Question

How and why does hybridization occur? For example, why does `NH_3` form an sp3 orbital?

Answer 1

Hybridization is a theory that takes a "holistic" view of the bonds in a molecule. Se the explanation portion of this answer.

Explanation:

Hybridization is a theory which starts from the consideration that when atoms combine to form a molecule, the orbitals combine to create an entirely new and different molecular orbital that does not resemble the original atomic orbitals.

This new hybrid molecular orbital "belongs" to the molecule as a whole, but its geometry is determined by the types of atomic orbitals in the central atom that were involved in the bonding of the molecule.

Thus is the case of carbon, the atom uses its 2`s` and all three 2`p` orbitals to create four identical bonding orbitals. The unique tetrahedral arrangement of these orbitals is known as `sp^3` hybridization.

By the way, `NH_3` results in `p^3` hybridization, as the nitrogen molecule uses only its three `2p` orbitals in bonding with three H atoms, and not the (full) `s` orbital.

I hope this tells you what you wanted to know.