By Lee Johnson – Updated Aug 30, 2022

Why Steric Numbers Matter

The geometry of a molecule is governed by the repulsion between electron pairs around its central atom. The steric number—the sum of bonded atoms and lone electron pairs—provides a quick, reliable way to predict that shape.

What Is a Steric Number?

It is the total count of:

• Atoms directly bonded to the central atom (each bond, regardless of single, double, or triple, counts as one)
• Lone‑pair electron groups on that atom

These electron groups arrange themselves to minimize repulsion, giving rise to the familiar VSEPR shapes:

Deriving the Steric Number from a Lewis Structure

1. Draw the Lewis structure. Place the central atom and connect it to the surrounding atoms with the correct number of bonds, using the molecular formula as a guide.

2. Count bonds. Each bond to the central atom adds one to the total, even if it is a double or triple bond.

3. Identify lone pairs. Any dots or electron pairs not involved in bonding around the central atom contribute one per pair.

4. Apply the formula.

Steric number = (number of bonded atoms) + (number of lone pairs)

Example: Water (H₂O)

The Lewis structure shows oxygen bonded to two hydrogens. Oxygen has six valence electrons; two are used in the O–H bonds, leaving four as two lone pairs.

Calculating:

steric number = 2 (bonds) + 2 (lone pairs) = 4

A steric number of four predicts a tetrahedral electron‑pair arrangement. In water, two of those positions are occupied by lone pairs, giving the observed bent molecular shape.

Key Takeaways

• Double and triple bonds count as one when determining the steric number.
• Only lone pairs on the central atom are considered—lone pairs on peripheral atoms do not affect the count.
• Once you can draw a Lewis structure, the steric number is almost instantaneous to compute.