VSEPR prediction: shape, bond angle and lone-pair effects

VSEPR prediction: shape, bond angle and lone-pair effects

VSEPR is most useful when you show the reasoning path instead of jumping to a memorised name. Count all electron domains around the central atom, including lone pairs and multiple bonds as domains. Arrange those domains as far apart as possible, then identify the positions occupied by atoms. Lone pairs occupy more space because their electron density is held by one nucleus rather than shared between two; their stronger repulsion compresses adjacent bond angles. This explains why methane is tetrahedral with an ideal angle near 109.5°, ammonia is trigonal pyramidal with a smaller angle, and water is bent with an even smaller angle.

Work it through

For CH₄, four bonding domains give a tetrahedral shape. For NH₃, one domain is a lone pair, leaving three bonded atoms in a trigonal-pyramidal shape. For H₂O, two lone pairs leave two bonded atoms in a bent shape. The sequence has the same electron-domain count but different molecular shapes because different numbers of domains are lone pairs.

Mastery target

Predict and compare CH₄, NH₃ and H₂O using electron-domain count, lone-pair repulsion and a precise molecular-shape name.