VSEPR Theory

VSEPR theory — Valence Shell Electron Pair Repulsion — predicts the three-dimensional shapes of molecules from a single simple rule: electron pairs around a central atom arrange themselves so as to be as far apart as possible. Developed by Ronald Gillespie and Ronald Nyholm in the 1950s, VSEPR is the standard tool for predicting molecular geometry from a Lewis structure alone, and it gives the right answer for the vast majority of small molecules without any quantum mechanics.

The Core Idea

Electron pairs in the valence shell of a central atom (whether bonding or lone) repel each other electrostatically. They settle into the geometry that puts them as far apart as possible. The molecular shape that results depends on:

• The number of bonding pairs (single, double, or triple bond all count as one pair for VSEPR).
• The number of lone pairs.
• The fact that lone pairs are ‘fatter’ (they spread out more, taking more space) than bonding pairs, so lone-pair-bonding-pair repulsion is greater than bonding-bonding repulsion.

Steric Number and Electron Geometry

The steric number is the total number of electron-pair domains (bonds + lone pairs) around the central atom. For each steric number, the electron pairs adopt a specific arrangement:

Steric number	Electron geometry	Bond angle (ideal)
2	Linear	180°
3	Trigonal planar	120°
4	Tetrahedral	109.5°
5	Trigonal bipyramidal	90° / 120°
6	Octahedral	90°

The molecular geometry — the shape described by the atom positions alone — depends on how many of those domains are lone pairs versus bonds. Lone pairs are invisible in the shape; they just push the atoms into a different arrangement.

Common Geometries

Steric #	Lone pairs	Molecular geometry	Example	Bond angle
2	0	Linear	BeCl₂, CO₂	180°
3	0	Trigonal planar	BF₃	120°
3	1	Bent	SO₂	~118°
4	0	Tetrahedral	CH₄	109.5°
4	1	Trigonal pyramidal	NH₃	107°
4	2	Bent	H₂O	104.5°
5	0	Trigonal bipyramidal	PCl₅	90° / 120°
5	2	T-shaped	ClF₃	~87°
6	0	Octahedral	SF₆	90°
6	2	Square planar	XeF₄	90°

Worked Example: Water

Water is H₂O. Lewis structure: oxygen is central, two O-H bonds, two lone pairs on oxygen. Steric number = 2 + 2 = 4. Electron geometry is tetrahedral. With two of the four corners occupied by lone pairs, the molecular geometry is bent.

The ideal tetrahedral angle is 109.5°, but the two lone pairs on oxygen exert extra repulsion on the bonding pairs, squeezing the H-O-H angle down to about 104.5°. This bent shape — combined with the high electronegativity of oxygen — gives water its polarity, hydrogen bonding, and unusual physical properties.

Worked Example: Ammonia

Ammonia is NH₃. Lewis structure: nitrogen central, three N-H bonds, one lone pair. Steric number = 3 + 1 = 4. Electron geometry is tetrahedral; with one corner taken by the lone pair, the molecular geometry is trigonal pyramidal. The bond angle is 107° — slightly less than 109.5° because the one lone pair pushes the bonding pairs together.

Limitations of VSEPR

• Transition metals. d-orbital chemistry doesn’t follow VSEPR cleanly. Crystal field theory and ligand field theory are the right tools for transition-metal complexes.
• Heavily strained molecules. Cyclopropane (60° angles) and other small rings show that the lowest-energy molecular geometry sometimes is not the one VSEPR predicts.
• Inert pair effect. Heavy main-group elements (lead, thallium, bismuth) sometimes keep their outer s electrons as a ‘inert’ lone pair that doesn’t behave like a normal lone pair, distorting predictions.
• Bond order subtleties. VSEPR treats double and triple bonds the same as single bonds in terms of count, which oversimplifies the geometry of conjugated and highly multiply-bonded species.

Why VSEPR Matters

• Reactivity. A molecule’s shape determines which functional groups are exposed, which face other molecules in collisions, and therefore which reactions are easy.
• Polarity. Bent, pyramidal, and asymmetric molecules can have net dipole moments; symmetric ones (linear, planar) don’t. This determines solubility, intermolecular forces, and boiling points.
• Biology. Enzymes and substrates fit by shape complementarity. A tetrahedral or planar fit at the active site can mean the difference between catalysis and no reaction.
• Drug design. Whether a drug binds its target depends on shape. VSEPR gives a fast first-pass estimate of a candidate molecule’s geometry before any computational chemistry.

Related study notes: Ionic vs Covalent Bonds, Electronegativity, Lewis Structures, Molecular Polarity.

Frequently Asked Questions