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Shapes and bond angles
There are four pairs of outer electrons around the central atom in each of these molecules. These pairs of electrons repel each other. In terms of the relative strength of repulsion:
|strongest||lone pair – lone pair|
|⇣||lone pair – bond pair|
|weakest||bond pair – bond pair|
The H—C—H bond angle in methane is the tetrahedral angle, 109.5°. This angle is obtained when all four pairs of outer electrons repel each other equally. The bond angles in ammonia and in water are less than 109.5° because of the stronger repulsion by their lone pairs of electrons.
|Molecule||Bonding pairs||Lone pairs||Bond angle||Shape|
Different sources may quote different bond angles for ammonia and water. The ones shown here are acceptable for AS and A Level examinations.