Which of the following is a polyatomic ion?

• A. Na^+
• B. Mg^{2+}
• C. NO_3^-
• D. Cl^-

Answer: (C)

Verified step by step guidance

1. Understand that a polyatomic ion is an ion composed of two or more atoms covalently bonded together that carry an overall charge.
2. Examine each option to determine if it consists of a single atom or multiple atoms: Na^+ is a sodium ion (single atom), Mg^{2+} is a magnesium ion (single atom), NO_3^- is the nitrate ion (multiple atoms: one nitrogen and three oxygens), and Cl^- is a chloride ion (single atom).
3. Recognize that ions like Na^+, Mg^{2+}, and Cl^- are monatomic ions because they consist of only one atom with a charge.
4. Identify NO_3^- as the polyatomic ion because it contains multiple atoms bonded together and carries a negative charge.
5. Conclude that the correct answer is NO_3^- since it fits the definition of a polyatomic ion.