Britannica Sources used in this answer

How do lone pairs of electrons affect the shape of a molecule?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram showing four types of crystal bonding: ionic bonding with sodium and chlorine ions, covalent bonding with carbon atoms sharing electrons, metallic bonding with iron ions surrounded by free electrons, and molecular bonding with hydrogen molecules held by weak electrical attraction. — chemical bonding Different types of bonding in crystals.

According to valence-shell electron-pair repulsion (VSEPR) theory, lone pairs of electrons significantly influence the shape of a molecule by exerting repulsive forces on other electron pairs.

The theory posits that all electron pairs—both bonding pairs and lone pairs—repel one another, and molecules adopt shapes that minimize these repulsions. Lone pairs affect molecular geometry in the following ways:

Volume Occupancy: Lone pairs are attached to only one central atom, causing them to spread over a greater volume than bonding pairs, which are constrained between two nuclei. Consequently, lone pairs exert stronger repulsive forces than bonding pairs.1
Repulsion Hierarchy: The strength of electron pair repulsions follows this order: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair.1
Bond Angle Distortion: Because lone pairs repel bonding pairs more strongly than bonding pairs repel each other, they push bonding pairs closer together. This results in bond angles that are smaller than those found in ideal geometric arrangements (e.g., the bond angle in water is 104° rather than the tetrahedral 109.5°).1