Why does bond angle decrease with an increasing number of lone pairs?

Bond angle decreases with an increasing number of lone pairs due to the greater repulsion they exert compared to bonding pairs.

In a molecule, the shape and bond angles are determined by the repulsion between electron pairs in the valence shell of the central atom. This is explained by the Valence Shell Electron Pair Repulsion (VSEPR) theory. According to this theory, electron pairs will arrange themselves to minimise repulsion and maximise the distance between them.

Lone pairs of electrons, which are not involved in bonding, occupy more space in the electron cloud around the central atom than bonding pairs. This is because they are held by only one nucleus and hence are more diffuse. As a result, they exert a greater repulsive force on the other electron pairs. This greater repulsion pushes the bonding pairs closer together, decreasing the bond angle.

For example, consider the molecules of water (H2O) and ammonia (NH3). In water, there are two bonding pairs (between the oxygen and hydrogen atoms) and two lone pairs on the oxygen atom. In ammonia, there are three bonding pairs (between the nitrogen and hydrogen atoms) and one lone pair on the nitrogen atom. According to the VSEPR theory, the bond angle in water should be less than in ammonia due to the greater number of lone pairs. This is indeed observed experimentally: the H-O-H bond angle in water is approximately 104.5 degrees, while the H-N-H bond angle in ammonia is approximately 107.3 degrees.

To further understand how molecular structures influence these observations, explore the concepts on simple molecular structures. Additionally, understanding the nature of polar and non-polar covalent bonds can provide deeper insights into why lone pairs affect bond angles differently. For a comprehensive review of how covalent bonding plays a crucial role in molecular geometry, see the detailed discussion on covalent bonding and molecular structures.

In summary, the presence of a greater number of lone pairs in a molecule leads to a decrease in bond angle due to the greater repulsion they exert on the bonding pairs. This is a fundamental concept in understanding the shapes of molecules and their chemical behaviour.