Fluorine is the most reactive of the halogens because it is at the top of the halogen group, which is the second to right group on the periodic table. With halogens, the higher an element is in the column, the more reactive it is.
Halogens are reactive because the outer shells that orbit the nucleus lack electrons. This makes them eager to grab electrons from wherever they can get them.
Fluorine, whose atomic number is 9, is a violently reactive element. It will even attack glass and noble gases, which are considered inert. Even water explodes when exposed to fluorine. Because of its reactivity, fluorine is never found free in nature, but is found as some type of fluoride.
At standard pressure and temperature, fluorine is a pale yellow, poison gas that should not be handled by a non-professional and should certainly not be inhaled or ingested. However, when fluorine is present as fluoride, it can be quite beneficial. Calcium fluoride is used as a flux in the metal industry, and pure crystals are used as prisms. It also produces hydrofluoric acid, which is used in the manufacture of ceramics. Stannous fluoride, a compound of tin and fluorine, protects tooth enamel.