atom
A polyatomic ion is an ion composed of more than one atom. For example, the ammonium ion consists of one nitrogen atom and four hydrogen atoms. Together, they comprise a single ion with a 1+ charge and a formula of NH 4 + . The carbonate ion consists of one carbon atom and three oxygen atoms, and it carries an overall charge of 2. The formula of the carbonate ion is CO 3 2- . The atoms of a polyatomic ion are tightly bonded together, so the entire ion behaves as a single unit. Figure below shows several models, and Table below lists many of the most common polyatomic ions.