Which of the following polyatomic ions cannot have a Lewis structure with an expanded octet?

• A. PO4^{3-}
• B. SO4^{2-}
• C. ClO4^-
• D. NO3^-

Answer: (D)

Verified step by step guidance

1. Recall that an expanded octet occurs when an atom has more than 8 electrons around it in its Lewis structure, which is possible for elements in period 3 or below because they have d orbitals available for bonding.
2. Identify the central atom in each polyatomic ion and determine its period in the periodic table: P in PO4^{3-} (period 3), S in SO4^{2-} (period 3), Cl in ClO4^- (period 3), and N in NO3^- (period 2).
3. Understand that elements in period 2, like nitrogen (N), cannot have an expanded octet because they lack accessible d orbitals to accommodate extra electrons beyond the octet rule.
4. Therefore, NO3^- cannot have a Lewis structure with an expanded octet, while PO4^{3-}, SO4^{2-}, and ClO4^- can have expanded octets due to their central atoms being in period 3.
5. Conclude that the correct answer is NO3^- because nitrogen is limited to an octet and cannot expand it, unlike phosphorus, sulfur, and chlorine.