Which of the following compounds contains a polyatomic ion?

• A. NaNO_3
• B. MgO
• C. KCl
• D. CaF_2

Answer: (A)

Verified step by step guidance

1. Understand what a polyatomic ion is: it is a charged species (ion) composed of two or more atoms covalently bonded, that acts as a single ion in chemical reactions.
2. Look at each compound and identify the ions present. For example, NaNO_3 consists of Na^+ and NO_3^- ions.
3. Recognize that NO_3^- (nitrate) is a common polyatomic ion, meaning NaNO_3 contains a polyatomic ion.
4. Check the other compounds: MgO contains Mg^{2+} and O^{2-} ions, both monatomic; KCl contains K^+ and Cl^- ions, both monatomic; CaF_2 contains Ca^{2+} and F^- ions, both monatomic.
5. Conclude that among the given options, only NaNO_3 contains a polyatomic ion (NO_3^-).