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You can see ball-and-stick models of methane, ammonia and water:
- all three molecules have four pairs of outer electrons around their central atom, so
- they all have a tetrahedral arrangement of electron pairs around the central atom.
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.
Summary of bond angles and shapes