shapes of molecules finding the lone pairs

Lone pairs and shapes of molecules

Ammonia is a molecule we met in gcse chemistry, its chemical formula is NH3. What would be 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 central atom, each contributes 1 electron. So we have 3 electrons in total.
  3. The total number of electrons in the valency shells is 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!!!.

Finding lone pairs

The ammonia molecule contains a lone pair or non-bonding pair of electrons However a tetrahedral molecule requires 4 atoms around the central atom. In this case 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 electrons are held in place by their attraction to the two nuclei of the bonding atoms. However lone pairs or non-bonding pairs 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 and as a consequence of this lone pairs take up more space than regular bonding pairs. This is shown below.

ammonia molecule has one lone pair of electrons

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 it will compress or squash down the other bonding pairs and reduce the bond angle to below 109.50. The single lone pair will force the 3 bonding pairs closer together; the new 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 shape of the molecule. So what shape is ammonia 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.

More lone pairs

water molecule with its 2 lone pairs of electrons As another example consider a molecule of water, H2O. What shape would a molecule of water have? Well as before simply use the VSEPR rules:

  1. Oxygen is the central atom and it is in group 6. It has 6 valency electrons
  2. Two hydrogens are bonded to the central atom, each contribute 1 electron. So we have 2 electrons in total.
  3. The total number of electrons in the valency shells is 8 electrons, dividing by 2 gives 4 electron pairs, so H2O is based on a tetrahedral shape. 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 is only 2 hydrogens. These 2 hydrogens will use 4 of the available 8 electrons and this obviously leaves 4 electrons, or two pairs. This means water has 2 lone 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

without its lone pairs the water molecule is said to be V-Shaped or bent

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


Practice questions

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

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