VSEPR theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory helps us predict the geometry of individual molecules. The theory argues that valence electrons want to spread out as much as possible around a central atom. This spread happens because electrons repel each other. Electrons that do not participate in bonding, i.e. lone pairs, repel other electrons more than electrons used for bonding. As a result, lone pair electrons tend to skew the structure to maximize the lone pair electron distance, even if it brings bonding pair electrons closer. Let's see how the VSEPR theory predicts the geometry of the water molecule.

Without VSEPR theory, the structure of water would be straight, i.e. with an 180-degree angle formed by the oxygen atoms. However, when we apply the VSEPR theory, considering electron repulsion, the structure bends. You can see this difference in the following figure.

Figure 1: The lewis structure of a water molecule, with and without VSEPR theory applied.