VSEPR Theory and Molecular Shapes: Complete Exam Guide

VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the three-dimensional shape of a molecule from a simple idea: electron pairs around a central atom repel each other and arrange themselves as far apart as possible. Count the bonding pairs and lone pairs, and the geometry follows.

This matters because shape decides everything a molecule does — its polarity, its boiling point, how it fits into enzymes and how it reacts. Two molecules with the same atoms but different shapes can behave completely differently, so predicting shape correctly is a core exam and real-world skill.

Key Concepts

• Electron domain: a region of electrons around the central atom — a single, double or triple bond each counts as one domain, and each lone pair counts as one domain.
• Steric number: total domains = bonding pairs + lone pairs; it fixes the electron geometry.
• Repulsion order: lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair. Lone pairs squeeze bond angles smaller.
• AXE notation: A = central atom, X = bonded atoms, E = lone pairs. Water is AX₂E₂; ammonia is AX₃E.
• Standard shapes: 2 domains → linear (180°); 3 → trigonal planar (120°); 4 → tetrahedral (109.5°); 5 → trigonal bipyramidal; 6 → octahedral (90°).
• Effect of lone pairs: NH₃ (AX₃E) is trigonal pyramidal with 107°; H₂O (AX₂E₂) is bent with 104.5° — both derived from the tetrahedron.

How It Works

1. Draw the Lewis structure and identify the central atom.
2. Count electron domains: each bond (single, double or triple) = 1, each lone pair = 1.
3. Match the steric number to the electron geometry (2 linear, 3 trigonal planar, 4 tetrahedral, 5 trigonal bipyramidal, 6 octahedral).
4. Remove lone pairs from the picture to name the molecular shape (the shape of the atoms only).
5. Adjust bond angles downward for each lone pair, since lone pairs repel more strongly.

Worked Example

Predict the shape and bond angle of ammonia, NH₃.

1. Lewis structure: nitrogen (5 valence electrons) bonded to 3 hydrogens, with 1 lone pair left on nitrogen.
2. Domains: 3 bonding pairs + 1 lone pair = steric number 4.
3. Electron geometry for 4 domains: tetrahedral (ideal 109.5°).
4. Molecular shape (atoms only, AX₃E): trigonal pyramidal.
5. The lone pair repels more strongly than bonding pairs, squeezing the H–N–H angle below 109.5°.

NH₃ is trigonal pyramidal with a bond angle of 107°.

Common Misconceptions

• Lone pairs can be ignored because they are not atoms. They cannot — lone pairs occupy space and repel bonding pairs, which is exactly why water is bent instead of linear.
• A double bond counts as two domains. It counts as one domain for shape purposes (though it repels slightly more strongly than a single bond).
• More atoms always means a bigger bond angle. H₂O has 3 atoms but a smaller angle (104.5°) than CH₄'s 109.5°, because water's two lone pairs compress the angle.

Real-World Applications

• Molecular polarity (and hence solubility and boiling points) follows directly from shape.
• Drug design depends on molecular shape fitting into protein binding sites.
• The bent shape of water explains its hydrogen bonding and life-supporting properties.
• Catalyst design uses the geometry of active sites to steer reactions.
• Materials science predicts crystal packing from molecular geometry.

Count the domains, pick the electron geometry, then hide the lone pairs to name the shape — adjusting angles down for lone-pair repulsion. That five-step routine predicts the geometry of almost any small molecule.