cation

A cation [ ˈkati̯oːn ] (pronounced: Kat-ion; from ancient Greek κατἰόν katión , German 'the descending' , present participle active neuter to ancient Greek κατἰέναι katiénai , German 'going down' ) is a positively charged ion . Since positively charged ions always move to the cathode during electrolysis , the name cations was chosen for them .

These arise from atoms or molecules by releasing electrons or absorbing hydrogen ions H ⁺ ( protons ).

Salts are always composed of cations and anions .

The exchange between different valued cations is described by the Gapon equation . For the chemical detection of cations, modern laboratory devices from instrumental analysis or even simple cation detection in test tubes are used.

Metal ions

Metal ions are simple cations that are formed from metal atoms by donating electrons ( oxidation ).

Examples of metal ions, sorted according to their valency :

univalent (monovalent): K ⁺ , Na ⁺ , Li ⁺ , Cu ⁺
bivalent (divalent): Mg ²⁺ , Ca ²⁺ , Ba ²⁺ , Cu ²⁺ , Fe ²⁺ , Zn ²⁺
trivalent: Al ³⁺ , Fe ³⁺
tetravalent (tetravalent): Pb ⁴⁺

Some metal ions such as iron, zinc or copper ions are important as trace elements in biology . For example, they are cofactors in metalloenzymes .

Compound cations

Examples of compound cations (molecular cations):

NH ₄⁺ ( ammonium ion)
H ₃ O ⁺ ( oxonium ion)
[NO] ⁺ ( nitrosyl cation )

Organic chemistry

Tetramethylammonium chloride {N [CH ₃ ] ₄⁺ Cl ^- } and triethylamine - hydrochloride {HN [C ₂ H ₅ ] ₃⁺ Cl ^- } are examples of organic ammonium salts, in which the nitrogen atom also has four binding partner, this, however, organic radicals or are partly also hydrogen atoms, as in hydrochlorides, hydrobromides or hydroiodides .

The charge of simple cations

Na → Na ⁺ + 1e ^-

A single positively charged sodium ion is created from the sodium atom by releasing an electron. Because there is less negative charge in the atom, the positive charge predominates.

Mg → Mg ²⁺ + 2e ^-

By releasing two electrons, a magnesium atom becomes a double positively charged magnesium ion.

Al → Al ³⁺ + 3e ^-

After releasing 3 electrons, the aluminum atom becomes a triple positively charged aluminum ion.

The charge of the metal ions results from the electron configuration (distribution of the electrons in the atomic shell). The aim of the emission of electrons is to achieve the same number of electrons as a noble gas ( noble gas configuration ). The number of electrons to be emitted depends on the number of outer electrons , which can be read off in the periodic table for the main group elements from their main group number.

Metal hydroxocations

In aqueous solutions, metal cations are either hydrated or, depending on the pH and type of metal cations and charge number, they form hydroxo complexes or isopolyoxocations with the general formula [M _x O _u (OH) _v (H ₂ O) _w ] ^{n +} , where not all types of ligands (H ₂ O, OH ^- , O ²⁻ ) must be bound. Cations with a smaller number of charges preferably form aqua (outdated aquo) and hydroxo complexes, cations with a higher number of charges preferably form oxo complexes. Several metal cations in the complex are bound by oxygen bridges. At a high pH, these complexes can react to form hydroxides that precipitate out of solution, or they form negatively charged hydroxo complexes ( e.g. aluminates ).

literature

AF Holleman , E. Wiberg , N. Wiberg : Textbook of Inorganic Chemistry . 91st – 100th, improved and greatly expanded edition. Walter de Gruyter, Berlin 1985, ISBN 3-11-007511-3 .
Hans Rudolf Christen: Fundamentals of general and inorganic chemistry. 8th edition. Otto Salle Verlag, Frankfurt am Main / Berlin / Munich 1985, ISBN 3-7935-5394-9 .
KH Tytko: Isopolyoxokationen - metal cations in aqueous solution, chemistry in our time , 13th year 1979, pp 184-194, ISSN 0009-2851 .

Individual evidence

^ Wilhelm Pape , Max Sengebusch (arrangement): Concise dictionary of the Greek language . 3rd edition, 6th impression. Vieweg & Sohn, Braunschweig 1914 ( zeno.org [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 κάτειμι káteimi , German , I go down ' specified).

[1] Wilhelm Pape , Max Sengebusch (arrangement): Concise dictionary of the Greek language . 3rd edition, 6th impression. Vieweg & Sohn, Braunschweig 1914 ( zeno.org [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 κάτειμι káteimi , German , I go down ' specified).