How do lone pairs affect molecular shapes differently than bonding pairs?

Lone pairs repel more strongly than bonding pairs, causing a distortion in the idealised molecular shape.

In a molecule, both lone pairs and bonding pairs of electrons occupy space around the central atom. However, they do not affect the molecular shape in the same way. The key difference lies in the fact that lone pairs of electrons repel more strongly than bonding pairs. This is due to the fact that lone pairs are located closer to the central atom and occupy more space, leading to a greater repulsion.

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a model used in chemistry to predict the geometry of individual molecules from the number of electron pairs surrounding their central atoms. According to this theory, electron pairs will arrange themselves in such a way that the repulsion between them is minimised. Bonding pairs, being shared by two atoms, are spread out more and hence cause less repulsion. On the other hand, lone pairs, being held by a single atom, are more localised and hence cause more repulsion.

This difference in repulsion causes a distortion in the idealised molecular shape. For example, in a molecule like ammonia (NH3), there are three bonding pairs and one lone pair of electrons around the nitrogen atom. According to VSEPR theory, the molecule should have a tetrahedral shape. However, due to the greater repulsion of the lone pair, the shape is distorted to a trigonal pyramid.

Furthermore, the presence of lone pairs can also affect the bond angles in a molecule. In water (H2O), for instance, the ideal bond angle in a tetrahedral arrangement is 109.5 degrees. However, due to the presence of two lone pairs on the oxygen atom, the bond angle is reduced to approximately 104.5 degrees.

In conclusion, while both lone pairs and bonding pairs play a role in determining the shape of a molecule, their effects are different. Lone pairs, due to their greater repulsion, can distort the idealised shape and reduce the bond angles. Understanding this concept is crucial for predicting the geometry of molecules.