Which of the following ionic compounds contains a polyatomic ion?

• A. CaCl2
• B. NaF
• C. CaCO3
• D. FeO

Answer: (C)

Verified step by step guidance

1. Step 1: Understand what a polyatomic ion is. A polyatomic ion is a charged species (ion) composed of two or more atoms covalently bonded, that acts as a single ion in ionic compounds. Examples include carbonate (CO3^{2-}), sulfate (SO4^{2-}), nitrate (NO3^{-}), and ammonium (NH4^{+}).
2. Step 2: Examine each compound to identify the ions present. For example, CaCl2 consists of Ca^{2+} and Cl^{-} ions, both of which are monatomic (single atom) ions.
3. Step 3: Check NaF, which contains Na^{+} and F^{-}, both monatomic ions as well.
4. Step 4: Look at CaCO3, which contains Ca^{2+} and CO3^{2-}. Here, CO3^{2-} is the carbonate ion, a polyatomic ion made of one carbon atom and three oxygen atoms bonded together.
5. Step 5: Finally, FeO contains Fe^{2+} and O^{2-}, both monatomic ions. Therefore, the compound that contains a polyatomic ion is CaCO3.