The prefix poly- means many and the prefix mono- means one.
Therefore, polyatomic ions are composed of many atoms, like NO−13 and PO−34.
The nitrate ion NO−13 is composed of one nitrogen atoms and three oxygen atoms arranged with an imbalance of one extra electron. The phosphate ion PO−34 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 exception being NH+14 the ammonium ion which is composed of one nitrogen and 4 hydrogen with an imbalance of one less electron.
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.
