What is the steric number of #NH_3#?

1 Answer
Jul 28, 2016

Answer:

#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

http://chem.libretexts.org/Textbook_Maps/General_Chemistry_Textbook_Maps

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.