A non-metallic, gaseous element, strongly acid or
   negative, or associated with chlorine, bromine, and iodine, in the
   halogen group of which it is the first member. It always occurs
   combined, is very active chemically, and possesses such an avidity for
   most elements, and silicon especially, that it can neither be prepared
   nor kept in glass vessels. If set free it immediately attacks the
   containing material, so that it was not isolated until 1886. It is a
   pungent, corrosive, colorless gas. Symbol F. Atomic weight 19.