Valence Shell Electron Pair Repulsion (VSEPR) Theory

Lewis theory tells us WHICH atoms are bonded to which (connectivity, via shared electron pairs), but it cannot by itself predict the three-dimensional SHAPE of a molecule. Valence Shell Electron Pair Repulsion (VSEPR) theory, used together with Lewis structures, fills this gap. Its important principles:

1. The shape of a molecule depends on the NUMBER of valence-shell electron pairs around its central atom.
2. There are two types of valence-shell electron pair: bond pairs (shared between two atoms) and lone pairs (not involved in bonding, localised entirely on the central atom).
3. Each pair of valence electrons around the central atom REPELS every other pair, so the pairs arrange themselves as far apart as possible in three-dimensional space, to MINIMISE the repulsion between them.
4. The repulsive interaction is not equal between all pair-types; it follows the order:

This ordering has a physical reason: a lone pair is localised on, and interacts with, only ONE nucleus (the central atom's), so it spreads out and occupies more angular space; a bond pair, by contrast, is shared between and pulled by TWO nuclei, so it is held more tightly and takes up less angular space. Because lone pairs occupy more space, they exert a GREATER repulsive push on neighbouring pairs than bond pairs do.

Predicting shapes. For a general molecule AB$_x$L (A = central atom, B = an atom bonded to A, x = number of B atoms, L = number of lone pairs on A), Table 10.6 tabulates the resulting shape for every combination the syllabus covers, from 2 to 7 total electron pairs: linear (AB₂, 0 lone pairs, e.g. BeCl₂, CO₂), trigonal planar (AB₃, 0 lone pairs, e.g. BF₃, NO₃⁻) versus bent/V-shaped (AB₂L, 1 lone pair, e.g. SO₂, O₃), tetrahedral (AB₄, 0 lone pairs, e.g. CH₄, SO₄²⁻) versus pyramidal (AB₃L, 1 lone pair, e.g. NH₃, H₃O⁺) versus bent (AB₂L₂, 2 lone pairs, e.g. H₂O), trigonal bipyramidal (AB₅, 0 lone pairs, e.g. PCl₅) versus seesaw (AB₄L, 1 lone pair, e.g. SF₄) versus T-shaped (AB₃L₂, 2 lone pairs, e.g. ClF₃, BrF₃) versus linear again (AB₂L₃, 3 lone pairs, e.g. XeF₂), octahedral (AB₆, 0 lone pairs, e.g. SF₆) versus square pyramidal (AB₅L, 1 lone pair, e.g. BrF₅, IF₅) versus square planar (AB₄L₂, 2 lone pairs, e.g. XeF₄), and finally pentagonal bipyramidal (AB₇, 0 lone pairs, e.g. IF₇). …