Patterns in polyatomic ion names:

• A few simple polyatomic anions have names ending in -ide.

  OH−   hydroxide ion

  CN−   cyanide ion

   
• Polyatomic cations formed from nonmetal atoms have names ending in -ium.

  NH4+   ammonium ion

  H3O+   hydronium ion

   
• Polyatomic anions containing oxygen have names ending in -ate or -ite.
   
  • The ending -ate is used for the most common ion form.

    NO3−   nitrate ion

    SO4−2   sulfate ion

     
  • The ending -ite is used for the ion that has one less oxygen atom.

    NO2−   nitrite ion

    SO3−2   sulfite ion

     
  • The prefix per- indicates one more oxygen atom than the form ending in -ate.
     
    • ClO₄⁻   perchlorate ion
     
  • The prefix hypo- indicates one less oxygen atom than the form ending in -ite.
     
    • ClO⁻   hypochlorite ion
   
• Polyatomic anions derived by adding H⁺ to an oxyanion are named by adding as a prefix the word hydrogen or dihydrogen, as appropriate.
   
  • HCO₃⁻   hydrogen carbonate ion