VSEPR Theory Foundation
Electron domains (bonding pairs, lone pairs, double bonds, triple bonds) repel each other and arrange themselves to maximise the distance between them, minimising electrostatic repulsion.
Each single bond, double bond, triple bond, or lone pair counts as one electron domain.
Repulsion Hierarchy
Electron Domain Repulsion Hierarchy
LP-LP > LP-BP > BP-BP
Lone pairs spread out more broadly (attracted to only one nucleus) and compress bond angles below their ideal values.
VSEPR Base Geometries (Electron Domains)
SL Geometries (2-4 Domains)
| Domains | BP | LP | Electron Geometry | Molecular Geometry | Bond Angle | Example |
|---|---|---|---|---|---|---|
| 2 | 2 | 0 | Linear | Linear | 180° | CO₂, BeCl₂ |
| 3 | 3 | 0 | Trigonal planar | Trigonal planar | 120° | BF₃, NO₃⁻ |
| 3 | 2 | 1 | Trigonal planar | Bent / V-shaped | ~118° | SO₂, O₃ |
| 4 | 4 | 0 | Tetrahedral | Tetrahedral | 109.5° | CH₄ |
| 4 | 3 | 1 | Tetrahedral | Trigonal pyramidal | ~107° | NH₃ |
| 4 | 2 | 2 | Tetrahedral | Bent / Angular | ~104.5° | H₂O |
Two Types of "Bent" Geometry (117° vs 104.5°)
A bent molecule from a trigonal planar base (e.g. SO₂) has a bond angle of ~118°. A bent molecule from a tetrahedral base (e.g. H₂O) has a bond angle of ~104.5°. These are frequently confused - pay attention to the number of electron domains!
Electron Domain vs Chemical Bond
A double or triple bond counts as ONE electron domain, not two or three. E.g. CO₂ has 2 electron domains (2 double bonds), not 4. Also, do not confuse electron domain geometry (includes lone pairs) with molecular geometry (atoms only). NH₃ is tetrahedral in electron domains but trigonal pyramidal in molecular shape.
Study this topic on the go
Get active recall flashcards, notes, and topic quizzes in ChemEasy, or build your revision schedule with ChemPlan IB.