How do polyatomic ions differ from monatomic ions?

Hint:The monatomic ion has only one atom that has an ion. In the case of polyatomic ions, there are two or more two atoms present and there is a charge present on the whole group. One can give examples of these types with elaboration.

Complete step by step answer:1) First of all, the main difference between the monatomic ion and polyatomic ions is the number of atoms present in that ion. Now let us see each type as below,
2) Monatomic ions:
The monatomic ions are the ions that possess only one atom in them. Now let us see some examples of the monatomic ion as below,
Ex. ${K^ + }$ ion: It is known as potassium ion which has only one atom potassium. As potassium has only one electron present in its outermost shell, it tends to donate one electron and form a cation.
$A{l^{3 + }}$ ion: The aluminum ion has three valence electrons in its outermost shell which it tends to donate and gain positive charge as the cation.
Other examples are oxide ion ${O^{2 - }}$ and nitride ion ${N^{3 - }}$.
3) Polyatomic ions:
The polyatomic ions are the ions that possess two or more atoms in their ionic structure. Now let us see some examples as below,
$N{H_4}^ + $ ion: The Ammonium ion has two atoms as nitrogen and four hydrogens as its constituents. This ion is formed by the protonation of the ammonia molecule.
$N{O_3}^ - $ ion: The nitrate ion has two atomic constituents as nitrogen and three oxygen atoms. This ion is a common component of fertilizers and explosives.
Some other examples include phosphate ion $P{O_4}^{3 - }$, carbonate ion $C{O_3}^{2 - }$.

Note:
The prefix poly- and mono- means many and single respectively and that can be an easy method to remember the differences. The alkali monoatomic ions are conductive in nature but polyatomic ions are not conductive in nature due to their size. The only exception to a polyatomic conductor is the hydroxide ion $\left( {^ - OH} \right)$ which has the highest conductivity.