Some elements are able to form more than one oxyanion (polyatomic ions that contain oxygen), each containing a different number of oxygen atoms. For example, chlorine can combine with oxygen in four ways to form four different oxyanions: ClO₄^-, ClO₃^-, ClO₂^-, and ClO^-. (Note that in a family of oxyanions, the charge remains the same; only the number of oxygen atoms varies.)

• The most common of the chlorine oxyanions is chlorate, ClO₃^-. In fact, you will generally find that the most common of an elements oxyanions has a name with the form (root)ate. These can be memorized from Table 1 below. The names of the other possible oxyanions are determined as follows (Table 2).

• The anion with one more oxygen atom than the (root)ate anion is named by putting per- at the beginning of the root and -ate at the end. For example, ClO₄^- is perchlorate.

• The anion with one fewer oxygen atom than the (root)ate anion is named with -ite on the end of the root. ClO₂^- is chlorite.

• The anion with two less oxygen atoms than the (root)ate anion is named by putting hypo- at the beginning of the root and -ite at the end. ClO^- is hypochlorite.

Table 1: Common Polyatomic ions that end in -ate

Table 2:

If you memorize that nitrate is NO₃^-, you know that NO₂^- is nitrite, because it has one less oxygen atom than nitrate. If you memorize that iodate is IO₃^-, you know that IO₄^- is periodate, because it has one more oxygen than iodate.

Some polyatomic ions like HCO₃^-, HSO₃^-, and HSO₄^- also have nonsystematic names that are often used (Table 3). You should avoid using these less accepted names, but because many people still use them, you should know them.

Table 3    Systematic and Nonsystematic Names for Some Polyatomic Ions  

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