Negatively charged ions are referred to as anions, as they migrate to the anode during electrolysis. Anions are formed by the absorption of an electron.

An anion is a negatively charged ion. It always moves in the direction of the positively charged anode in an electric field, e.g. during electrolysis. Anions are formed from atoms or molecules by accepting or releasing electrons from hydrogen ions H+.

Anions, e.g. oxides (O2-) or sulphide anions (S2), can form a variety of compounds by combining with different cations. Entire classes of compounds are therefore derived from the individual anions, in this example the oxides and sulphides, 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 sulphate ion, but on the other hand it also stands for the substance class of sulphates.

As almost every 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 substance classes are derived.

Acid residue ions or acid residues are derived from the respective acids. According to the following scheme, an acid HnA releases protons and an acid residue An- is formed:

In the literature, completely deprotonated ions (S2-, SO42- and CO32-) are referred to as acid residues, while incompletely deprotonated ions such as HCO3- are referred to as base residues.

The term dissociation is used to describe a reaction in which a compound in solution is split into its individual ions. This scheme describes a dissociation and not a reaction in a solution.