Electron pair repulsion plays a crucial role in determining the molecular geometry of compounds by influencing the spatial arrangement of atoms within a molecule.

This concept is central to VSEPR (Valence Shell Electron Pair Repulsion) theory, which predicts the shapes of individual molecules based on the electrostatic repulsion between electron pairs. According to VSEPR theory, the electron pairs surrounding a central atom repel each other and strive to maximize their distance from one another. This behavior arises because electrons carry a negative charge and, as like charges, they repel each other.

The molecular geometry is influenced by the number of bonding electron pairs and lone pairs present in the valence shell of the central atom. For instance, if there are two electron pairs in the valence shell, they will arrange themselves linearly to achieve maximum separation. With three electron pairs, the arrangement results in a trigonal planar geometry, while four electron pairs will lead to a tetrahedral configuration.

It is essential to recognize that not all electron pairs exert equal repulsive forces. Lone pairs of electrons (non-bonding pairs) exhibit stronger repulsion compared to bonding pairs. This is due to their closer proximity to the central atom and the greater spatial volume they occupy. Consequently, lone pairs can push the bonding pairs closer together, distorting the overall molecular shape. For example, in water ($\text{H}_2\text{O}$), there are two bonding pairs and two lone pairs associated with the oxygen atom. While the presence of four electron pairs might suggest a tetrahedral shape, the stronger repulsion from the lone pairs causes the molecular shape to be bent or V-shaped.

In conclusion, electron pair repulsion is a fundamental principle for understanding molecular geometry. It clarifies why molecules adopt specific shapes, as electron pairs arrange themselves to minimize repulsion by maximizing their separation. Additionally, the type of electron pair—whether bonding or lone—significantly influences the final shape of the molecule. Grasping this concept is vital for predicting the structures and properties of molecules in chemistry.