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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 element’s 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

EXAMPLE 1 - Naming Polyatomic Ions:    Write names that correspond to the following formulas for polyatomic ions: (1) PO₃³⁻ (with barium ions in glass) and (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, SO₄²⁻.  

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 and (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).