Vsepr

Science

Definition

VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the three-dimensional shape of a molecule based on the idea that electron pairs around a central atom repel each other and arrange themselves as far apart as possible to minimize repulsion.

How It Works

  1. Draw the Lewis structure of the molecule to identify bonding pairs and lone pairs around the central atom.
  2. Count the total number of electron groups (bonding pairs + lone pairs) around the central atom.
  3. Determine the electron geometry based on the number of electron groups (2 = linear, 3 = trigonal planar, 4 = tetrahedral, 5 = trigonal bipyramidal, 6 = octahedral).
  4. Determine the molecular geometry by considering only the positions of the atoms (lone pairs affect shape but are not visible in molecular geometry).
  5. Predict bond angles based on the geometry, noting that lone pairs compress bond angles slightly.

Examples

  • Methane (CH₄) has 4 bonding pairs and 0 lone pairs → tetrahedral shape, 109.5° bond angles
  • Water (H₂O) has 2 bonding pairs and 2 lone pairs → bent shape, ~104.5° bond angle
  • Ammonia (NH₃) has 3 bonding pairs and 1 lone pair → trigonal pyramidal, ~107° bond angles
Key Fact

Electron groups: 2→linear (180°), 3→trigonal planar (120°), 4→tetrahedral (109.5°).

Related Terms

Lewis structures Covalent bonds Dipole-dipole Electronegativity

Study This Concept

Practice VSEPR with free review games in these units:

AP Chemistry Unit 2: Molecular and Ionic Bonding