Rules for Naming Monatomic Ions Positive ions--Cations 3.Cations formed from nonmetal atoms have names that end in -ium NH:ammonium ion HO:hydronium ion4 3 : NH ammon ion : H O hydro ium n n ium io   Positive ions--Cations 3. Cations formed from nonmetal atoms have names that end in -ium Rules for Naming Monatomic Ions