Which of the following is a polyatomic ion?

• A. NO_3^-
• B. Cl^-
• C. Na^+
• D. O_2

Answer: (A)

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 multiple atoms or a single atom:
3. Cl⁻ is a chloride ion, which is a single atom with a negative charge, so it is a monatomic ion.
4. Na⁺ is a sodium ion, a single atom with a positive charge, also a monatomic ion.
5. O₂ is a molecule made of two oxygen atoms but it is neutral, not an ion.
6. NO₃⁻ (nitrate ion) consists of one nitrogen atom and three oxygen atoms bonded together with an overall negative charge, making it a polyatomic ion.