Editing Polyatomic ion (section)

==Nomenclature of polyatomic anions==
There are several patterns that can be used for learning the nomenclature of polyatomic anions. First, when the prefix ''bi'' is added to a name, a hydrogen is added to the ion's formula and its charge is increased by 1, the latter being a consequence of the hydrogen ion's +1 charge. An alternative to the ''bi-'' prefix is to use the word hydrogen in its place: the anion derived from {{chem2|H+}}. For example, let us consider the carbonate({{chem2|link=carbonate|CO3(2-)}}) ion:

:{{chem2|H+}} + {{chem2|link=carbonate|CO3(2-)}} → {{chem2|link=bicarbonate|HCO3-}},

which is called either bicarbonate or hydrogen carbonate. The process that forms these ions is called [[protonation]].

Most of the common polyatomic anions are [[oxyanion]]s, conjugate bases of [[oxyacid]]s (acids derived from the [[oxide]]s of [[Nonmetal (chemistry)|non-metallic elements]]). For example, the [[sulfate]] anion, {{chem2|auto=yes|SO4(2-)}}, is derived from {{chem2|link=sulfuric acid|H2SO4}}, which can be regarded as {{chem2|link=sulfur trioxide|SO3}} + {{chem2|link=water|H2O}}.

The second rule is based on the [[oxidation state]] of the central atom in the ion, which in practice is often (but not always) directly related to the number of oxygen atoms in the ion, following the pattern shown below. The following table shows the [[chlorine]] [[oxyanion]] family:
{| class="wikitable"
|-
! Oxidation state
| −1
| +1
| +3
| +5
| +7
|-
! Anion name
| [[chloride]]
| [[hypochlorite]]
| [[chlorite]]
| [[chlorate]]
| [[perchlorate]]
|-
! Formula
| {{chem2|Cl-}}
| {{chem2|ClO-}}
| {{chem2|ClO2-}}
| {{chem2|ClO3-}}
| {{chem2|ClO4-}}
|-
! Structure
| [[File:Chloride-ion-3D-vdW.png|50px|The chloride ion]]
| [[File:Hypochlorite-ion-3D-vdW.png|70px|The hypochlorite ion]]
| [[File:Chlorite-ion-3D-vdW.png|70px|The chlorite ion]]
| [[File:Chlorate-ion-3D-vdW.png|70px|The chlorate ion]]
| [[File:Perchlorate-ion-3D-vdW.png|70px|The perchlorate ion]]
|}

As the number of oxygen atoms bound to chlorine increases, the chlorine's oxidation number becomes more positive. This gives rise to the following common pattern: first, the ''-ate'' ion is considered to be the base name; adding a ''per-'' prefix adds an oxygen, while changing the ''-ate'' suffix to ''-ite'' will reduce the oxygens by one, and keeping the suffix ''-ite'' and adding the prefix ''hypo-'' reduces the number of oxygens by one more, all without changing the charge. The naming pattern follows within many different oxyanion series based on a standard root for that particular series. The ''-ite'' has one less oxygen than the ''-ate'', but different ''-ate'' anions might have different numbers of oxygen atoms.

These rules do not work with all polyatomic anions, but they do apply to several of the more common ones. The following table shows how these prefixes are used for some of these common anion groups.

{| class="wikitable"
|-
| [[bromide]]
| [[hypobromite]]
| [[bromite]]
| [[bromate]]
| [[perbromate]]
|-
| {{chem2|Br-}}
| {{chem2|BrO-}}
| {{chem2|BrO2-}}
| {{chem2|BrO3-}}
| {{chem2|BrO4-}}
|-
| [[iodide]]
| [[hypoiodite]]
| [[iodite]]
| [[iodate]]
| [[periodate]]
|-
| {{chem2|I-}}
| {{chem2|IO-}}
| {{chem2|IO2-}}
| {{chem2|IO3-}}
| {{chem2|IO4-}} or {{chem2|IO6(5-)}}
|-
| [[sulfide]]
| [[hyposulfite]]
| [[sulfite]]
| [[sulfate]]
| [[persulfate]]
|-
| {{chem2|S(2-)}}
| {{chem2|S2O2(2-)}}
| {{chem2|SO3(2-)}}
| {{chem2|SO4(2-)}}
| {{chem2|SO5(2-)}} or {{chem2|S2O8(2-)}}
|-
| [[selenide]]
| [[hyposelenite]]
| [[Selenite (ion)|selenite]]
| [[selenate]]
|
|-
| {{chem2|Se2-}}
| {{chem2|Se2O2(2-)}}
| {{chem2|SeO3(2-)}}
| {{chem2|SeO4(2-)}}
|
|-
| [[Telluride (chemistry)|telluride]]
| [[hypotellurite]]
| [[tellurite (ion)|tellurite]]
| [[tellurate]]
|
|-
| {{chem2|Te2-}}
| {{chem2|TeO2(2-)}}
| {{chem2|TeO3(2-)}}
| {{chem2|TeO4(2-)}}
|
|-
| [[nitride]]
| [[hyponitrite]]
| [[nitrite]]
| [[nitrate]]
| [[peroxynitrate|pernitrate]]
|-
| {{chem2|N3-}}
| {{chem2|N2O2(2-)}}
| {{chem2|NO2-}}
| {{chem2|NO3-}}
| {{chem2|NO4-}}
|-
| [[phosphide]]
| [[hypophosphite]]
| [[phosphite]]
| [[phosphate]]
| [[perphosphate]]
|-
| {{chem2|P3-}}
| {{chem2|H2PO2-}}
| {{chem|PO|3|3-}}
| {{chem|PO|4|3-}}
| {{chem|PO|5|3-}}
|-
| [[arsenide]]
| [[hypoarsenite]]
| [[arsenite]]
| [[arsenate]]
|
|-
| {{chem|As|3-}}
| {{chem|AsO|2|3-}}
| {{chem|AsO|3|3-}}
| {{chem|AsO|4|3-}}
|
|-
|}

Some oxo-anions can [[dimer (chemistry)|dimer]]ize with loss of an oxygen atom. The prefix ''pyro'' is used, as the reaction that forms these types of chemicals often involves heating to form these types of structures.<ref>{{GoldBookRef|file=P04959|title=pyro}}</ref> The prefix ''pyro'' is also denoted by the prefix ''di-'' . For example, dichromate ion is a dimer.

{|class=wikitable
| [[sulfite]]
| [[pyrosulfite]]
|-
| {{chem|S|O|3|2-}}
| {{chem|S|2|O|5|2-}}
|-
| [[sulfate]]
| [[pyrosulfate]]
|-
| {{chem|S|O|4|2-}}
| {{chem|S|2|O|7|2-}}
|-
| [[phosphite anion|phosphite]]
| [[pyrophosphite]]
|-
| {{chem|P|O|3|3-}}
| {{chem|P|2|O|5|4-}}
|-
| [[phosphate]]
| [[pyrophosphate]]
|-
| {{chem|P|O|4|3-}}
| {{chem|P|2|O|7|4-}}
|-
| [[arsenate]]
| [[pyroarsenate]]
|-
| {{chem|As|O|4|3-}}
| {{chem|As|2|O|7|4-}}
|-
| [[Chromate and dichromate|chromate]]
| [[dichromate]]
|-
| {{chem|CrO|4|2-}}
| {{chem|Cr|2|O|7|2-}}
|-
| [[carbonate]]
| [[dicarbonate]]
|-
| {{chem|CO|3|2-}}
| {{chem|C|2|O|5|2-}}
|-
|selenite
|pyroselenite
|-
|{{chem|SeO|3|2-}}
|{{chem|Se|2|O|5|2-}}
|}