The ground state and the excited state outer electronic configurations of phosphorus (Z = 15) are represented below. P (ground state)
sp3d hybrid orbitals filled by electron pairs denoted by five Cl atoms.
Now the five orbitals (i.e., one s, three p and one d orbitals) are available for hybridization to yield a set of five sp3d hybrid orbitals which are directed towards the five corners of a trigonal bipyramid.
As the axial bond pairs suffer more repulsive interaction from the equatorial bond pairs, axial bonds have been found to be slightly longer than the equatorial bonds.
Trigonal bipyramidal geometry of PCl5 molecule.