Which of the following is classified as a polyatomic cation?

• A. NO_3^-
• B. SO_4^{2-}
• C. CO_3^{2-}
• D. NH_4^+

Answer: (D)

Verified step by step guidance

1. Understand that a polyatomic ion is a charged species composed of two or more atoms covalently bonded together that carry an overall charge.
2. Identify the charge and composition of each given ion: \(\mathrm{NO_3^-}\) (nitrate), \(\mathrm{SO_4^{2-}}\) (sulfate), \(\mathrm{CO_3^{2-}}\) (carbonate), and \(\mathrm{NH_4^+}\) (ammonium).
3. Note that the first three ions (\(\mathrm{NO_3^-}\), \(\mathrm{SO_4^{2-}}\), \(\mathrm{CO_3^{2-}}\)) are all negatively charged polyatomic ions (anions).
4. Recognize that \(\mathrm{NH_4^+}\) (ammonium) is a polyatomic ion with a positive charge, making it a polyatomic cation.
5. Conclude that among the options, \(\mathrm{NH_4^+}\) is the only polyatomic cation, while the others are polyatomic anions.