Final answer:
Expanded-octet molecules cannot be represented by the valence-shell orbital model because this model does not account for the participation of d orbitals in bonding, which allows atoms from the third period and beyond to exceed the traditional octet rule.
Step-by-step explanation:
Expanded-octet molecules cannot be properly represented by the valence-shell orbital model because this model is based on the octet rule, which states that atoms tend to combine in such a way that they each have eight electrons in their valence shell. However, expanded-octet molecules involve elements from the third period or beyond of the periodic table, and these atoms have accessible d orbitals in their valence shells which can participate in bonding, allowing for more than eight electrons.
For example, phosphorus in phosphorous pentachloride (PCl5) can form five covalent bonds, resulting in a valence shell with ten electrons. This ability to utilize d orbitals and extend beyond eight electrons in the valence shell does not conform to the constraints of the valence-shell orbital model that the second-row elements are subject to.
Molecules such as PCl5 demonstrate expanded octets, and though the Lewis structure can depict such scenarios, the valence-shell orbital model cannot easily account for these additional electrons because it does not incorporate the use of d orbital interactions in bonding.