Topic 3 · page 13 of 15
Lone pairs and the shapes of NH3 and H2O
Methane, ammonia and water all have four electron pairs. They have different shapes and angles because lone pairs repel more than bonding pairs. This is the most common explanation question in the topic.
Read first: Shapes with no lone pairs
Same four pairs, three different shapes 3.183.16
| Species | Counting | Bonding pairs | Lone pairs | Shape | Bond angle |
|---|---|---|---|---|---|
| CH₄ | 4 + 4 = 8 | 4 | 0 | Tetrahedral | 109.5° |
| NH₃ | 5 + 3 = 8 | 3 | 1 | Trigonal pyramidal | 107° |
| H₂O | 6 + 2 = 8 | 2 | 2 | Bent (non-linear) | 104.5° |
Each of the three species has four electron pairs round the central atom, so the pairs point to the corners of a tetrahedron. The lone pairs are not drawn as atoms, so the shape of the molecule is described by the atoms only:
- With four bonding pairs the four atoms make a tetrahedron.
- With one lone pair, the three atoms and the central atom make a pyramid.
- With two lone pairs, the two atoms and the central atom make a bent shape.
Why the angle goes down 3.163.18
Lone pairs repel more than bonding pairs. Each lone pair therefore pushes the bonding pairs closer together, and the angle falls by about 2.5°.
| Species | Lone pairs | Estimate from 109.5° | Angle |
|---|---|---|---|
| CH₄ | 0 | 109.5° | 109.5° |
| NH₃ | 1 | 109.5° − 2.5° | 107° |
| H₂O | 2 | 109.5° − 2 × 2.5° | 104.5° |
Worked exampleNH₃ against CH₄
Explain why the bond angle in NH₃ is smaller than in CH₄.
- Both molecules have four electron pairs round the central atom, in a tetrahedral arrangement.
- CH₄ has four bonding pairs. NH₃ has three bonding pairs and one lone pair.
- A lone pair repels more than a bonding pair, because it is closer to the central atom.
- The lone pair pushes the bonding pairs closer together, so the H–N–H angle (107°) is smaller than H–C–H (109.5°).
The same steps explain why NH₃ (107°) is smaller than NH₄⁺ (109.5°). NH₄⁺ has four bonding pairs and no lone pairs, while NH₃ has three bonding pairs and a lone pair. And H₂O, with two lone pairs, has the smallest angle of all.
Predicting by analogy 3.19
| Species | Counting | Pairs | Shape | Angle | Like |
|---|---|---|---|---|---|
| NCl₃ | 5 + 3 = 8 | 3 bonding, 1 lone | Pyramidal | about 107° | NH₃ |
| OF₂ | 6 + 2 = 8 | 2 bonding, 2 lone | Bent | about 104.5° | H₂O |
| H₃O⁺ | 6 + 3 − 1 = 8 | 3 bonding, 1 lone | Pyramidal | about 107° | NH₃ |
| NH₂⁻ | 5 + 2 + 1 = 8 | 2 bonding, 2 lone | Bent | about 104.5° | H₂O |
In H₃O⁺ and NH₂⁻ the charge step matters: subtract 1 for the positive ion and add 1 for the negative ion. In each case the total is 8, so there are 4 pairs.
MethodPredict the shape of a species
- Count the pairs, using the counting method (do not forget the charge).
- Split into bonding pairs and lone pairs.
- Match the numbers to a known species: 2 bonding, 0 lone is linear. 3 and 0 is trigonal planar. 4 and 0 is tetrahedral. 3 and 1 is pyramidal. 2 and 2 is bent. 5 and 0 is trigonal bipyramidal. 6 and 0 is octahedral.
- Give the shape and the angle.
Check yourself
- Explain why the H–N–H bond angle in NH₃ (107°) is smaller than the H–N–H angle in NH₄⁺ (109.5°).
- Explain the shape of a water molecule and state the bond angle.
- Put CH₄, NH₃ and H₂O in order of bond angle, largest first, and explain the order.
- Predict the shape and bond angle of NCl₃, and of OF₂, showing the electron count.
- Predict the shape and bond angle of H₃O⁺ and NH₂⁻, showing the electron count.
- A student says: "water is linear because it has two bonds, and ammonia is trigonal planar because it has three". Give the correct shapes and the error in each.
Answers cover these until you have tried the questions
- NH₄⁺ has four bonding pairs and no lone pairs. NH₃ has three bonding pairs and one lone pair. A lone pair repels more than a bonding pair, so in NH₃ the bonding pairs are pushed closer together, giving 107° rather than 109.5°.
- O has 6 outer electrons and two hydrogens add 2, giving 8 electrons, or 4 pairs: 2 bonding pairs and 2 lone pairs. The pairs repel and get as far apart as possible. Lone pairs repel more than bonding pairs, so the two bonding pairs are pushed closer together than 109.5°. The shape is bent (non-linear), with a bond angle of 104.5°.
- CH₄ (109.5°) > NH₃ (107°) > H₂O (104.5°). All three have four pairs. The number of lone pairs rises from 0 to 1 to 2, and lone pairs repel more than bonding pairs, so each extra lone pair pushes the bonding pairs closer together by about 2.5°.
- NCl₃: 5 + 3 = 8, which is 4 pairs: 3 bonding and 1 lone, so pyramidal, about 107°. OF₂: 6 + 2 = 8, which is 4 pairs: 2 bonding and 2 lone, so bent, about 104.5°.
- H₃O⁺: 6 + 3 − 1 = 8, which is 4 pairs: 3 bonding and 1 lone, so pyramidal, about 107°. NH₂⁻: 5 + 2 + 1 = 8, which is 4 pairs: 2 bonding and 2 lone, so bent, about 104.5°.
- Water is bent (104.5°). The error is that the two lone pairs on oxygen also count as electron pairs, so there are four pairs, not two. Ammonia is trigonal pyramidal (107°). The error is that the lone pair on nitrogen counts, so there are four pairs, not three. Trigonal planar would need three bonding pairs and no lone pair.