VSEPR Theory
Predict 3D molecular shapes from electron pairs
Core Principle
Electron pairs repel to maximize distance
Regions of electron density (bonding + lone pairs) arrange themselves to minimize repulsion
Geometries by Electron Groups
2 groups → Linear (180°)
Example: CO₂, BeCl₂
3 groups → Trigonal planar (120°)
Example: BF₃, NO₃⁻
4 groups → Tetrahedral (109.5°)
Example: CH₄, NH₄⁺
5 groups → Trigonal bipyramidal
Example: PCl₅ (90°, 120°, 180°)
6 groups → Octahedral (90°, 180°)
Example: SF₆
Example: H₂O
Central atom O: 2 bonding pairs (to H) + 2 lone pairs = 4 electron groups
Electron geometry: Tetrahedral
Molecular geometry: Bent (only atoms counted)
Bond angle: ~104.5° (compressed from 109.5° by lone pairs)
Lone pairs repel more than bonding pairs → smaller bond angles
Key Rules
- Multiple bonds count as ONE electron region
- Lone pairs occupy more space: LP-LP > LP-BP > BP-BP repulsion
- Molecular shape describes only atom positions, not lone pairs