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Tetrahedral molecules

What is here?

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.
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chemdoodle.png atom labels
ball and stick model
space-filling model
perspective view

methane, CH4

ammonia, NH3

water, H2O

Shapes

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

Molecule
Bonding pairs
Lone pairs
Bond angle
Shape
CH4 4 0 109.5° tetrahedral
NH3 3 1 107.0° trigonal pyramidal
H2O 2 2 104.5° bent line
⚠ Different sources may quote different bond angles for ammonia and water. The ones shown here are acceptable for AS and A Level examinations.
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