Identify the polyatomic ions from the following.
NH4+
Hydroxide ion
An ion is an atom with positive or negative charge over it. A polyatomic ion is the one which has more than one atom within the ionic group. The formula of hydroxide ion is OH−. O− and H− make it polyatomic. Similarly, NH4+ is also a polyatomic ion.
Note: Don't get confused between the charge on the ion and the atomicity. Here Al3+ and O2− have +3 and -2 charges respectively, but each one is a single atom.