A weak bond and strong ones
A fluorine atom has nine protons and seven outer electrons, one short of the full shell of neon. It is the smallest of the halogens and the most electronegative element there is, 3.98 on Pauling's scale: no atom takes electrons more readily. Two atoms make F₂, a pale yellow gas, but the bond between them is oddly weak, about 159 kJ per mole, because the small atoms sit so close that their filled shells repel each other. An easily broken bond, and bonds to everything else that are the strongest of their kind: hence the most reactive element known.
Hydrogen explodes in it even in the dark; sulphur, phosphorus and many metals catch fire; glass, sand and water burn in a stream of it; and the first compounds of a noble gas, made from xenon in 1962, all contained fluorine. Yet the bond fluorine makes with carbon, about 485 kJ per mole, is so strong that fluorocarbons are among the most inert substances ever made.
Locked in minerals
Nothing so reactive can exist free in nature; every atom of fluorine on Earth is bound. The crust holds about 0.06 % by mass, mostly as fluorite, CaF₂, also called fluorspar, and as fluorapatite, the phosphate of most phosphate rock and, in a slightly different form, of bones and teeth. Cryolite, Na₃AlF₆, once mined in Greenland, was the third ore.
Fluorite heated with sulphuric acid gives hydrogen fluoride, and from that comes almost every fluorine compound industry uses. The element itself is still made as Moissan made it, by electrolysing potassium fluoride dissolved in liquid hydrogen fluoride, and is stored in nickel vessels, whose surface fluoride protects the metal beneath.
From toothpaste to the bomb
Fluorine became an industrial element in the Second World War, for one purpose: uranium hexafluoride, the only compound of uranium that turns to gas a little above room temperature, was what the Manhattan Project needed to separate uranium-235 from uranium-238. The plants built for it made the first tonnes of fluorine, and the seals that resisted it were cut from a polymer found by accident at DuPont in 1938 — polytetrafluoroethylene, later Teflon.
Chlorofluorocarbons were the refrigerants and aerosol propellants of the mid-century until they were found to be destroying stratospheric ozone; the Montreal Protocol of 1987 phased them out. A fluoride ion swapped into the apatite of tooth enamel makes it resist acid, hence the fluoride in toothpaste and in many water supplies. About a fifth of modern drugs contain fluorine, because the C–F bond stops the body breaking the molecule down too soon; and the fluorinated surfactants called PFAS persist in water and blood so long that they are known as forever chemicals.
Discovery and name
Fluorspar had been used by smelters since at least the sixteenth century, when Georgius Agricola described it as a flux that made ores flow; the Latin fluere gave the mineral its name and later the element's. Scheele made hydrofluoric acid from it in 1771, and in 1810 André-Marie Ampère wrote to Humphry Davy that the acid must contain an unknown element analogous to chlorine, and later suggested the name.
Isolating it took seventy-six years and cost lives, since nothing would take fluorine from its compounds and every attempt at electrolysis ended with the gas attacking the apparatus or the chemist: Davy was made ill by hydrogen fluoride, the Knox brothers were invalided, Paulin Louyet and Jérôme Nicklès died of it. On 26 June 1886 Henri Moissan, in Paris, electrolysed potassium hydrogen fluoride in anhydrous hydrogen fluoride at −50 °C, in a platinum U-tube stoppered with fluorite, and collected a gas that set silicon alight. He received the Nobel Prize in 1906.