VSEPR Theory
Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used to predict the three-dimensional geometry of molecules based on the principle that electron pairs in the valence shell of a central atom repel each other and arrange themselves as far apart as possible to minimize repulsion. The theory considers both bonding pairs and lone pairs, with lone pairs exerting greater repulsive force than bonding pairs, which distorts ideal bond angles. VSEPR theory was developed by Ronald Gillespie and Ronald Nyholm in 1957 and remains one of the most useful and accessible tools for predicting molecular shape.
Worked Example
Problem
Use VSEPR theory to predict the shape of NH₃ (ammonia). (N has 3 bonding pairs and 1 lone pair)
Solution
Step 1: Draw the Lewis structure of NH₃: N is central with 3 N–H bonds and 1 lone pair. Step 2: Count electron domains around N: 3 bonding pairs + 1 lone pair = 4 electron domains. Step 3: 4 electron domains → tetrahedral electron geometry (ideal bond angles 109.5°). Step 4: The molecular geometry considers only atoms, not the lone pair. With 3 bonding pairs and 1 lone pair → trigonal pyramidal shape. Step 5: The lone pair compresses the H–N–H bond angles below 109.5°. Actual H–N–H bond angle = 107°.
Answer
NH₃ is trigonal pyramidal with H–N–H bond angles of approximately 107° due to lone pair repulsion.
VSEPR Geometries for Different Electron Domain Arrangements
| Electron Domains | Bonding Pairs | Lone Pairs | Molecular Geometry | Bond Angle |
|---|---|---|---|---|
| 2 | 2 | 0 | Linear | 180° |
| 3 | 3 | 0 | Trigonal planar | 120° |
| 3 | 2 | 1 | Bent (V-shaped) | < 120° |
| 4 | 4 | 0 | Tetrahedral | 109.5° |
| 4 | 3 | 1 | Trigonal pyramidal | < 109.5° |
| 4 | 2 | 2 | Bent (V-shaped) | < 109.5° |
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Related Terms
Lewis Structure
A Lewis structure (also called a Lewis dot structure or electron dot diagram) is a two-dimensional representation of a molecule that shows the arrangement of atoms, bonding electron pairs (as lines or pairs of dots between atoms), and lone (non-bonding) electron pairs. Developed by Gilbert N. Lewis in 1916, these diagrams are essential tools for predicting molecular geometry, reactivity, and understanding bond types. Lewis structures obey the octet rule — most atoms in a molecule strive to have 8 electrons in their valence shell, with the notable exception of hydrogen (which requires only 2).
Molecular Geometry
Molecular geometry (or molecular shape) refers to the three-dimensional spatial arrangement of atoms within a molecule, determined by the positions of the atoms — not the lone pairs — around the central atom. The geometry is predicted using VSEPR theory or hybridization models and directly influences physical properties such as polarity, reactivity, phase of matter, colour, magnetism, and biological activity. Common geometries include linear, bent, trigonal planar, trigonal pyramidal, tetrahedral, and octahedral.
Hybridization
Hybridization is a theoretical concept in chemistry describing the mixing of atomic orbitals of similar energy within the same atom to form new hybrid orbitals of equivalent energy and shape, oriented to minimize electron repulsion. Developed by Linus Pauling in 1931, hybridization explains molecular geometry that cannot be accounted for by simple orbital overlap — for example, carbon's four equivalent C–H bonds in methane despite having distinct 2s and 2p orbitals. The type of hybridization (sp, sp², sp³, sp³d, sp³d²) determines bond angles, molecular geometry, and the presence of pi bonds.
Acronym for Valence Shell Electron Pair Repulsion. "Valence" from Latin "valentia" (strength), "repulsion" from Latin "repellere" (to push back). The theory was formally published by Ronald J. Gillespie and Ronald Sydney Nyholm in 1957.