A pale-yellow, highly corrosive, poisonous, gaseous halogen element, the most electronegative and most reactive of all the elements, existing as a diatomic gas (F₂) and used in a wide variety of industrially important compounds. Atomic number 9; atomic weight 18.9984; melting point -219.67°C; boiling point -188.12°C; specific gravity of liquid 1.50 (at boiling point); valence 1. See Periodic Table.