VSEPR Theory: Molecular Geometry, Bond Angles and Polarity

VSEPR (valence shell electron pair repulsion) theory says that regions of electron density around a central atom spread as far apart as possible. You count the electron domains on the central atom, which fixes the electron geometry, and then ignore the lone pairs when you name the molecular geometry, which describes only where the atoms sit.

Count domains first

An electron domain is a bond or a lone pair. A single, double or triple bond each counts as one domain, because all the electrons in a multiple bond occupy the same region. So CO₂ (O=C=O) has two domains on carbon, and H₂C=O has three.

1. Draw the Lewis structure.
2. Count domains on the central atom.
3. Count how many of those are lone pairs.
4. Read the shape from the table below.

The shape table

The MCAT most often tests the first six rows. Water, ammonia and methane are the standard trio: all three have four domains and tetrahedral electron geometry, but the molecular shapes differ because of lone pairs.

Why lone pairs squeeze bond angles

A lone pair is held by one nucleus only, so its cloud is larger and repels more strongly than a bonding pair. The order is lone pair/lone pair greater than lone pair/bonding pair greater than bonding pair/bonding pair. That is why the angle shrinks from 109.5 degrees in methane to about 107 degrees in ammonia and about 104.5 degrees in water.

From shape to polarity

A molecule is polar only if it has polar bonds and the bond dipoles do not cancel. The dipoles are vectors, so symmetry decides the result.

• CO₂: two polar C=O bonds point opposite ways and cancel. Nonpolar overall.
• H₂O: bent, so the two O-H dipoles add up. Polar.
• BF₃: three polar bonds at 120 degrees cancel. Nonpolar.
• NH₃: pyramidal, dipoles do not cancel. Polar.
• CCl₄: tetrahedral and symmetric, so nonpolar even though each C-Cl bond is polar.
• CHCl₃: tetrahedral but not symmetric, so polar.

Polarity then predicts boiling point, solubility ("like dissolves like") and the strength of intermolecular forces. Electronegativity trends from the periodic table note tell you which bonds are polar in the first place.

Hybridization shortcut

The number of domains matches the hybridization: 2 domains is sp, 3 is sp², 4 is sp³. Lone pairs count, so the nitrogen in NH₃ and the oxygen in water are both sp³.

Common traps

• Naming the electron geometry when asked for the molecular geometry. Water is bent, not tetrahedral.
• Counting a double bond as two domains. It is one.
• Assuming polar bonds mean a polar molecule. Symmetric molecules cancel.
• Forgetting that bent molecules from the 3-domain and 4-domain rows have different angles.

One-minute recap

Count electron domains, subtract lone pairs to name the shape, and remember that lone pairs compress angles below the ideal value. Polarity needs polar bonds plus an asymmetric shape. Domains of 2, 3, 4 give sp, sp², sp³.