A level Chemistry · Edexcel 9CH0 · Topic 2C
Shapes of molecules
Key points
- Electron-pair repulsion: pairs of electrons around a central atom repel and move as far apart as possible.
- Lone pairs repel more strongly than bonding pairs, so each lone pair cuts bond angles by about 2.5°.
- Key shapes: linear 180° (CO₂, BeCl₂); trigonal planar 120° (BF₃); tetrahedral 109.5° (CH₄); trigonal pyramidal 107° (NH₃); bent/V-shaped 104.5° (H₂O); trigonal bipyramidal 90° and 120° (PCl₅); octahedral 90° (SF₆).
- Count electron pairs (bonding + lone); a double bond counts as one bonding region.
- For ions, add electrons for negative charge and remove for positive charge before counting, e.g. NH₄⁺ is tetrahedral.
- Exam tip: always state the shape name, the angle and the reason (the numbers of bonding and lone pairs, and that lone pairs repel more than bonding pairs).
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The shape of a molecule decides whether it is polar, which intermolecular forces it has and even how it reacts. Electron-pair repulsion theory lets you predict any shape from a dot-and-cross diagram.
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