What is the steric number of the central atom in the following species ? a) Carbon dioxide, CO2 b) Ammonia, NH3 c) Ammonium ion d) Formaldehyde

The steric number is used to express how many regions of electron density surround a given atom.

In other words, the steric number represents the number of lone pairs present on, and the number of atoms bonded to the central atom of a molecule.

Part a)

Carbon is the central atom of the carbon dioxide molecule. The molecule's Lewis structure looks like this

As you can see, the central atom is bonded to two oxygen atoms, and no lone pairs are present on it, which means that its steric number will b equal to 2.

Part b)

Nitrogen is the central atom of the ammonia molecule. As you can see from its Lewis structure

nitrogen is bonded to three hydrogen atoms and has one lone pair of electrons on it, which implies that its steric number will be equal to

#SN_("nitrogen") = 3 + 1 = 4#

Part c)

This time, the nitrogen atom is bonded to four hydrogen atoms, but because that lone pair of electrons is no longer present, its steric number will also be equal to 4.

Part d)

The carbon atom is attached to two hydrogen atoms through single bonds, and to an oxygen atom through a double bond. Since no lone pairs are present, its steric number will be equal to 3.