that I know only VBT and MOT so I have asked the question in the context of these two theories.