Nonmetallic chemical element , chemical symbol F, atomic number 9.
The lightest halogen , it is the most reactive element, forming compounds with all others except helium, neon, and argon (the lighter noble gas es). Its only valence is 1, in F 2 (the diatomic molecule) and fluorides. A toxic, pale yellow gas with a pungent odour, it can be produced only by electrolysis under special conditions. Its chief source is fluorite ; it also occurs in cryolite, fluorapatite, seawater, bones, and teeth. Hydrogen fluoride (HF) is a raw material for many other fluorides. Its water solution, hydrofluoric acid, is used to clean metals and to polish, etch, or frost glass. Other fluorides are useful catalyst s and raw materials. Sodium fluoride (NaF) is added to water and tin fluoride (SnF 2 ) to dental-care products to reduce dental caries (see fluoridation of water ). Fluorocarbons are hydrocarbon s in which some hydrogen atoms have been replaced by fluorine atoms; examples include Teflon .