VSEPR Theory
Valence‑shell electron‑pair repulsion (VSEPR) theory predicts molecular geometry by minimising repulsion between electron pairs.
What is VSEPR Theory?
Valence‑shell electron‑pair repulsion (VSEPR) theory is a model used to predict the three‑dimensional arrangement of atoms in a molecule. The central idea is that electron pairs (both bonding pairs and lone pairs) repel each other and will arrange themselves to be as far apart as possible.
Electron Groups
An electron group can be:
- A single bond (σ bond)
- A double bond (counts as one group)
- A triple bond (counts as one group)
- A lone pair
Geometry Table
| Electron Groups | Lone Pairs | Geometry | Bond Angle | Example |
|---|---|---|---|---|
| 2 | 0 | Linear | 180° | CO₂ |
| 3 | 0 | Trigonal planar | 120° | BH₃ |
| 3 | 1 | Bent | < 120° | SO₂ |
| 4 | 0 | Tetrahedral | 109.5° | CH₄ |
| 4 | 1 | Trigonal pyramidal | < 109.5° | NH₃ |
| 4 | 2 | Bent | < 109.5° | H₂O |
| 5 | 0 | Trigonal bipyramidal | 90°, 120° | PCl₅ |
| 5 | 1 | See‑saw | 90°, < 120° | SF₄ |
| 5 | 2 | T‑shaped | 90° | ClF₃ |
| 5 | 3 | Linear | 180° | XeF₂ |
| 6 | 0 | Octahedral | 90° | SF₆ |
| 6 | 1 | Square pyramidal | 90° | IF₅ |
| 6 | 2 | Square planar | 90° | XeF₄ |
Lone‑Pair Repulsion
Lone pairs occupy more space than bonding pairs, so they exert greater repulsion:
Lone–lone > lone–bonding > bonding–bonding
This is why NH₃ (107°) and H₂O (104.5°) have bond angles smaller than the ideal tetrahedral 109.5°.
Related molecules: Water, Ammonia, Methane, Carbon Dioxide
Try the tool: Geometry Predictor