Question

What is the steric number of `NH_3`?

Answer 1

`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, `"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 `"sp"^3` hybridized.