A nonmetallic element, the lightest of the halogens; it is a pale yellow, corrosive gas, that is highly toxic. Symbol, F. It is the most electronegative and reactive of all the elements. Fluorine occurs chiefly in fluorite (CaF₂) and cryolite (Na₂AIF₆), but is widely distributed in other minerals. Used in producing uranium and many hightemperature plastics.