Which of the following is a polyatomic ion?

• A. Mg^{2+} (magnesium ion)
• B. NO_3^- (nitrate)
• C. Cl^- (chloride ion)
• D. Na^+ (sodium ion)

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. Examine each option to determine if it consists of multiple atoms or just a single atom:
3. Mg^{2+} is a magnesium ion, which is a single atom with a charge of +2, so it is a monatomic ion.
4. Cl^- is a chloride ion, a single chlorine atom with a charge of -1, so it is also a monatomic ion.
5. Na^+ is a sodium ion, a single sodium atom with a charge of +1, so it is a monatomic ion.
6. NO_3^- (nitrate) consists of one nitrogen atom and three oxygen atoms bonded together, carrying an overall charge of -1, making it a polyatomic ion.