Fluorine

Fluorine [ˈfluːoːɐ̯] is a chemical element with the symbol F and atomic number 9. In the periodic table it is in the 7th main group and thus belongs to the 17th IUPAC group, the halogens, of which it is the lightest. Under normal conditions it exists in gaseous form as the diatomic molecule F2 and is the most reactive of all elements. It reacts with all elements except the noble gases helium and neon. Fluorine is colourless and appears pale yellow when strongly compressed. It is the most electronegative of all elements and always has the oxidation state -1 in compounds with other elements.

The name of the element is derived from the Latin fluores. This term designated the most important naturally occurring mineral fluorite (fluorspar), which is used in metallurgy as a flux to lower the melting point of ores (in the original context: lapides igni liquescentes (fluores)).

Elemental fluorine is very toxic and highly corrosive. Even in low concentrations, its pervasive odor can be noticed. Its salts (fluorides and various fluorocomplex salts such as sodium monofluorophosphate) are also very toxic in higher concentrations, but are administered in traces for the prophylaxis of dental caries. For this reason (fluorine and fluorides are involved in the formation of bones and teeth) they are sometimes added to drinking water or table salt (fluoridation).

History

The first fluorine salt described was the naturally occurring calcium fluoride (fluorspar). It was described by Georgius Agricola in 1530 and mentioned by him in 1556 as an aid to smelting ores. It makes ore melts and slags more fluid, makes them flow (flux).

In 1771, Carl Wilhelm Scheele began to study fluorspar and its properties in greater detail, as well as the hydrofluoric acid formed from it during acid treatment. He researched the reactions that occur when hydrofluoric acid acts on glass, forming silicon tetrafluoride and fluorosilicic acid. Another property he discovered in fluorspar was fluorescence, which is named after the mineral.

In a letter to the editor of the Philosophical Magazine, signed only with the abbreviation "E.B.", the writer complained in 1808 about what he considered to be an inconsistent approach to the naming of new elements. In an addendum, he proposed the name fluorine for the basic element bound in fluoric acid. In a letter to Humphry Davy dated August 25, 1812, André-Marie Ampère expressed the idea that, as in hydrochloric acid, the radical in hydrofluoric acid was bound to hydrogen (fluorine, sometimes also fluorin, by analogy with chlorine for chlorine). After that, many chemists tried to isolate the element, but this was difficult because of its reactivity and toxicity. On June 26, 1886, Henri Moissan succeeded for the first time in producing and characterizing elemental fluorine. He obtained it by electrolysis of a solution of potassium hydrogen difluoride in liquid hydrogen fluoride at low temperatures in a specially designed apparatus (partly made of fluorspar). For this achievement, Moissan received the Nobel Prize in Chemistry in 1906.

Fluorine production took off during the Second World War, on the one hand due to the development of nuclear weapons in the USA (Manhattan Project), since the isotopic enrichment of ²³⁵ uranium takes place via gaseous uranium hexafluoride (UF6), which is produced with the help of elemental fluorine. On the other hand, I.G. Farben operated a fluorine electrolysis cell in Gottow at that time, the product of which was supposedly intended only for the production of a new incendiary agent (chlorine trifluoride) for incendiary bombs. Whether it would have been possible in Germany at the time to use this fluorine production to enrich ^{235 uranium has been the subject of} controversy.

Occurrence

In the earth's crust, fluorine is a relatively common element with 525 ppm. Due to its reactivity, it is rarely found in nature as an element, but almost exclusively bound as fluoride in the form of some minerals. An exception is stinkspar (a uraniferous fluorite variety) from Wölsendorf, among others, as well as villiaumite, in which small amounts of elemental fluorine are formed by radiolysis, which causes a strong odour due to released fluorine during mechanical processing. Seawater contains little dissolved fluoride because in the presence of calcium, solubility is limited by the formation of sparingly soluble calcium fluoride. The most common fluorine minerals are fluorite CaF2 and fluorapatite Ca5(PO4)_3F. Most of the fluorite is bound in fluorapatite, but this contains only a small mass fraction of fluorine of about 3.8 %. Therefore, fluorapatite is not mined for its fluorine content, but primarily as a source of phosphate. The main source of fluorine and fluorine compounds is fluorite. Larger fluorite deposits exist in Mexico, China, South Africa, Spain and Russia. Fluorite is also found in Germany, for example in the Wölsendorf mentioned at the beginning.

Another naturally occurring fluorine mineral is cryolite Na3AlF6. The originally important cryolite deposits at Ivigtut on Greenland have been exploited. The cryolite required in aluminium production is now produced chemically.

Fluoride ions also occur in some rare minerals, in which they replace the hydroxide groups. Examples are asbestos and the gemstone topaz Al2SiO4(OH, F)₂, sellaite MgF2 and bastnäsite (La,Ce)(CO3)F. An overview is given in the category:Fluorine mineral.

A few organisms can incorporate fluorides into fluoroorganic compounds. The South African bush gifblaar and other plant species of the genus Dichapetalum can synthesize fluoroacetic acid and store it in their leaves. This serves as a defense against predators, for which fluoroacetic acid is lethal. The poisonous effect is triggered by interruption of the citrate cycle.