A non-metallic, gaseous element, strongly acid or negative, and 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.
- A pale-yellow, highly corrosive, poisonous, gaseous halogen element, the most electronegative and most reactive of all the elements, used in a wide variety of industrially important compounds. Atomic number 9; atomic weight 18.9984; freezing point -219.62°C; melting point -223°C; boiling point -188.14°C; specific gravity of liquid 1.108 (at boiling point); valence 1. See Table at element.