Question

How do you determine which orbitals are used in hybridization?

Answer 1

Hybridization is a method of expressing the geometry and shape of any chemical compound. Hybridization is generally written for polyatomic molecules which contain more than one atom. A promotion concept is generally used to explain the hybridization.

Complete answer:
There are some basic rules which should be followed to write the hybridization of any compound.
The atom which is present in minimum number is considered as the central atom.
Write the electronic configuration of the elements to determine the valence electron present in atom.
Maintain the vacant orbital by promoting the paired electron to the higher orbital to generate the vacant orbital required for bond formation.
The type of orbital which is used during bond formation is considered as hybridization of that molecule.
For example- in the molecule of ammonia $N{H_3}$, Nitrogen atom is considered as central atom which have electronic configuration $1{s^2}2{s^2}2{p_x}^12{p_y}^12{p_z}^1$. From the electronic configuration we see that nitrogen has three unpaired electrons in its $\left( {p - } \right)$ orbitals. Molecules of $N{H_3}$ contain only three hydrogen atoms so nitrogen is not required to increase its valency by promotion. Now all the four above orbitals combine with each other to form four hybrid orbitals named as $s{p^3}$.
These hybrid orbitals combine with three hydrogen atoms and one orbital contains a lone pair of electrons which remain located to the central nitrogen atom.
Hence, we can see that hybridization of molecules is based on the orbital which undergoes the combination to form hybrid orbitals of equivalent energy status.

Note:
Valence orbits are only allowed to undergo the hybridization process. The orbitals which have small differences in their energy state easily undergo hybridization. The process of promotion to generate a vacant orbital is not mandatory in hybridization.