In short: Ammonia, NH₃, is pyramidal with a bond angle of 107°. Nitrogen has three bonding pairs and one lone pair, and the lone pair repels the bonding pairs more strongly than they repel each other, so the angle is squeezed below 109.5°.
Shapes of molecules are some of the most reliable marks in A Level Chemistry. They are also easy to lose, because the shape name and the bond angle both have to be right.
Spot the mistake
In the clip, an answer about ammonia lists the number of bonding pairs, the number of lone pairs, the shape and the bond angle. Three parts are right. One is not.
The mistake is the bond angle. The answer gave 109.5°, which is the angle in methane. Ammonia's angle is 107°.
Rich's tip: Check every part of the answer, not just the shape name.
Electron pair repulsion theory
Electron pairs around a central atom repel each other. They spread out as far apart as possible, and that sets the shape of the molecule. Count the pairs, then work out the shape.
Methane: 109.5°
Carbon in CH₄ has four bonding pairs and no lone pairs. The four pairs spread out as far apart as possible, giving a tetrahedral shape with a bond angle of 109.5°.
Ammonia: 107°
Nitrogen in NH₃ has three bonding pairs and one lone pair. The four pairs point towards the corners of a tetrahedron, but the shape of a molecule is named from the positions of the atoms. With three hydrogen atoms around the nitrogen, the shape is pyramidal. The bond angle is 107°.
Water: 104.5°
Oxygen in H₂O has two bonding pairs and two lone pairs. The shape is bent (you may also see it called non-linear or V-shaped), and the bond angle is 104.5°.
Why the angle gets smaller
Lone pairs are closer to the central atom and are not shared between two nuclei, so they repel more strongly than bonding pairs. The order is:
lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair
Each lone pair squeezes the bonding pairs closer together. So the bond angle goes from 109.5° in methane, to 107° in ammonia, to 104.5° in water.
How to explain a shape in the exam
- State the number of bonding pairs and lone pairs on the central atom.
- Say that the electron pairs repel each other as far apart as possible.
- Say that lone pairs repel more than bonding pairs (if there are any).
- Name the shape and give the bond angle.
For ammonia, that is: three bonding pairs and one lone pair; the pairs repel; the lone pair repels the bonding pairs more than they repel each other; so the shape is pyramidal and the bond angle is 107°.
Common mistakes
- Giving 109.5° for ammonia or water.
- Writing "the bonds repel" instead of "the electron pairs repel".
- Forgetting that the lone pair repels more, which is the mark for the smaller angle.
- Calling the shape "tetrahedral" when there is a lone pair. The ammonium ion, NH₄⁺, is tetrahedral (109.5°), but ammonia is not.
If you want to practise shapes of molecules, Dexter, the A Level Chemistry tutor I built, can ask you for shapes and bond angles on the molecules in your course and check your explanation for the points you missed.