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An anion [ ˈanioːn ] (pronounced: An-ion; from ancient Greek ἀνἰόν anión , German 'das Aufsteigende' , present participle active neuter to ancient Greek ἀνιέναι aniénai , German ' to ascend' ) describes a negatively charged ion in chemistry . The name refers to the fact that in an electrical field, for example during electrolysis , it always moves towards the positively charged anode . Anions are formed from atoms or molecules by accepting electrons or releasing hydrogen ions H + ( protons ).

Anions, e.g. B. Oxide (O 2− ) or sulfide anions (S 2− ) can form a variety of compounds when combined with different cations . Therefore, whole 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 always consist of cations and anions, according to which they can be divided into the corresponding substance classes such as sulfates , carbonates , chlorides , etc. The term sulphate has two aspects in this context: it stands for a single sulphate ion on the one hand, but also for the sulphate class of compounds on the other.

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 any atom or molecule can be negatively charged under suitable conditions, there are many possible anions. The following list therefore only contains anions from which important classes of substances are derived.

Acid residue ions

Acid residue ions or acid residues are derived from the respective acids . According to the following scheme, an acid H n A releases protons and an acid residue A n− is formed :

This scheme describes a dissociation and not a reaction in a solution.

In addition to completely deprotonated ions such as S 2− , SO 4 2− and CO 3 2− , incompletely deprotonated ions such as HCO 3 - are also referred to as acid residues in the literature.

Simple element anions

According to the rules of chemical nomenclature, the name of such anions ends, mainly in binary, i.e. H. Find only two-element connections on -id . When naming these compounds, e.g. B. sodium chloride , the cationic part is always mentioned first, while the anionic part is always mentioned last.

Anions of the 7th main group

Anions of the 6th main group

Anions of the 5th main group

Anions of the 4th main group

Anions of the 3rd main group

Anions of the 1st main group (*)

Other element anions

(*) No stable element anions are known from the second main group, only a few intermetallic phases with more electropositive metals such as the berylids M x Be y .

Complex anions

Molecular anions containing oxygen

According to the rules of chemical nomenclature, the names of such anions end i. d. Usually on -it or -at , whereby different prefixes such as hypo- ( German under- ) or per- ( German over- ) can be added to differentiate between the various oxidation states of the central atom .   

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

8. Main group

7th main group

6th main group

5th main group

4th main group

3rd main group


More molecular anions

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

Halogen complexes

Organic anions

The following names denote the anion and salt of the specified acid:

More anions

Free electrons can also act as anions. The corresponding salts, which can be crystallized using correspondingly strong complexing agents (more precisely: cryptands ), are called electrides .

Furthermore, superatom anions are also known.

Web links

Wiktionary: Anion  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ Wilhelm Pape , Max Sengebusch (arrangement): Concise dictionary of the Greek language . 3rd edition, 6th impression. Vieweg & Sohn, Braunschweig 1914 ( [accessed on March 6, 2019]; note: here, as is customary in ancient Greek dictionaries, it is not the infinitive, but the 1st person singular present active ancient Greek ἄνειμι áneimi , German , I climb up ' specified).