VSEPR Molecular Geometry

You’ll be able to

Count electron domains around a central atom to find its geometry
Distinguish electron-domain geometry from molecular (atom) geometry
Predict bond angles and how lone pairs distort them

Electron pairs push each other as far apart as possible

VSEPR (Valence Shell Electron Pair Repulsion) rests on one idea: negatively charged electron domains around a central atom repel and spread out to maximize their separation. A domain is any single bond, double bond, triple bond, or lone pair — a multiple bond still counts as just one domain. Count the domains and the geometry is set.

Electron-domain geometry by domain count

2 → linear (180°) · 3 → trigonal planar (120°) · 4 → tetrahedral (109.5°)

This is the arrangement of ALL domains. The shape named by the atoms (molecular geometry) can differ when some domains are lone pairs.

Lone pairs change the shape you name

You name the molecular geometry from the positions of the atoms only, but the lone pairs are still there, pushing. Starting from a tetrahedral electron arrangement: 0 lone pairs → tetrahedral (CH₄), 1 lone pair → trigonal pyramidal (NH₃), 2 lone pairs → bent (H₂O). Starting from trigonal planar: 1 lone pair → bent (SO₂). Same domain count, different named shapes depending on how many domains are invisible lone pairs.

Lone pairs squeeze bond angles

A lone pair is held by only one nucleus, so its cloud is fatter and pushes harder than a bonding pair. Each lone pair therefore compresses the remaining bond angles below the ideal. From the perfect tetrahedral 109.5°, methane (0 lone pairs) stays at 109.5°, ammonia (1 lone pair) drops to about 107°, and water (2 lone pairs) drops further to about 104.5°.

Worked example

Determine the electron-domain geometry, molecular geometry, and approximate bond angle of water, H₂O.

1.Draw the Lewis structure: O is central, bonded to two H atoms, with two lone pairs remaining on O.
2.Count domains on O: 2 bonding pairs + 2 lone pairs = 4 electron domains.
3.Four domains give a tetrahedral electron-domain geometry (109.5° ideal).
4.Only 2 of the 4 domains are atoms, so the molecular geometry — named from atoms — is bent.
5.The two lone pairs press the H–O–H angle inward from 109.5° to roughly 104.5°.

Answer: Tetrahedral electron-domain geometry, bent molecular geometry, bond angle ≈ 104.5°.

Tip

Always draw the Lewis structure first — VSEPR is impossible without knowing the lone pairs. The number that fixes the geometry is the total electron domains; the number of those that are lone pairs then tells you which named shape and how squeezed the angle is.

Checkpoint

Ammonia (NH₃) has three N–H bonds and one lone pair on nitrogen. What is its molecular geometry?

Checkpoint

What is the molecular geometry and bond angle of carbon dioxide, O=C=O?

Checkpoint

The sulfur atom in SO₂ has two S–O bonds and one lone pair. Compared with the ideal 120° of its trigonal-planar electron arrangement, the actual O–S–O bond angle is:

On the exam

Keep two questions separate on the exam: "How many total domains?" fixes the electron-domain geometry and ideal angle; "How many of those are lone pairs?" fixes the named molecular shape and how far the real angle is squeezed below ideal.

Answer the 3 checkpoints as you read.