Which of the following is a polyatomic ion: CH3COOH, CR2O7^2-, Na^+, or H2?
Polyatomic ions are ions with charge that are made up of two or more atoms.
#[H_3O]^+# #[SO_4]^(2-)# #[CO_3]^(2-)#
Notice that all of these examples have a charge attached to them. This is what makes them ionic.
Also, all of these ions are made up of more than 2 ions.
Looking through the problem that was given, we have 4 different choices.
First, discard the ones that do not have a charge attached to them.
Second, discard the ones that do not have two or more ions in them.
Finally, you are left with the last choice: