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Unit II — Bonding Theories · VSEPR Theory

VSEPR Theory & Molecular Shapes

CompleteFundamentals of Chemistry

Electron pairs repel and arrange themselves as far apart as possible — this fixes molecular shape.

Full notes

Definition: VSEPR: the valence-shell electron pairs (bonding and lone) around a central atom repel each other and adopt a geometry that maximizes their separation.

1. Main idea

  • Lone pair–lone pair repulsion > lone pair–bond pair > bond pair–bond pair.
  • Ratio of repulsion: lp–lp : lp–bp : bp–bp ≈ 1 : 0.8 : 0.6 → angles get compressed by lone pairs.
  • Double/triple bonds behave like one 'super' electron domain repelling more strongly.

2. Predicting shapes of the molecules in the syllabus

MoleculeElectron domainsLone pairsShape
H₂O42Bent (104.5°)
NH₃41Trigonal pyramidal (107°)
PCl₅50Trigonal bipyramidal
SF₆60Octahedral
SF₄51See-saw
ClF₃52T-shaped
H₃O⁺41Trigonal pyramidal
Note: Exam point: write the electron-pair arrangement first (from total domains) and then the molecular shape (ignoring lone pairs).

Exam notes

Domain count = σ bonds + lone pairs. Geometry from domains; molecular shape excludes lone pairs; lone pairs compress angles.

Important points

  • VSEPR fails for transition-metal complexes — use crystal field theory there.

Common mistakes

  • Reporting electron-pair geometry instead of molecular shape.
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