In unit 7 we have studied the formation of molecules through interatomic bonding. However, one question which still lingers in our mind at this stage, is why do different molecules have different shapes. For example, BCI3 and PCI3 are both tetra atomic covalent molecules, yet they have different shapes. BCI3 is trigonal planar and PCI3 has pyramidal shape. In this unit, we shall try to provide answer to the above question by using the concept of Hybridisation of atomic orbitals. Before we look into the details of hybridisation concept we shall have to understand as to how the atoms participating in bonding involve their atomic orbitals for forming the covalent bonds. We shall also study the formation of sigma covalent bonds and the pi covalent bonds.