How do polyatomic ions differ from monatomic ions?

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

Therefore, are composed of many atoms, like ##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 exception being ##NH_4^(+1)## 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.

I hope this was helpful. SMARTERTEACHER