Oxyanions



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

Ion Name Ion Name
NO3- nitrate ClO3- chlorate
SO42- sulfate BrO3- bromate
PO43- phosphate IO3- iodate

Table 2:

Relationship General Name Example Name Example Formula
one more oxygen atom than (root)ate   per(root)ate   perchlorate ClO4-  
  (root)ate   chlorate   ClO3-  
one less oxygen atom than (root)ate   (root)ite   chlorite   ClO2-  
two less oxygen atoms than (root)ate   hypo(root)ite   hypochlorite ClO-  

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  

Formula Systematic (Preferred) Name Nonsystematic Name
HCO3- hydrogen carbonate   bicarbonate  
HSO4-   hydrogen sulfate bisulfate
HSO3- hydrogen sulfite   bisulfite

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