Which of the following statements correctly identifies the sulfate ion?

• A. SO4 2-
• B. NO3-
• C. CO3 2-
• D. PO4 3-

Answer: (A)

Recognise a polyatomic ion by its formula and overall charge. The sulfate ion has four oxygen atoms coordinated to a sulfur atom and an overall charge of 2−, so its formula is SO4 2−. The sulfur is in a high oxidation state (+6) and the four oxygens create resonance structures, but the key identifying features are four oxygens and the −2 overall charge. The other ions are different: nitrate has three oxygens and a −1 charge (NO3−), carbonate has three oxygens and a −2 charge (CO3 2−), and phosphate has four oxygens and a −3 charge (PO4 3−). Therefore, the correct identification is SO4 2−.