The prefix poly- means many and the prefix mono- means one.
Therefore, polyatomic ions 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.
