The ground state and the excited state outer electronic configurations of phosphorus (Z = 15) are 1s2 2s2 2p6 3s2 3p3 and 1s2 2s2 2p6 3s1 3p3 3d1 respectively.
Now the five orbitals (i.e., one s, three p and one d orbitals) are available for hybridisation to yield a set of five sp3d hybrid orbitals which are directed towards the five corners of a trigonal bipyramidal. It should be noted that all the bond angles in trigonal bipyramidal geometry are not equivalent. In PCl5, the five sp3d orbitals of phosphorus overlap with the singly occupied p orbitals of chlorine atoms to form five P-Cl sigma bonds. Three P-Cl bonds lie in one plane and make an angle of 120° with each other; these bonds are termed as equatorial bonds. The remaining two P-Cl bonds-one lying above and the other lying below the equatorial plane, make an angle of 90° with the plane. These bonds are called axial bonds. As axial bonds suffer more repulsion than equatorial bonds, these are found to be slightly longer and hence slightly weaker than equatorial bonds.