Fluorine is a chemical element (symbol F), of atomic number 9, atomic mass 18.99, the first element in the halogen group. A pale yellow gas of density 1.696 g/litre (0°C, 1 atm), liquefaction point -188.1°C, freezing point -212.62°C.
Fluorine is prepared by the electrolysis of cryolite or from fluorite (calcium fluoride). Fluorine is a highly reactive oxidiser and is therefore highly toxic. It is used as a fuel or propellant, eventually mixed with oxygen (in a fluorine-hydrogen mixture) as catalyst.
Fluorine is a component of fluorapatite, present in bones, in hydrofluoric acid and fluorinated derivatives such as the (chloro)- fluorocarbons, known commercially as freons, fluorinated polymers such as teflon (PTFE, poly tetrafluoroethylene) and vinylidene polyfluoride (see vinyl).