How are polyatomic ions similar?

Polyatomic ions are ions which consist of more than one atom. For example, nitrate ion, NO3-, contains one nitrogen atom and three oxygen atoms. The atoms in a polyatomic ion are usually covalently bonded to one another, and therefore stay together as a single, charged unit.

Explanation:

A polyatomic ion has two or more covalently bonded atoms that act as a single ion. The polyatomic ion forms ionic bonds with other ions and acts externally as a unit, just like monatomic ions.