VSEPR Theory
Valence Shell Electron Pair Repulsion Theory
VSEPR Principle
Core Concept: Electron pairs around a central atom repel each other and arrange themselves to minimize repulsion, achieving maximum distance from each other.
1. Draw Lewis Structure
Determine total valence electrons and draw bonds
2. Count Electron Domains
• Bonding domains: Single, double, or triple bonds (each counts as ONE)
• Lone pairs: Non-bonding electron pairs
3. Determine Electron Geometry
Based on total number of electron domains
4. Determine Molecular Geometry
Based on bonding domains only (ignore lone pairs for shape name)
Electron Domain Geometries
2 Electron Domains
Electron Geometry: Linear
Bond Angle: 180°
Example: BeCl₂, CO₂
3 Electron Domains
Electron Geometry: Trigonal Planar
Bond Angle: 120°
Example: BF₃, SO₃
4 Electron Domains
Electron Geometry: Tetrahedral
Bond Angle: 109.5°
Example: CH₄, NH₄⁺
5 Electron Domains
Electron Geometry: Trigonal Bipyramidal
Bond Angles: 90°, 120°, 180°
Example: PCl₅, AsF₅
6 Electron Domains
Electron Geometry: Octahedral
Bond Angle: 90°, 180°
Example: SF₆, PF₆⁻
Molecular Geometries with Lone Pairs
| Domains | Bonding | Lone Pairs | Molecular Shape | Bond Angle | Example |
|---|---|---|---|---|---|
| 2 | 2 | 0 | Linear | 180° | CO₂ |
| 3 | 3 | 0 | Trigonal Planar | 120° | BF₃ |
| 3 | 2 | 1 | Bent | <120° | SO₂ |
| 4 | 4 | 0 | Tetrahedral | 109.5° | CH₄ |
| 4 | 3 | 1 | Trigonal Pyramidal | ~107° | NH₃ |
| 4 | 2 | 2 | Bent | ~104.5° | H₂O |
| 5 | 5 | 0 | Trigonal Bipyramidal | 90°, 120° | PCl₅ |
| 5 | 4 | 1 | See-Saw | <120°, <90° | SF₄ |
| 5 | 3 | 2 | T-Shaped | <90° | ClF₃ |
| 5 | 2 | 3 | Linear | 180° | XeF₂ |
| 6 | 6 | 0 | Octahedral | 90° | SF₆ |
| 6 | 5 | 1 | Square Pyramidal | <90° | BrF₅ |
| 6 | 4 | 2 | Square Planar | 90° | XeF₄ |
Bond Angle Modifications
Lone Pair Repulsion
Repulsion Strength: LP-LP > LP-BP > BP-BP
Lone pairs occupy more space than bonding pairs, compressing bond angles
CH₄: 109.5° (no LP)
NH₃: 107° (1 LP)
H₂O: 104.5° (2 LP)
Multiple Bonds
Double and triple bonds contain more electron density, slightly repel other bonds more
H₂CO (formaldehyde): H-C-H angle < 120°
Electronegativity Effects
More electronegative substituents attract bonding electrons, slightly reducing bond angles
Worked Examples
Example 1: H₂O (Water)
Step 1: Draw Lewis structure
O has 6 valence e⁻, each H has 1 → total = 8 e⁻
Step 2: Count electron domains around O
2 bonding domains (O-H bonds)
2 lone pairs
Total = 4 electron domains
Step 3: Electron geometry
Tetrahedral
Step 4: Molecular geometry (2 bonds, 2 LP)
Bent, ~104.5°
Example 2: SF₄ (Sulfur Tetrafluoride)
Step 1: Lewis structure
S has 6 valence e⁻, each F has 7 → S forms 4 bonds, 1 LP remains
Step 2: Count electron domains
4 bonding domains (S-F bonds)
1 lone pair
Total = 5 electron domains
Step 3: Electron geometry
Trigonal Bipyramidal
Step 4: Lone pair placement
Lone pairs prefer equatorial positions (less repulsion)
Step 5: Molecular geometry (4 bonds, 1 LP)
See-Saw shape
Example 3: XeF₄ (Xenon Tetrafluoride)
Step 1: Lewis structure
Xe has 8 valence e⁻, forms 4 bonds, 2 LP remain
Step 2: Count electron domains
4 bonding domains
2 lone pairs
Total = 6 electron domains
Step 3: Electron geometry
Octahedral
Step 4: Lone pair placement
2 LP occupy opposite positions (trans)
Step 5: Molecular geometry (4 bonds, 2 LP)
Square Planar, 90°
Common Mistakes
Forgetting Lone Pairs
Always count ALL electron domains, not just bonds!
Multiple Bonds = Multiple Domains
Double and triple bonds count as ONE electron domain!
Confusing Electron vs Molecular Geometry
Electron geometry uses all domains; molecular shape ignores LP positions!