Naming Polyatomic Ions

Some of the names and charges of common polyatomic cations and anions are listed in Table IV.

Table IV

Notice that there are a lot more polyatomic anions than cations. Most polyatomic anions, consist of a nonmetallic element combined with different numbers of oxygen atoms (these polyatomic anions are called oxoanions.) Even though it seems there is no simple rule in naming these ions, in fact, here are some guidelines to follow:

• When an element forms two different oxoanions, the ion with the lesser number of oxygen atoms ends with –ite and the one with more oxygen atoms ends with –ate. Examples are the ions in blue in Table IV.
• When an element forms more than two oxoanions, the prefixes hypo- and per- are used to indicate the one with the fewest number of oxygens and the most number of oxygens, respectively. Examples are the oxoanions of the halogens (in orange in Table IV). Similarly, BrO_4^- is called perbromate ion and IO^- is called hypoiodite ion.
• When H⁺ is added to an oxoanion, the name of the hydrogen containing polyatomic anion begins with the word 'hydrogen' or 'dihydrogen'. An older but still commonly used naming system is to add the prefix bi- to denote the presence of hydrogen. Examples are the ions in green in Table IV.

It should be noted that the acetate and oxalate ions (in purple) are organic ions. They follow the naming system of organic compounds. They are included for reference here, as they are commonly used in Chem 111A, 112A, 151 and 152.

Elements in the same group of the periodic chart have similar chemical properties. Very often, they form similar polyatomic ions. One can draw analogy in the names and formulas of the polyatomic ions of elements in the same group. For example, if one knows that chlorate ion is ClO_3^- , then, an educated guess for the formula of bromate ion is BrO_3^- and for iodate ion is IO_3^- .