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.
Use your mouse (or finger on touch devices) to move or scale molecules. Double tap to stop and start each animation.
Options
atom labels
ball and stick model
space-filling model
perspective view
methane, CH4
ammonia, NH3
water, H2O
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 |
|---|---|---|---|---|
| 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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