The VSEPR stands for ¡°Valence Shell Electron Pair Repulsion¡±, the theory is used for predicting the structure of molecules. The theory can be used in two main steps. First, the number of all bonding pairs and lone pairs is calculated, next, these pairs of electrons are rearranged in order to minimize the repulsion between them.
Different number of bonding pairs and lone pairs of electrons can form different structure. When there are only two bonding pairs around the central atom, for instance, the structure is likely to be linear, where the angle between the two bonds is 180¡ã. When there are three bonding pairs and no lone pairs of electrons, the shape of this molecule is called trigonal planar, where the angle between each bond is 120¡ã. When it comes to more than three pairs, for example four pairs of electrons, the shape becomes 3-Dimensional, the shape of a molecule which has four bonding pairs around the central atom is called tetrahedral, where the angle between each bond is now 109.5¡ã, in order to minimize the repulsion between the electrons. However, when lone pairs of electrons are involved in the molecule, the shape changes slightly, this is because that the repulsion between a lone pair and a bonding pair is greater than bonding pair and bonding pair. In the a molecule of water, for example, two lone pairs and two bonding pairs are around the oxygen, therefore, the angle between each bond is 104¡ã due to the greater repulsion between the two lone pairs. Also in the molecules of ammonia (NH3), since there is one lone pair around the nitrogen, the angle between each bond is 107¡ã.
In conclusion, the repulsion between lone pair and lone pair is usually greater than it between lone pair and bonding pair, and is greater than bonding pair and bonding pair. It is the repulsion between the electrons caused the different shapes of the molecules.