Final answer:
Molecules XeF₄ and SF₆ must be described by a structure that has an expanded octet, as Xenon and Sulfur have the ability to utilize d orbitals beyond the third period. Thus, the answer is d. ii and iii.
Step-by-step explanation:
In determining which molecules must be described by a structure that has an expanded octet, we must consider the elements involved in the molecules and verify if they have the ability to accommodate more than eight electrons in their valence shell.
Typically, nonmetal atoms in the third period (row) of the periodic table or beyond have d orbitals available that can be used to expand the valence shell past the octet rule.
- ClO₂⁻ (Chlorite ion): Chlorine belongs to the third period of the periodic table and can expand its octet when forming certain compounds. However, in the case of ClO₂⁻, it does not necessarily require an expanded octet to satisfy its valence requirements.
- XeF₄ (Xenon tetrafluoride): Xenon is a noble gas with available d orbitals, and XeF₄ is known to utilize an expanded octet structure. Xenon's ability to form compounds with an expanded octet is a notable exception to the octet rule.
- SF₆ (Sulfur hexafluoride): Sulfur is also in the third period and in SF₆, sulfur forms six bonds with fluorine atoms, resulting in an expanded octet structure for the sulfur atom.
Therefore, the molecules that must be described by a structure that has an expanded octet are XeF₄ and SF₆. The correct answer to the student's question is d. ii and iii.