Science
Vsepr.
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 · 5 phases
Step by step.
- Draw the Lewis structure of the molecule to identify bonding pairs and lone pairs around the central atom.
- Count the total number of electron groups (bonding pairs + lone pairs) around the central atom.
- Determine the electron geometry based on the number of electron groups (2 = linear, 3 = trigonal planar, 4 = tetrahedral, 5 = trigonal bipyramidal, 6 = octahedral).
- Determine the molecular geometry by considering only the positions of the atoms (lone pairs affect shape but are not visible in molecular geometry).
- Predict bond angles based on the geometry, noting that lone pairs compress bond angles slightly.
Examples
Real-world.
- 1 Methane (CH₄) has 4 bonding pairs and 0 lone pairs → tetrahedral shape, 109.5° bond angles
- 2 Water (H₂O) has 2 bonding pairs and 2 lone pairs → bent shape, ~104.5° bond angle
- 3 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°).