Naming Ions

As a starting point, consider a handful of common polyatomic anions. These show up repeatedly in nature.

some common polyatomic anions
CN-	cyanide
N3-	azide
OCN-	cyanate
C2H3O2-	acetate
C2O4-2	oxylate
CHO2-	formate

Notice the endings for these ions. The names end in either -ide or -ate. These ending are very common in the names of anions. As we will see, the ending can be an important tipoff as to the oxidation number on the central atom in an anion.

Polyatomic oxyanions

The naming of oxyanions looks to be an overhelming task due to the large number of different ions which occur in nature. But fortunately, we have a very systematic method for naming these beasts. If you can simply see and learn the patterns, you will find the task of naming the ions quite simple.

Consider a few common ions found in nature:

Polyatomic Oxyanions Containing:
boron		carbon		nitrogen		oxygen
BO3-3	orthoborate	CO3-2	carbonate	NO3-	nitrate	O2-2	peroxide
BO2-	metaborate	HCO3-	hydrogen carbonate (bicorbonate)	NO2-	nitrite	OH-	hydroxide
B4O7-2	tetraborate
		silicon		phosphorus		sulfur		chlorine
		SiO4-4	orthosilcate	PO4-3	phosphate (orthophosphate)	SO4-2	sulfate	ClO4-	perchlorate
		SiO3-2	metasilcate	PO3-3	phosphite	SO3-2	sulfite	ClO3-	chlorate
		Si2O5-2	disilicate	HPO4-2	hydrogen phosphate	S2O3-2	thiosulfate	ClO2-	chlorite
				H2PO4-	dihydrogen phosphate	HSO4-	hydrogen sulfate (bisulfate)	ClO-	hypochlorite
				aresenic		selenium		bromine
				AsO4-3	orthoarsenate	SeO4-2	selenate	BrO4-	perbromate
				AsO3-	metaarsenate	SeO3-2	selenite	BrO3-	bromate
				HAsO4-2	hydrogen arsenate	HSeO4-	hydrogen selenate	BrO2-	bromite
				H2AsO4-	dihydrogen arsenate			BrO-	hypobromite
						tellurium		iodine
						TeO4-2	tellurate	IO4-	periodate
						TeO3-2	tellurite	IO3-	iodate
								IO2-	iodite
								IO-	hypoiodite

For convenience, the ions have been grouped by the element of the central atom. This is a long list of ions to be sure! But there is a simple pattern to the naming of the ions. Learn the pattern, and the naming of oxyanions will be simple!

Changing the number of oxygen atoms
(changing the oxidation number of the central atom)

Consider the oxyanions of chlorine:

chlorine		whatever
ClO4-	perchlorate	one extra O	per - ate
ClO3-	chlorate	normal # of O	- ate
ClO2-	chlorite	one fewer O	- ite
ClO-	hypochlorite	two fewer O	hypo - ite

So, as an example, you can see that an ion with one extra oxygen atom is signified by adding the prefix "per-" to the name of the ion. This is even aparent in the naming of the ion peroxide! Another common ion in nature is the cyonate ion which has the formula OCN^-.

Addition of H⁺ to an ion

Another important patterns include the addition of an H⁺ to the ion. This doesn't change the oxidation number on the central atom, but the name is changed by adding "hydrogen" to the name. Examples of this pattern can be found in the ions HCO₃^- (hydrogen carbonate) and HAsO₄^-2 (hydrogen arsenate). If two hydrogens are added (H₂PO₄^- for example), the word dihydrogen is added to the name of the ion.

some hydrogen added anions
OH-	hydroxide (short for hydrogen oxide)
HSO4-	hydrogen sulfate (or bisulfate)
HCO3-	hydrogen carbonate (or bicarbonate)
H2PO4-	dihydrogen phosphate

Older references will often use the prefix "bi-" to indicate the adition of an H⁺. To understand this, consider the salts sodium carbonate (Na₂CO₃) and sodium hydrogen carbonate (or sodium bicarbonate - NaHCO₃). In sodium bicarbonate, the ratio of carbonate units to sodium ions is doubled compared to sodium carbonate.

meta- and ortho- prefixes
(Changing the number of oxygen atoms without changing the oxidation number of the central atom)

Some oxyanions (such as those of boron, phosphorus and arsenic) can occur with different numbers of oxygen atoms, but the same oxidation number on the central atom. In these cases, the form with the larger number of oxygen atoms is given the prefix "ortho-" and the form with the smaller number of oxygen atoms is given the prefix "meta-". The pratical difference is that the "meta-" ions generally occur only under anhydrous conditions. And since water is very common on Earth, the "ortho-" forms are more commonly encountered. When no prefix is indicated, it can be safely assumed that the more common "ortho-" form is implied.

Replacement of oxygen with sulfur

Many ions exist which are just like the oxyanions shown above, except that one or more oxygen atoms has been preplaced by sulfur. In the table above, one such example is given - S₂O₃^-2 - which is thiosulfate. The prefix "thio-" means sulfur. One way to think of the thiosulfate ion is a reguar sulfate ion in which one oxygen atom has been replaced by sulfur.

Many other examples of thiosubstituted ions exist in nature. Some examples include:

some thio- ions
AsS4-3	thioarsenate
AsS3-3	thioarsenite
CS3-2	trithiocarbonate
SCN-	thiocyanate

Thiosubstituted ions can be formed under geological conditions where oxygen isn't present. Notice that the pattern of -ate and -ite is retained in the ion name despite the substitution of sulfur.

Replacement of oxygen with fluorine

Another common substitution to find in ions is the substitution of fluorine for oxygen. Because oxygen has an oxidation number of -2 and fluorine has only an oxidation number of -1 in compounds, it generally takes two fluorine atoms per oxygen in the ion. With that in mind, the naming pattern is very similar.

some fluo- ions
SiF6-2	fluosilicate
BF4-	fluoborate
PF6-	fluophosphate
GeF6-2	fluogermanate

Potassium fluosilicate is also know as the mineral hieritite. And potassium fluoborate is known as the mineral avogadrite. Many minerals have common names which are used by geologists. The geologist's names for ionic compounds sometimes seem more confusing that the chemist's names for the compounds, but keep in mind that geologists are often concerned not only with composition, but also crystaline structure.

Polymeric oxyanions

Some oxyanions are built from repeating structural units. They are common with central atoms which form neutral polymeric structures with oxygen linkages such as SiO₂. Notice that in the polyatomic anions with an -ate ending, the central atoms ahve the same oxidation number as in the regular -ate ions. Consider some examples:

polymeric oxyanions
Si2O5-2	disilicate
B4O7-2	tetraborate

Disilicate has a structure which looks like this:

Oxyanions with metal centers

Several oxyanions occur in nature where the central atom is a metal atom. Some examples of these are:

metal centered oxyanions
WO4-2	tungstate
MnO4-2	manganate
MnO4-	permanganate
CrO4-2	chromate
Cr2O7-2	dichromate
MoO4-2	molybdate
AuO2-	aurate

Other polyatomic anions with metal centers

Many metal atoms can form anion complexes with chloride, cyanide or other species. Naming these species is particularly simple! Once again, it centers around the oxidation number on the central atom. A Greek prefix is used to give the number of ligand species in the ion and a roman numeral is used to indicate the oxidation number on the central atom. (CN^- as a ligand is treated as an atom with and oxidation number of -1.) Sometimes these ions are not anions at all but have positive charges. These are typically ions which are complexed with ammonia (treated as an atom with an oxidation number of 0) or with insufficient negatively charged ligands to produce a nuetral species. A few examples will be helpful:

metal complex anions and cations
Fe(CN)6-4	hexacyanoferrate (II)
Fe(CN)6-3	hexacyanoferrate (III)
Cu(CN)4-3	tetracyanocuprate (I)
NiCl4-2	tetrachloronicklate (II)
Cu(NH3)4+2	tetramincopper (II)
Ag(NH3)2+	diaminsilver (I)

A Note in Closing

Many, many other anions are found in nature. Only a handful are shown in this page to give an introduction to the patterns used in naming these ions. Keep these patterns in mind as you study your text and encounter ions in the world around you!

Name the ion
SO4-2 borate carbonate nitrate phosphate sulfate	PO4-3 borate carbonate nitrate phosphate sulfate	BO3-3 borate carbonate nitrate phosphate sulfate	NO3- borate carbonate nitrate phosphate sulfate	CO3-2 borate carbonate nitrate phosphate sulfate
BrO4- perbromate bromate bromite hypobromite bromide	BrO- perbromate bromate bromite hypobromite bromide	BrO2- perbromate bromate bromite hypobromite bromide	BrO3- perbromate bromate bromite hypobromite bromide	Br- perbromate bromate bromite hypobromite bromide
SO3-2 sulfate sulfite hydrogen sulfate thiosulfate fluosulfite	SF6-2 sulfate sulfite hydrogen sulfate thiosulfate fluosulfite	S2O3-2 sulfate sulfite hydrogen sulfate thiosulfate fluosulfite	SO4-2 sulfate sulfite hydrogen sulfate thiosulfate fluosulfite	HSO4- sulfate sulfite hydrogen sulfate thiosulfate fluosulfite
PO3-3 orthophosphate metaphosphate phosphite thiophosphate fluophosphate	PO3- orthophosphate metaphosphate phosphite thiophosphate fluophosphate	PF6- orthophosphate metaphosphate phosphite thiophosphate fluophosphate	PS3- orthophosphate metaphosphate phosphite thiophosphate fluophosphate	PO4-3 orthophosphate metaphosphate phosphite thiophosphate fluophosphate
WO4-2 tungstate aurate plumbate stanate osmate	PbO3-2 tungstate aurate plumbate stanate osmate	SnO3-2 tungstate aurate plumbate stanate osmate	OsO4-2 tungstate aurate plumbate stibate osmate	AuO2- tungstate aurate plumbate stibate osmate