Which of the following polyatomic ions has an overall charge of -2?

• A. NO_3^{-} (nitrate)
• B. SO_4^{2-} (sulfate)
• C. PO_4^{3-} (phosphate)
• D. NH_4^{+} (ammonium)

Answer: (B)

Verified step by step guidance

1. Review the charge on each polyatomic ion given in the problem: nitrate (NO_3^{-}), sulfate (SO_4^{2-}), phosphate (PO_4^{3-}), and ammonium (NH_4^{+}).
2. Recall that the superscript on each ion indicates its overall charge. For example, NO_3^{-} has a charge of -1, SO_4^{2-} has a charge of -2, PO_4^{3-} has a charge of -3, and NH_4^{+} has a charge of +1.
3. Identify which ion has the charge of -2 by comparing the charges: look for the ion with the superscript 2-.
4. Confirm that SO_4^{2-} (sulfate) is the ion with the overall charge of -2.
5. Understand that the charge on polyatomic ions is important for balancing chemical equations and forming neutral compounds.