VSEPR theory and molecular shapes

The shape of a molecule is set by the number of electron pairs (bonding and lone) around the central atom, which arrange themselves as far apart as possible.

A multiple bond is treated as a single electron-pair region (a 'super pair') when predicting shape.

Repulsion order: lone pair-lone pair > lone pair-bond pair > bond pair-bond pair, because lone pairs are held by only one nucleus and spread out more.

Without lone pairs: 2 pairs linear (BeCl₂, 180°), 3 trigonal planar (BF₃, 120°), 4 tetrahedral (CH₄, 109.5°), 5 trigonal bipyramidal (PCl₅), 6 octahedral (SF₆).

With lone pairs and 4 regions: NH₃ (3 bond pairs, 1 lone pair) is trigonal pyramidal with about 107°; H₂O (2 bond pairs, 2 lone pairs) is bent with about 104.5°.

With 5 regions, lone pairs take equatorial positions: SF₄ (4 bp, 1 lp) is see-saw and ClF₃ (3 bp, 2 lp) is T-shaped. Background from the Class 12 noble-gas chapter, not in this chapter's table: XeF₂ (2 bp, 3 lp) is linear by the same rule.

With 6 regions: BrF₅ (5 bp, 1 lp) is square pyramidal and XeF₄ (4 bp, 2 lp) is square planar.

SO₂ and O₃, each with 3 regions including one lone pair, are bent.

Molecular shape names describe the positions of atoms only; lone pairs affect the shape but are not part of its name.