# What is the steric number for the "O" atom in water?

Jun 29, 2016

$4$

#### Explanation:

An atom's steric number tells you how many regions of electron density surround the atom in a given molecule.

A region of electron density is simply

• a single, double, or triple bond $\to$ all three count as one region of electron dnesity
• a lone pair of electrons

This means that in order to find an atom's steric number, all you have to do is see how many lone pairs of electrons it has and to count the number of bonds it forms with other atoms.

A good starting point here will be to draw the Lewis structure of water, $\text{H"_2"O}$.

Each water molecule contains $2$ atoms of hydrogen and $1$ atom of oxygen. The molecule will have a total of $8$ valence electrons

• one from each of the two hydrogen atoms
• six from oxygen

The oxygen atom will be the central atom. It will form two single bonds, one with each hydrogen atom, that account for $4$ valence electrons, and has $2$ lone pairs of electrons that account for the other $4$ valence electrons.

As you can see, the oxygen atom is surrounded by $4$ regions of electron density, $2$ single bonds and $2$ lone pairs of electrons.

As a result, the atom's steric number will be equal to $4$.