Define polyatomic ion. Give one example

polyatomic ion, also known as a molecular ion, is a charged species (ion) composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. The prefix poly- means many in Greek, but even ions of two atoms are commonly referred to as polyatomic. In older literature, a polyatomic ion is also referred to as a radical, and less commonly, as a radical group.

For example, a hydroxide ion is made of one oxygen atom and one hydrogen atom: its chemical formula is (OH)−. It has a charge of −1. An ammonium ion is made up of one nitrogen atom and four hydrogen atoms: its chemical formula is (NH4)+. It has charge of +1.

There are many examples, but here are a few:

Acetate : CH3COO-
Nitrate: NO3-
Hydroxide : OH-