How does VSEPR theory predict molecular shapes?

Valence-shell electron-pair repulsion (VSEPR) theory is a qualitative model used to predict the geometric arrangement of atoms in a molecule by focusing on the behavior of electron pairs around a central atom.12

How VSEPR Predicts Molecular Shape

The theory operates on the principle that all electron pairs—both bonding pairs and lone pairs—repel one another. To minimize these electrostatic repulsions, electron pairs adopt positions that maximize their mutual separation around the central atom. The prediction process generally follows these steps:12

• Identify Electron Pairs: Determine the total number of electron pairs (bonding and lone pairs) surrounding the central atom.2
• Determine Electron Geometry: Arrange these pairs in a configuration that maximizes their separation (e.g., four pairs adopt a tetrahedral disposition).2
• Define Molecular Shape: The final shape of the molecule is determined by the spatial arrangement of the atoms themselves, rather than the electron pairs.2

Accounting for Variations

VSEPR accounts for variations in bond angles by assuming that repulsion strengths differ: lone pair–lone pair repulsions are greatest, followed by lone pair–bonding pair, with bonding pair–bonding pair repulsions being the weakest. This explains why molecules like water (H₂O) and ammonia (NH₃) exhibit bond angles slightly smaller than the ideal tetrahedral angle of 109.5°.2

While VSEPR is a powerful rule of thumb, it has limitations: it does not predict bond lengths, nor does it quantify the exact magnitude of bond angle decreases.3