Fluorine was the most difficult halogen to isolate because it is so chemically reactive. H. Moissan first isolated elemental fluorine in 1886, more than seventy years after the first attempts. Moissan received the 1906 Nobel Prize for Chemistry for this work. The technique that he developed, electrolysis of potassium fluoride in anhydrous liquid hydrogen fluoride, is still used today, with some modifications. The name fluorine comes from the mineral fluorspar, or calcium fluoride, in which it was found. Fluorspar also provided the term "fluorescence," because the mineral gave off light when it was heated. Hydrofluoric acid has been used since the 1600s to etch glass. However, it, as well as fluorine, must be handled with care because it causes painful skin burns that heal very slowly. Fluorine and fluoride compounds are toxic.
Fluorine is so reactive that it forms compounds with the noble gases, which were thought to be chemically inert. Fluorine compounds have been extremely important in the twentieth century. Uranium for the first atomic bomb and for nuclear reactors was enriched in the 235 isotope, as compared to the more abundant 238 isotope, by gaseous diffusion. Molecules of a uranium atom with six fluorine atoms exist as a gas. Less massive gases will pass through a porous barrier faster than more massive ones. After passage through thousands of barriers the uranium hexafluoride gas was substantially enriched in the 235 isotope.
Fluoride ions in low concentrations have been shown to prevent cavities in teeth. Toothpaste may contain "stannous fluoride," and municipal water supplies are often fluoridated. However, too high a concentration of fluoride will cause new permanent teeth to have enamel that is mottled. Chlorofluorocarbons were developed and used as refrigerants, blowing agents for polyurethane foam, and propellants in spray cans. Their use became widespread because they are chemically inert. Once the active fluorine is chemically bound the resulting molecule is generally stable and unreactive. The polymer polytetrafluoroethylene is made into Teflon, a non-stick coating.