What is the difference between a polyatomic ion and a monatomic ion?
The prefix "mono" suggests to you that a monoatomic ion consists of only one atom.
Example of monoatomic ions: `Na^(1+), H^+`
The prefix "poly" suggests to you that a polyatomic ion consists of more than only one atom, like 2,3, 4 or more.
Take as example the hydroxyl ion `OH^-` that consists of 2 atomes, one oxygen atom and one hydrogen atom.
The acetate ion `CH_3COO` contains 7 atoms, 2 Carbons, 3 Hydrogens and 2 Oxygens.
Hence, a monoatomic ion contains only one kind of atom while polyatomic ion contains more than one atom.