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During the hybridization of certain elements such as phosphorus during the formation of $\rm PCl_5$, the atom first gets changed to the excited state. In the case of $\rm PCl_5$, the paired electron in the 3s orbital gets promoted to 3d orbital. Similarly in $\rm SF_6$, the paired electrons in 3s and 3p orbitals get promoted to the 3d orbital. Why does this happen? Shouldn't the electrons get promoted to the 4s orbital as it is lower in energy than the 3d orbital, and also nearer to the ground state orbitals?

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The ground state of phosphorus is $\mathrm{[Ne] 3s^2 3p^3}$.

In order for $\mathrm{PCl_5}$ to exist with the molecular shape we know from experiment, a hybrisation occurs in $\mathrm{P}$'s valence electrons, that is:

$$\mathrm{sp^4}$$

These are five atomic orbitals, identical in all respects.

This explains the molecule's trigonal bipyridal shape, shown at the bottom of that (an answer of mine) link.

Shouldn't the electrons get promoted to the 4s orbital as it is lower in energy than the 3d orbital, and also nearer to the ground state orbitals?

The $\mathrm{d}$ orbital doesn't enter the equation, if we assume a set of five $\mathrm{sp^4}$ hybridised orbitals. The $\mathrm{sp^4}$ have energies in between the $\mathrm{s}$ and $\mathrm{p}$ energy levels.

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  • $\begingroup$ $sp^4$?........ $\endgroup$
    – ACB
    Commented Aug 12, 2021 at 8:06
  • $\begingroup$ @ACB Five atomic orbitals, each with one unpaired electron 'occupying' them. $5$ valence electrons, and $5$ $\mathrm{sp^4}$ hybridised electrons. $\endgroup$
    – Gert
    Commented Aug 12, 2021 at 8:12
  • $\begingroup$ hmm there is only 3 p orbitals so where did the extra p come from? $\endgroup$
    – Babu
    Commented Aug 12, 2021 at 8:13
  • $\begingroup$ @Buraian There are no 'extra' $p$: the valence electrons for $\mathrm{P}$ are $\mathrm{3s^2 3p^3}$. That's FIVE electrons. $\endgroup$
    – Gert
    Commented Aug 12, 2021 at 8:15
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    $\begingroup$ I know only $sp^3d$ after $sp^3$. $\endgroup$
    – ACB
    Commented Aug 12, 2021 at 8:20

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