Final answer:
PCl5 is an exception to the octet rule due to the presence of three unpaired electrons in the phosphorus atom's 3d orbital, requiring the use of d orbitals to form more than eight bonds. In contrast, PCl3 follows the octet rule by forming three sigma bonds with three chlorine atoms and having a complete valence shell.
Step-by-step explanation:
Phosphorus pentachloride (PCl5) is an exception to the octet rule, while phosphorus trichloride (PCl3) is not because of their different electron configurations. In PCl3, the phosphorus atom forms three sigma bonds with three chlorine atoms, using three of its valence electrons. The remaining two valence electrons form a lone pair, giving the phosphorus atom a total of eight valence electrons, satisfying the octet rule.
On the other hand, in PCl5, the phosphorus atom forms five sigma bonds with five chlorine atoms, using five of its valence electrons. However, this leaves the phosphorus atom with three unpaired electrons in its 3d orbital, exceeding the octet rule. To accommodate the extra electrons, the phosphorus atom utilizes its 3d orbital to form five hybrid orbitals (sp3d), which are involved in the P-Cl bonds.
Therefore, PCl5 is an exception to the octet rule because it requires the use of d orbitals to form more than eight bonds, while PCl3 follows the octet rule, with all the atoms achieving a complete valence shell through the formation of sigma bonds and lone pairs.