Shapes of Molecules: VSEPR Theory

Predicting the three-dimensional shapes of molecules is a fundamental skill in A-Level Chemistry. The theory that allows us to do this is called VSEPR — Valence Shell Electron Pair Repulsion theory. It is based on a single, elegant principle: electron pairs around a central atom arrange themselves as far apart as possible to minimise repulsion.

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The Core Principle

All electron pairs in the valence shell of the central atom — whether they are bonding pairs (shared with another atom) or lone pairs (not shared) — repel each other because they are all regions of negative charge. The shape adopted by the molecule is the one that places these electron pairs as far apart as possible.

However, there is a crucial hierarchy of repulsion strengths:

Lone pair–lone pair repulsion > Lone pair–bonding pair repulsion > Bonding pair–bonding pair repulsion

Lone pairs occupy more space than bonding pairs because they are held closer to the central atom (they are not being "pulled" towards another nucleus). This means lone pairs squash the bonding pairs closer together, reducing bond angles.

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Shapes with No Lone Pairs