What is a polyatomic ion?

The prefix poly- means many and the prefix mono- means one.

Therefore, polyatomic ions are composed of many atoms.

Examples include: #NO_3^(-1)# and #PO_4^(-3)#.

The nitrate ion #NO_3^(-1)# is composed of one nitrogen atoms and three oxygen atoms arranged with an imbalance of one extra electron.

The phosphate ion #PO_4^(-3)# is composed of one phosphate ion and four oxygen atoms arranges with an imbalance of three extra electrons.

Most polyatomic ions are negative with the main exceptions being #NH_4^(+1)# and #H_3O^(+1)#. The ammonium ion which is composed of one nitrogen and 4 hydrogen with an imbalance of one less electron. The hydronium ion is created when water takes on a dissociated #H^+# ion in an acid solution.

Monatomic ions are single atoms with a positive or negative charge like Calcium #Ca^(+2)# having two less electrons and Chloride #Cl^(-1)# having one more electron.

I hope this was helpful.
SMARTERTEACHER