Postulates

Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used in chemistry to predict the three-dimensional geometry of individual molecules based on the number of electron pairs surrounding a central atom. The fundamental premise is elegantly simple: electron pairs — whether they form bonds or exist as lone pairs — repel one another because they carry the same negative charge. These pairs arrange themselves in three-dimensional space to minimize repulsion, and the resulting arrangement determines the shape of the molecule. VSEPR theory categorises electron pairs into two distinct types: bonding pairs (BP), which are shared between two atoms to form covalent bonds, and lone pairs (LP), which belong exclusively to the central atom and are not involved in bonding. The critical insight of VSEPR theory is that these two types of electron pairs do not repel equally. Lone pairs occupy more space around the nucleus than bonding pairs because they are attracted by only one nucleus rather than two, making them 'fatter' and exerting stronger repulsive forces on neighbouring electron pairs. By counting the total number of electron pairs (both bonding and lone) around a central atom and then applying the repulsion hierarchy, chemists can accurately predict molecular geometries for a vast range of compounds — from the linear shape of carbon dioxide ( $\ce{CO2}$ ) to the bent geometry of water ( $\ce{H2O}$ ) and the trigonal pyramidal shape of ammonia ( $\ce{NH3}$ ). VSEPR theory remains one of the most powerful yet conceptually accessible tools in structural chemistry.

Key takeaways

Electron Pairs Occupy the Valence Shell and Repel Each Other
The Repulsion Hierarchy: lp–lp > lp–bp > bp–bp
Multiple Bonds Are Treated as Single Electron Domains
Electron-Domain Geometry vs. Molecular Geometry

Electron Pairs Occupy the Valence Shell and Repel Each Other

All electron pairs — both bonding pairs and lone pairs — in the valence shell of the central atom repel one another due to electrostatic (Coulombic) forces between like charges. This repulsion causes the electron pairs to spread as far apart as possible in three-dimensional space, adopting a geometry that minimises the total repulsion energy of the molecule.

The Repulsion Hierarchy: lp–lp > lp–bp > bp–bp

Not all repulsions are equal. A lone pair–lone pair (lp–lp) repulsion is the strongest because both electron clouds are diffuse and each is held by only one nucleus, causing them to occupy a large volume and push against each other intensely. A lone pair–bonding pair (lp–bp) repulsion is intermediate in strength. A bonding pair–bonding pair (bp–bp) repulsion is the weakest, since both pairs are held between two nuclei and are therefore more compact and directed. This hierarchy explains why bond angles in molecules with lone pairs are compressed relative to ideal geometries — for example, the H–N–H angle in $\ce{NH3}$ is 107.3° rather than the tetrahedral ideal of 109.5°, and the H–O–H angle in $\ce{H2O}$ is just 104.5°.

Multiple Bonds Are Treated as Single Electron Domains

For the purpose of predicting geometry, a double bond (two shared electron pairs) or triple bond (three shared electron pairs) is treated as a single electron domain or 'super pair.' The multiple bond occupies one region of space around the central atom. However, because double and triple bonds contain more electron density than single bonds, they exert slightly greater repulsive force on adjacent electron pairs. For example, in $\ce{CO2}$ the central carbon has two double-bond domains (no lone pairs), giving a linear geometry with a bond angle of 180°.

Electron-Domain Geometry vs. Molecular Geometry

The electron-domain geometry describes the arrangement of all electron pairs (bonding and lone) around the central atom. The molecular geometry describes only the positions of the atoms, ignoring lone pairs. These two geometries are identical when there are no lone pairs (e.g., tetrahedral $\ce{CH4}$ , electron-domain geometry = molecular geometry = tetrahedral). When lone pairs are present, the molecular geometry is a subset of the electron-domain geometry. For example, $\ce{NH3}$ has a tetrahedral electron-domain geometry but a trigonal pyramidal molecular geometry because one of the four domains is a lone pair. $\ce{H2O}$ has a tetrahedral electron-domain geometry but a bent molecular geometry because two of the four domains are lone pairs.

The Key Equation

\text{Steric Number} = (\text{number of atoms bonded to central atom}) + (\text{number of lone pairs on central atom})

SN — Steric Number (dimensionless): The total number of electron domains (bonding pairs + lone pairs) around the central atom. SN=2 gives linear, SN=3 trigonal planar, SN=4 tetrahedral, SN=5 trigonal bipyramidal, SN=6 octahedral electron-domain geometry.
BP — Bonding Pairs (dimensionless): The number of atoms directly bonded to the central atom. Each bonded atom, regardless of bond order (single, double, or triple), counts as one bonding domain.
LP — Lone Pairs (dimensionless): Non-bonding electron pairs localised on the central atom. Each lone pair counts as one electron domain and exerts stronger repulsion than bonding pairs due to its greater spatial diffuseness.

Worked Example

Problem. Predict the molecular geometry and approximate bond angles of a water molecule ( $\ce{H2O}$ ) using VSEPR theory. Oxygen is the central atom.

\angle \text{H–O–H} \approx 104.5^\circ

Answer. Molecular geometry: bent (V-shaped). H–O–H bond angle: approximately 104.5°.

Figure

Diagram

Origins · 1 minRonald J. Gillespie and how this was discovered

Ronald J. Gillespie

1924–2021 · British-Canadian

Ronald Gillespie, working with Ronald Sydney Nyholm at University College London in 1957, reformulated and greatly expanded the VSEPR model into the systematic, widely-used theory taught today. Their landmark 1957 paper in the Quarterly Reviews of the Chemical Society established the rules for lone-pair vs. bonding-pair repulsion hierarchies and extended the model to cover molecules of main-group elements across the periodic table. Gillespie later refined the model further, introducing the concept of electron localisation functions (ELF) to provide quantum mechanical support for VSEPR predictions. Gillespie was still publishing research papers on VSEPR theory and chemical bonding well into his 90s, making him one of the longest-active theoretical chemists in history. He reportedly kept a molecular model set on his desk until very late in life.

Molecular Shapes from VSEPR — Once the VSEPR postulates are understood, the next step is to systematically catalogue all molecular geometries (linear, trigonal planar, tetrahedral, trigonal bipyramidal, octahedral, and their lone-pair variants) and learn to name them from steric number and lone-pair count.
Lewis Dot Structures — VSEPR predictions always begin with a correct Lewis structure to identify bonding pairs and lone pairs on the central atom. Without an accurate Lewis structure, the steric number and geometry cannot be determined.
Molecular Polarity and Dipole Moments — The three-dimensional geometry predicted by VSEPR determines whether bond dipoles cancel (non-polar molecule, e.g. CO₂) or reinforce (polar molecule, e.g. H₂O). Geometry and polarity are therefore inseparable concepts.
Orbital Hybridisation (sp³, sp², sp) — Hybridisation theory provides a quantum mechanical explanation for the geometries predicted by VSEPR. The steric number directly corresponds to the type of hybridisation: SN=4 gives sp³, SN=3 gives sp², SN=2 gives sp.