Which of the following is an example of a polyatomic ion?

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

Answer: (B)

Verified step by step guidance

1. Understand what a polyatomic ion is: it is an ion composed of two or more atoms covalently bonded together that carry an overall charge.
2. Look at each option and identify whether it consists of a single atom or multiple atoms: Mg^{2+} is a magnesium ion (single atom), Cl^- is a chloride ion (single atom), Na^+ is a sodium ion (single atom), and NO_3^- is the nitrate ion (multiple atoms: one nitrogen and three oxygen atoms).
3. Recognize that ions like Mg^{2+}, Cl^-, and Na^+ are monatomic ions because they contain only one atom each.
4. Identify NO_3^- as a polyatomic ion because it contains multiple atoms bonded together and carries a charge.
5. Conclude that NO_3^- is the correct example of a polyatomic ion among the given options.