Themes > Science > Chemistry > Miscellenous > Help file Index > Naming rules > Polyatomic ion names

Polyatomic ion names are more irregular than the monoatomic ion names. There is a series of rules for ions consisting of a nonmetal and oxygen, but there are a fair number you will just have to remember.

For nonmetals mixed with oxygen(s), the rules are as follows:

  1. If a nonmetal forms two possible anions with oxygen, the one with the largest number of oxygens has the suffix -ate attached to the root of the nonmetal. The one with the fewer number of atoms has the suffix -ite.
    • SO4-2 is the sulfate ion
    • SO3-2 is the sulfite ion
  2. If the nonmetal forms more than two, the prefix per- is attached to the ion with the most oxygens, the prefix hypo- to the one with the fewest.
    • ClO4- is the perchlorate ion
    • ClO3- is the chlorate ion
    • ClO2- is the chlorite ion
    • ClO- is the hypochlorite ion
The rest of the ions are fairly irregular. Some common ones follow:
  • OH-: hydroxide
  • CN-; cyanide
  • SCN-: thiocyanate
  • C2H3O2-: acetate
  • CrO4-2: chromate. (See rules above)
  • Cr2O7-2: dichromate
  • MnO4-: permanganate

Example: What are the names of the NO3- and NO2- ions?

Solution: Here we have two ions that have a nonmetal (N) and oxygen. We use the root of the nonmetal, nitrogen to get nitr-, then attach a -ate to the one with the most oxygens and a -ite to the one with the fewest

  • NO3-: nitrate
  • NO2-: nitrite


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