VSEPR (Valence Shell Electron Pair Repulsion) theory is a fundamental concept in chemistry that helps predict the shapes of molecules and ions. It provides a simple yet powerful model for understanding the three-dimensional arrangement of atoms in a molecule based on the repulsion between electron pairs.

The theory is based on the idea that electron pairs, whether they are bonding or non-bonding, repel each other and strive to be as far apart as possible. This repulsion determines the overall molecular geometry. By considering the number of electron pairs around the central atom, chemists can determine the molecular shape.

According to VSEPR theory, the first step is to determine the Lewis structure of the molecule, which shows the arrangement of atoms and the bonding and non-bonding electron pairs. The number of electron pairs is then used to predict the geometry.

The basic principles of VSEPR theory are as follows:

• Electron pairs, whether bonding or non-bonding, repel each other and tend to adopt positions that minimize repulsion.

• Multiple electron pairs are treated as individual pairs and are considered in determining the molecular shape.

• The repulsion between electron pairs is ranked in the following order: non-bonding pair (lone pair) - lone pair > non-bonding pair - bonding pair > bonding pair - bonding pair.