Question 7
Which of these is a polyatomic ion — that is, an ion made of more than one atom?
The right answer is the one marked above.
SO42−. The sulfate ion is one sulfur atom and four oxygen atoms bound together, carrying a charge of −2 as a single unit.
Na+, Cl− and Mg2+ are each a single atom that has lost or gained electrons. The number of charges is not what makes an ion polyatomic — Mg2+ carries two charges but is still one atom.