# What is the steric number of NH_3?

Jul 28, 2016

$4$

#### Explanation:

For a given molecule, the steric number is calculated by counting how many regions of electron density surround the central atom.

A region of electron density can be

• a single, double, or triple bond -- all three count as one region of electron density
• a lone pair of electrons

In order to calculate the steric number of nitrogen, which is the central atom in ammonia, ${\text{NH}}_{3}$, you must first draw the molecule's Lewis structure.

The molecule will have a total of $8$ valence electrons, $5$ coming from nitrogen and $1$ coming from each of the three hydrogen atoms.

The nitrogen atom will form single bonds with the three hydrogen atoms, with each of these bonds accounting for $2$ valence electrons. The remaining $2$ valence electrons will be placed on the nitrogen atom as a lone pair of electrons.

The Lewis structure for ammonia will thus looks like this

Notice that the central atom is surrounded by a total of $4$ regions of electron density

• three single bonds
• one lone pair of electrons

You can thus say that the nitrogen atom has a steric number equal to $4$. The steric number tells you the hybridization of the central atom.

In this case, a steric number equal to $4$ implies that the nitrogen atom is ${\text{sp}}^{3}$ hybridized.