Fluorine is the smallest, lightest, and most reactive member of the halogen group.
It is a pale yellow gas whose molecules contain two atoms. It is, however, far too reactive to be found in nature in its diatomic form. Fluorine is also found in small quantities in seawater, teeth, bones, and blood. It was long known that fluorine compounds existed, but because of its extreme reactivity, pure fluorine had never been isolated. It took 75 years of continuous effort on the part of many chemists before it was finally isolated by the French chemist Henri Moissan in 1886.