What Is VSEPR Theory?
VSEPR theory — Valence Shell Electron Pair Repulsion theory — is a model used to predict the three-dimensional geometry of molecules based on the idea that electron pairs around a central atom repel one another and arrange themselves to be as far apart as possible.
Developed by Ronald Gillespie and Ronald Nyholm in the 1950s (building on earlier work by Nevil Sidgwick and Herbert Powell), VSEPR theory is remarkably simple yet accurate. It requires only a Lewis structure and the count of electron pairs around the central atom.
Core Principle: Electron Pair Repulsion
Electron pairs — whether bonding pairs or lone pairs — are negatively charged and repel each other. They adopt the geometry that maximizes the angles between them, minimizing repulsion and lowering the molecule's energy.
There are two key terms: - Electron geometry: The arrangement of all electron pairs (bonding + lone pairs) around the central atom - Molecular geometry: The arrangement of only the atoms (bonding pairs), ignoring lone pairs
Electron Pair Geometries
The number of electron groups (counting each bonding pair/bond and each lone pair as one group) around the central atom determines the basic geometry:
| Electron Groups | Electron Geometry | Ideal Bond Angles |
|---|---|---|
| 2 | Linear | 180° |
| 3 | Trigonal planar | 120° |
| 4 | Tetrahedral | 109.5° |
| 5 | Trigonal bipyramidal | 90°, 120° |
| 6 | Octahedral | 90° |
Note: Multiple bonds (double or triple) count as a single electron group for VSEPR purposes, since all the electrons in a multiple bond are between the same two atoms.
Lone Pairs Change Molecular Shape
Lone pairs occupy more space than bonding pairs because they are not constrained by a second nucleus pulling them into a defined direction. This means lone pairs exert greater repulsion on neighboring electron pairs, compressing bond angles.
Repulsion hierarchy: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair
The Water Molecule: A Classic Example
Oxygen in H₂O has 4 electron groups: 2 bonding pairs + 2 lone pairs → electron geometry = tetrahedral.
But molecular geometry describes only atom positions: H–O–H angle ≈ 104.5° (compressed from ideal 109.5° by the two lone pairs). Molecular shape = bent (angular).
VSEPR Molecular Geometries at a Glance
2 Electron Groups
- Linear: No lone pairs (e.g., CO₂, BeH₂). Bond angle = 180°.
3 Electron Groups
- Trigonal planar: 0 lone pairs (e.g., BF₃, SO₃). Bond angle ≈ 120°.
- Bent: 1 lone pair (e.g., SO₂, O₃). Bond angle < 120°.
4 Electron Groups
- Tetrahedral: 0 lone pairs (e.g., CH₄, SiCl₄). Bond angle = 109.5°.
- Trigonal pyramidal: 1 lone pair (e.g., NH₃, PCl₃). Bond angle ≈ 107°.
- Bent: 2 lone pairs (e.g., H₂O, H₂S). Bond angle ≈ 104.5°.
5 Electron Groups
- Trigonal bipyramidal: 0 lone pairs (e.g., PCl₅). Angles: 90° (axial–equatorial) and 120° (equatorial–equatorial).
- See-saw: 1 lone pair (e.g., SF₄).
- T-shaped: 2 lone pairs (e.g., ClF₃).
- Linear: 3 lone pairs (e.g., XeF₂). Bond angle = 180°.
6 Electron Groups
- Octahedral: 0 lone pairs (e.g., SF₆). Bond angle = 90°.
- Square pyramidal: 1 lone pair (e.g., BrF₅).
- Square planar: 2 lone pairs (e.g., XeF₄). The two lone pairs adopt axial positions to minimize repulsion.
Step-by-Step: Applying VSEPR
Determine the shape of ammonia (NH₃):
- Draw the Lewis structure: N is central with 3 N–H bonds and 1 lone pair.
- Count electron groups: 3 bonding + 1 lone pair = 4 groups.
- Electron geometry = tetrahedral (4 groups).
- Lone pairs: 1 → push bonding pairs slightly closer together.
- Molecular geometry = trigonal pyramidal.
- Predicted H–N–H bond angle ≈ 107° (experimental: 107.8°). ✓
VSEPR and Molecular Polarity
Molecular geometry is critical for determining whether a molecule is polar or nonpolar:
- CO₂ is linear (O=C=O): the two C=O dipoles point in exactly opposite directions and cancel → nonpolar molecule.
- H₂O is bent: the two O–H dipoles do not cancel → polar molecule with a net dipole moment.
- CH₄ is perfectly tetrahedral: all four C–H dipoles cancel → nonpolar.
- CHCl₃ (chloroform) is tetrahedral but asymmetric: the three C–Cl dipoles do not cancel the one C–H dipole → polar.
A symmetric geometry with identical terminal atoms → nonpolar; any asymmetry → polar.
Limitations of VSEPR
VSEPR is a qualitative model with notable limitations:
- It cannot predict exact bond angles without additional quantum mechanical calculation.
- It does not explain why certain geometries are adopted at a quantum level (molecular orbital theory does this better).
- It struggles with transition metal complexes, where d-electron effects dominate geometry.
- Molecules like BH₃ and AlCl₃ dimerize in reality, which VSEPR cannot predict.
Despite these limitations, VSEPR remains an essential tool for students, educators, and practicing chemists because of its predictive power and simplicity.
Real-World Significance of Molecular Shape
Molecular geometry is not just an academic exercise — it determines the function of molecules in the real world:
- Enzymes and drug binding: A drug molecule must have the precise 3D shape to fit into an enzyme's active site (lock-and-key model). The efficacy of many medications depends on molecular geometry.
- DNA double helix: The specific angles and geometries of base pairs (planar aromatic rings) allow hydrogen bonds to form with exactly the right spacing.
- Greenhouse gases: CO₂ and H₂O are greenhouse gases partly because their molecular geometry allows them to absorb infrared radiation with specific vibrational modes.
- Taste and smell: Olfactory receptors detect molecules based on shape; two molecules with the same formula but different 3D geometry can smell entirely different.
VSEPR theory connects the abstract world of electron pairs to the tangible shapes that define how molecules interact — making it one of chemistry's most useful and enduring concepts.