What are polyatomic ions? Give examples.

Hint: There are many types of ions like monatomic ion, diatomic ion, polyatomic ion, etc. The mono, di, and poly tell the number of atoms in the ion. $NO_{3}^{-}$ is a nitrate ion that is regarded as a polyatomic ion.

Complete answer:
Ions are those molecules or compounds that have either a positive or negative charge. They are classified as monatomic in which only one atom is present with either positive or negative charge, diatomic in which two atoms are present, and polyatomic in which more than two atoms are present.
So, polyatomic elements can be regarded as those ions in which more than two atoms are present with either positive or negative charge.
Some examples are:
$NO_{3}^{-}$, this is nitrate ion and it is regarded as a polyatomic ion because there is one nitrogen and three oxygen atoms and it has a negative charge.
$SO_{4}^{2-}$, this is sulfate ion and it is regarded as a polyatomic ion because there is one sulfur and four oxygen atoms and it has a -2 charge.
$NH_{4}^{+}$, this is ammonium ion or ammonium cation and it is regarded as a polyatomic ion because it has one nitrogen and four hydrogen atoms and it has a positive charge.
${{C}_{2}}{{H}_{3}}O_{2}^{-}$, this acetate ion, and it is regarded as polyatomic ion because it has two carbon, three hydrogen, and two oxygen atoms and it has a negative charge.

Note:
In monatomic ions only one atom is present like $N{{a}^{+}},\text{ }{{F}^{-}},\text{ C}{{\text{l}}^{-}}$, etc and in diatomic ions there are two atoms present like $C{{N}^{-}},\text{ O}{{\text{H}}^{-}},\text{ O}_{2}^{-}$. The polyatomic ions are used as ligands in coordination compounds.