The octet rule and expanded octet rule determine 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 (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 3s and 3p orbitals, 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.
Some of the third-period elements (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 3s and 3p orbitals, 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.