Ion

In chemistry, an anion [ˈanioːn] (pronounced an-ion; from the ancient Greek ἀνἰόν anión, German 'das Hinaufsteigende', present participle active neuter to the ancient Greek ἀνιέναι aniénai, German 'to ascend') refers to a negatively charged ion. The term refers to the fact that it always moves towards the positively charged anode in the electric field, for example in electrolysis. Anions are formed from atoms or molecules by the absorption or release of electrons from hydrogen ions H+ (protons).

Anions, e.g. oxide (O2-) or sulfide anions (^S2-), can form a variety of compounds by combining with different cations. Therefore, entire classes of compounds are derived from the individual anions, in this example the oxides and sulfides, which form many ores and rocks and are therefore particularly important in mineralogy and metallurgy.

All salts also always consist of cations and anions, according to which they can be divided into the corresponding substance classes such as sulphates, carbonates, chlorides, etc. The term sulphate therefore has two aspects in this context: On the one hand, it stands for a single sulfate ion, but on the other hand, it also stands for the compound class of sulfates.

For the qualitative identification of anions in solutions, anion detection as well as modern laboratory equipment and methods of instrumental analysis are available in chemistry.

Since almost every atom or molecule can become negatively charged under suitable conditions, there are very many possible anions. Therefore, the following list only includes anions from which important classes of substances are derived.

Simple element anions

According to the rules of chemical nomenclature, the name of such anions, which are mainly found in binary compounds, i.e. compounds consisting of only two elements, ends in -id. When naming these compounds, e.g. sodium chloride, the cationic part is always named first and the anionic part last.

Anions of the 7th main group

Halide X-

Fluoride F-
Chloride Cl-
Bromide Br-
Iodide I-
Astatid At-

Anions of the 6th main group

Hydroxide OH-
Oxide O2-
Hyperoxide O2-
Peroxide O22-
Ozonide O3-
Hydrogen sulphide HS-
Sulphide ^S2-
Disulfide S22-
Selenide Se2-
Diselenide Se22-
Telluride Te2-
Polonide Po2-

Anions of the 5th main group

Nitride N3-
Phosphide P3-
Arsenide As3-
Antimonide Sb3-
Bismutide Bi3-

Anions of the 4th main group

Carbide C4-
Silicide Si4-
Germanid Ge4-

Anions of the 3rd main group

Boride Bx-

Anions of the 1st main group(*)

Hydride H-
Alkalis

Other element anions

Zintl Phases
Auride Au-
Platinide Pt2-

(*) No stable element anions are known from the second main group, only some intermetallic phases with more electropositive metals like the beryllides MxBey.

Complex anions

Oxygen-containing molecular anions

According to the rules of chemical nomenclature, the names of such anions usually end in -it or -at, although various prefixes such as hypo- (German unter-) or per- (German über-) may be added to distinguish the different oxidation states of the central atom.

Another way of designating anions of this type, although rather unusual, is according to the rules of complex chemistry, in which compounds with molecular anions containing oxygen are called oxo compounds and their oxygen atoms are treated as free ligands, so that, for example, sulfites are designated as trioxosulfates, sulfates as tetraoxosulfates, and so on.

8. main group

Hydrogen xenate HXeO4-
Xenat XeO42-
Perxenate XeO64-

7th main group

Hypochlorite ClO-
Chlorite ClO2-
Chlorate ClO3-
Perchlorate ClO4-
Hypobromite BrO-
Bromite BrO2-
Bromate BrO3-
Perbromate BrO4-
Hypoiodite IO-
Iodite IO2-
Iodate IO3-
Periodate IO4-

6. main group

Hydrogen sulphite HSO3-
Sulphite SO32-
Hydrogen sulphate HSO4-
Sulphate SO42-
Thiosulphate S2O32-
Dithionate S2O62-
Peroxodisulphate S2O82-
Selenite SeO32-
Selenate SeO42-
Tellurate TeO42-

5. main group

Hyponitrite NO-
Nitrite NO2-
Nitrate NO3-
Dihydrogen phosphate H2PO4-
Hydrogen phosphate HPO42-
Phosphate PO43-
Arsenate AsO43-
Antimonate SbO43-

4th main group

Hydrogen carbonate HCO3-
Carbonate CO32-
Silicate SiO44-
Germanate GeO44-
Stannate SnO32-

3rd main group

Borate BO33- or B4O72-
Aluminate

Subgroups

Chromate CrO42-
Dichromate Cr2O72-
Molybdate MoO42-
Niobate NbO3-
Permanganate MnO4-
Perrhenate ReO4-
Rhenat ReO42-
Tantalate TaO3-
Technetat TcO42-
Pertechnetate TcO4-
Titanate TiO3-
Wolframat WO42-
Zirconate

Other molecular anions

The rules of chemical nomenclature are inconsistent here - the name of some of these anions ends in -id, as with element anions, while the name of others ends in -at:

Cyanate OCN-
Thiocyanate SCN-
Cyanide CN-
Amide NH2-
Azide N3-

Halogen complexes

Tetrachloroaurate AuCl4-
Tetrachloroaluminate AlCl4-
Tetrafluoroborate BF4-
Hexachlorophosphate PCl6-
Hexachloroplatinate PtCl62-
Hexachloroosmat OsCl62-
Hexachloroiridate IrCl62-
Hexafluoroantimonate SbF6-
Hexafluoroarsenate AsF6-
Hexafluorophosphate PF6-
Hexafluorosilicate SiF62-
Hexafluorotitanate TiF62-
Hexafluorozirconate ZrF62-