Which of the following is a common polyatomic anion?

• A. NO_3^- (nitrate)
• B. PO_4^{3-} (phosphate)
• C. NH_4^+ (ammonium)
• D. SO_4^{2-} (sulfate)

Answer: (A)

Verified step by step guidance

1. Understand that a polyatomic ion is a charged species composed of two or more atoms covalently bonded, that acts as a single ion in chemical reactions.
2. Identify the charge and composition of each ion given: NO_3^- (nitrate), PO_4^{3-} (phosphate), NH_4^+ (ammonium), and SO_4^{2-} (sulfate).
3. Recognize that common polyatomic anions are negatively charged ions composed of multiple atoms, such as nitrate (NO_3^-), phosphate (PO_4^{3-}), and sulfate (SO_4^{2-}).
4. Note that ammonium (NH_4^+) is a polyatomic ion but it carries a positive charge, so it is a polyatomic cation, not an anion.
5. Conclude that the common polyatomic anions from the list are NO_3^-, PO_4^{3-}, and SO_4^{2-}, with NO_3^- (nitrate) being a very common example.