Five Pairs of Electrons

PCl₅ has three chlorine atoms around a central phosphorus atom arranged in a plane 120° apart and two atoms above and below the plane with a bond angle of 90° This shape is known as trigonal bipyramidal.The octet rule and expanded octet rule determines how many valence electrons there are. In the PCl₅ molecule, the central phosphorus atom is bonded to five Cl atoms, thus having 10 bonding electrons and violating the octet rule (trigonal bipyramidal).Some of the third-period elements (Si, P, S, and Cl) bond to more than four other atoms because for n=3, the d sublevel exists, and it has five d orbitals.Although the energy of empty 3d-orbitals is ordinarily higher than that of the 4s orbital, that difference is small and the additional d orbitals can accommodate more electrons. Therefore, the d orbitals participate in bonding with other atoms and an expanded octet is produced.