Question

What is the steric number of $N{H_3}$?

Answer 1

Steric number is the number of atoms, groups, or lone pairs around the central atom in a molecule. It determines the molecular geometry. Find out the number of lone pairs and bonded atoms in $N{H_3}$ using Lewis structure and calculate the steric number using the formula:
Steric number $=$(number of lone pairs on central atom)$+$(number of atoms bonded to central atom)

Complete answer:
The steric number is defined as the number of atoms bonded to a central atom of a molecule plus the number of lone pairs attached to the central atom. The steric number is used in the VSEPR theory to determine the molecular geometry of a molecule.
The formula for calculating the steric number $=$(number of lone pairs on central atom)$+$(number of atoms bonded to the central atom)
Drawing Lewis's structure is important to calculate the steric number.
First we will draw the Lewis structure of $N{H_3}$ as follows:

In the structure of ammonia, we can see that the nitrogen atom forms single bonds with three hydrogen atoms, with each of these bonds accounting for 2 valence electrons. The remaining 2 valence electrons are placed on the nitrogen atom as a lone pair of electrons.
Steric number $=$(number of lone pairs on central atom)$+$(number of atoms bonded to central atom)
(For ammonia number of lone pairs$= 1$ and number of bonded atoms $= 3$)
Steric number$= 1 + 3$
$\Rightarrow 4$
Therefore the steric number of ammonia is $4$

Note:
The steric number also tells the hybridization of the central atom. In the case of ammonia, the steric number is $4$ and this implies that the hybridization of nitrogen atoms is $s{p^3}$. It also determines the bonded atom lone pair arrangement, the shape that maximizes the distances between the valence-shell electron pairs.