Phosphorus forms binary compound with halogen: the trihalides, PX3, and the pentahalides, PX5 where X denotes a halogen atom. In contrast, nitrogen can form only trihalides (NX3). Unlike nitrogen, phosphorus has a 3d sub shell that can be used for valence shell expansion. We can explain the bonding in PCl5 by assuming a hybridization process involving 3s, 3p, and 3d orbitals of phosphorus, that is, an sp3d hybridization. The five sp3d hybrid orbitals also account for the trigonal bi-pyramid geometry of the PCl5 molecule
Phosphorus trichloride is prepared by heating white phosphorus in an atmosphere of chlorine: