Topic 3 · page 11 of 15

Electron-pair repulsion theory

Electron pairs round a central atom repel and move as far apart as they can. That one idea fixes every shape. This page gives the theory, the two definitions you must know, and a counting method that works every time.

13 min read3.163.173.19

Read first: Covalent bonds and dot-and-cross diagrams

The theory 3.16

Molecules and ions have shapes because electron pairs repel each other. The idea has five parts:

  1. The electron pairs around the central atom repel each other.
  2. They arrange themselves as far apart as possible, which makes the repulsion as small as it can be.
  3. Both bonding pairs and lone pairs count.
  4. A double bond counts as one bonding region.
  5. Lone pairs repel more than bonding pairs.

A picture to hold: balloons tied together at a centre push each other to the corners of a shape. The shape of the molecule is the arrangement that gives the largest separation of the pairs.

Why do lone pairs repel more? A lone pair is held by one nucleus only, so it is closer to the central atom and spreads out more. It takes up more space than a bonding pair, which is held between two nuclei.

Bond length and bond angle 3.17

A water molecule with the oxygen at the top and two hydrogens below. The bond angle H-O-H of 104.5 degrees is marked between the two bonds, and the bond length is marked from the oxygen nucleus to one hydrogen nucleus.
Bond length and bond angle in water.

In water the bond angle H–O–H is 104.5°. Water has one bond angle. Methane has six, but all of them are 109.5°.

Counting the electron pairs 3.163.19

This method gives the number of pairs on the central atom for any species, so you can predict a shape you have never seen.

MethodWorking out a shape

  1. Count the outer electrons of the central atom.
  2. Add one electron for each atom bonded by a single bond (H or a halogen).
  3. Adjust for the charge: add one for each negative charge and subtract one for each positive charge.
  4. Divide by 2. This gives the number of electron pairs.
  5. Split them into bonding pairs (one per atom bonded) and lone pairs. The numbers give the shape and the angle.

Worked exampleCounting pairs

NH₃: N has 5 outer electrons, and 3 hydrogen atoms add 3. 5 + 3 = 8, which is 4 pairs. There are 3 bonding pairs, so there is 1 lone pair.

NH₄⁺: 5 + 4 = 9. The charge is +1, so subtract 1, giving 8, which is 4 pairs. All 4 are bonding pairs, so there are 0 lone pairs.

H₂O: O has 6, and 2 hydrogen atoms add 2. 6 + 2 = 8, which is 4 pairs. There are 2 bonding pairs, so there are 2 lone pairs.

BCl₃: B has 3, and 3 chlorine atoms add 3. 3 + 3 = 6, which is 3 pairs. All 3 are bonding pairs, so there are 0 lone pairs.

The shapes that follow from these numbers are on the next two pages. For a species with a double bond, count bonding regions round the central atom instead. CO₂ has two regions round the carbon (two double bonds). Each carbon in C₂H₄ has three regions (two C–H bonds and one C=C).

Check yourself

  1. State the principle of electron-pair repulsion theory, and say why the pairs adopt a particular arrangement.
  2. State what is meant by the terms bond length and bond angle.
  3. Use the counting method to find the number of electron pairs, bonding pairs and lone pairs on the central atom in NH₄⁺ and in BCl₃.
  4. How many bonding regions are there round the carbon atom in CO₂, and why?
  5. Using the 2.5° rule, estimate the bond angles in NH₃ and in H₂O, starting from 109.5°.
  6. A student writes: "the lone pair repels the hydrogen atoms more because lone pairs are stronger". Give two corrections.

Answers cover these until you have tried the questions

  1. Electron pairs round a central atom repel each other, and they get as far apart as possible so that the repulsion is as small as possible. Both bonding pairs and lone pairs count, and lone pairs repel more than bonding pairs.
  2. Bond length is the distance between the nuclei of two bonded atoms. Bond angle is the angle between two bonds from the same atom.
  3. NH₄⁺: 5 + 4 − 1 = 8, which is 4 pairs: 4 bonding pairs and 0 lone pairs. BCl₃: 3 + 3 = 6, which is 3 pairs: 3 bonding pairs and 0 lone pairs.
  4. Two regions. Each C=O double bond counts as one bonding region, because the two shared pairs in a double bond are in the same direction from the carbon, so they act together.
  5. NH₃ has one lone pair: 109.5° − 2.5° = 107°. H₂O has two lone pairs: 109.5° − 2 × 2.5° = 104.5°.
  6. (1) Repulsion is between electron pairs, so say that the lone pair repels the bonding pairs, not the hydrogen atoms. (2) A lone pair repels more because it is closer to the central atom, held by only one nucleus, so it takes up more space. "Stronger" is not the reason.