Question

Why a p orbital is left unhybridised in sp2d hybridisation?

Answer 1

The main function of unhybridized orbitals is usually to form double bonds. Consider each of the following cases:

- sp3 hybridization has 4 sp3 hybridized orbitals and 0 unhybridized orbitals. These allow molecules to form 4 sigma bonds.
- sp2 hybridization has 3 sp2 hybridized orbitals and 1 unhybridized p orbital. This allows molecules to form 3 sigma and 1 pi bond (remember that a double bond has a 1 sigma AND 1 pi bond)
- sp hybridization has 2 sp hybridized orbitals and 2 unhybridized p porbitals. This allows molecules to from 2 sigma and 2 pi bonds (allowing 2 double bonds).

Based on this information, you can see that the total number of orbitals is conserved, with 4 total. This matches the fact that we begin with 1 s and 3 p orbitals, thus ensuring that the number of electrons remains the same as well.

To answer your final question, if hybridized orbitals "use up" all the electrons (as is the case with sp3 hybridization), there will indeed be no unhybridized orbitals. This is the same as saying there are 8 electrons available and no need to form double bonds (which require unhybridized p orbitals).