Why does bent molecular geometry occur in molecules with lone pairs?

Bent molecular geometry occurs in molecules with lone pairs due to the repulsion between electron pairs.

In more detail, the shape of a molecule is determined by the Valence Shell Electron Pair Repulsion (VSEPR) theory. This theory states that electron pairs around a central atom will arrange themselves in such a way as to minimise repulsion. This is because electrons are negatively charged and, as like charges repel, they will try to get as far away from each other as possible.

In molecules with lone pairs, these lone pairs of electrons also contribute to the shape of the molecule. Lone pairs are pairs of valence electrons that are not shared with another atom and are not involved in a chemical bond. They occupy more space than bonding pairs and therefore exert a greater repulsive force. This means that they will push the bonding pairs of electrons closer together, causing the molecule to have a bent or V-shaped geometry.

For example, consider the water molecule (H2O). The oxygen atom in water has two lone pairs of electrons and two bonding pairs. According to the VSEPR theory, the electron pairs will arrange themselves to be as far apart as possible. However, because the lone pairs occupy more space, they push the hydrogen atoms closer together, resulting in a bent molecular geometry.

In conclusion, the presence of lone pairs in a molecule can significantly influence its shape. The repulsion between electron pairs, particularly the stronger repulsion from lone pairs, causes the molecule to adopt a bent geometry. This is a fundamental concept in understanding the structure and properties of molecules.