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 element’s oxyanions has a name with the form (root)ate. These can be memorized from See below. The names of the other possible oxyanions are determined as follows (see below).

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.

Common Polyatomic ions that end in -ate

NO₃^-  nitrate

SO₄^2-  sulfate

PO₄^3-  phosphate

ClO₃^-  chlorate

BrO₃^-  bromate

IO₃^-  iodate

Different number of oxygen atoms

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.

Systematic and Nonsystematic Names for Some Polyatomic Ions  

EXAMPLE 1 - Naming Polyatomic Ions:    Write names that correspond to the following formulas for polyatomic ions:

(1) PO₃^3- (with barium ions in glass)

(2) HSO₄⁻ (in perfumes).

Solution:

1. PO₃³⁻ is phosphite. PO₄³⁻ is phosphate, and this ion has one less oxygen. 

2. HSO₄^- is hydrogen sulfate. This could be called by its nonsystematic name, bisulfate, but it is preferable to use the systematic name, which shows that one H⁺  ion has been added to sulfate, SO₄^2-.

EXAMPLE 2 - Formulas for Polyatomic Ions:    Write formulas that correspond to the following names for polyatomic ions:

(1) bromite ion, used in the production of cloth

(2) dihydrogen hypophosphite ion, used with manganese(II) ions as a food additive.

Solution:

1. Bromite ion is BrO₂⁻. Bromate is BrO₃⁻. Bromite has one less oxygen atom than bromate.

2. Dihydrogen hypophosphite ion is H₂PO₂⁻. Hypophosphite has two less oxygen atoms than phosphate, PO₄³⁻. The dihydrogen part of the name indicates that two H⁺ ions have been added to hypophosphite, PO₂³⁻.  The two H⁺ ions neutralize two of the three minus charges, leaving −1.

Exercise 1 - Naming Polyatomic Ions:    Write names that correspond to the following formulas for monatomic ions: IO₃⁻, used with calcium ions as a dough conditioner, ClO₄⁻, used in jet fuel, and HCO₃⁻, used in antacids.

Exercise 2 - Formulas for Polyatomic Ions:    Write formulas that correspond to the following names for polyatomic ions: sulfite ion (preservative) and hydrogen phosphate ion (in animal feed supplements).