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.