Lone pairs and shapes of molecules

Ammonia is a molecule we met in gcse chemistry; its chemical formula is NH₃. What is the shape of an ammonia molecule? Well if we simply use the VSEPR rules we have been using so far we have:

1. N is the central atom and it is in group 5. It has 5 valency electrons
2. Three hydrogen atoms are bonded to the nitrogen central atom and each hydrogen atom contributes 1 electron to any covalent bond formed to the nitrogen atom. So we have 3 electrons in total from the three hydrogen atoms.
3. The total number of electrons in the valency shells is therefore 8 electrons; dividing by 2 gives 4 electron pairs; so the shape of a NH₃ molecule will be based on a tetrahedral structure with bond angles of 109.5⁰!!!

However a tetrahedral molecule requires 4 atoms around the central atom but in the example above using ammonia there are only 3 hydrogen atoms around the central nitrogen atom not 4. The reason for this is that ammonia contains a lone pair or a non-bonding pair of electrons. When deciding on the shape of a molecule it is vital that you locate any of these lone pairs.

In a normal covalent bond between two atoms each atom contributes one electron to the covalent bond and these two electrons are held in place by their attraction to the two positively charged nuclei of the bonding atoms. However lone pairs or non-bonding pairs of electrons are different. Here we have 2 electrons in the lone pair but they are being held in place by the attraction of only one nucleus. This means that the electrons in a lone pair are not held as tightly as those in a bonding pair of electrons and as a consequence of this lone pairs take up more space than regular bonding pairs of electrons. This is outlined in the diagram below.

In a normal tetrahedral molecule with no lone pairs and 4 bonding pairs of electrons all the bond angles would be 109.5⁰. However since the lone pair takes up more space than a bonding pair of electrons it will compress or repel the other three bonding pairs of electrons and reduce the bond angle between them to below 109.5⁰. The single lone pair will force the 3 bonding pairs closer together; the new bond angle between them will be 107⁰.

In deciding on the shape of the molecules we need to be aware of the presence of any lone pairs of electrons but they are not taken into account when deciding on the overall shape of the molecule. So what shape is the ammonia molecule then? The image below shows the ammonia molecule without its lone pair of electrons. The shape is no longer tetrahedral as this requires 4 atoms around the central atom. If you look at the molecule it resembles a pyramid with triangular sides. So its shape is described as trigonal pyramidal or simply pyramidal.

More lone pairs of electrons

As another example consider a molecule of water (H₂O). What would be the shape of a molecule of water? Well as before to work out the shape simply use the VSEPR rules:

1. Oxygen is the central atom and it is in group 6 of the periodic table. It has 6 valency electrons.
2. Two hydrogen atoms are bonded to the central atom so each hydrogen atom will contribute 1 electron to bond with the oxygen atom so we have 2 electrons in total from the hydrogen atoms.
3. The total number of electrons in the valency shells is therefore 8 electrons with 6 electrons from the oxygen atom and 2 electrons from the hydrogen atoms; so as before dividing by 2 gives 4 electron pairs. Four electron pairs means the shape of the water molecule will be based on a tetrahedral arrangement. However like ammonia it will have lone pairs. For a tetrahedral shaped molecule we need 4 atoms around the central atom and in this case there are only 2 hydrogen atoms. These 2 hydrogen atoms will use 4 of the available 8 electrons in the valency shell and this obviously leaves 4 electrons; or two lone pairs of electrons. This means that a molecule of water has 2 lone pairs of electrons and 2 bonding pairs of electrons.

Remember that we do not consider the lone pairs when deciding on the final shape of the molecule. So try to imagine a water molecule without its lone pairs of electrons. Without the presence of the lone pairs the water molecule is described as having a V-shape or bent shape. This is shown below

We mentioned earlier that lone pairs of electrons require more space than bonding pairs. In the ammonia molecule there is a single lone pair of electrons and this compressed or squashed down the angles between the bonding pairs by over 2⁰ from 109.5 to 107⁰. Water has 2 lone pairs of electrons and this means that the bonding pairs are going to even more compressed. In the diagram above you can see that the bond angle between the hydrogen atoms and the oxygen is squashed down from 109.5 ⁰ to 104.5 ⁰

Key Points

• To find the shape of a molecule it is necessary to check to see if it contains any lone or non-bonding pairs of electrons.
• Lone pairs of electrons take up more space than bonding pairs of electrons. This can have a dramatic effect on the expected bond angles in a molecule. The diagram below summarises how lone pairs of electrons affect bond angles in molecules.

• In deciding on the final shape of a molecule the lone pairs are NOT taken into account. Only the bonding pairs of electrons connected to other atoms are considered when taking into account the final shape of a molecule.

Check your understanding - Questions on shape and lone pairs in small molecules