Fluorine is a halogen group element known by the atomic symbol F. Its atomic number is 9, and its atomic weight is 18.998. It has a melting point of -363.3 °F (-219.6 °C), a boiling point of -306.6 °F (-188.1 °C), and typically occurs as a pale yellow, diatomic gas.

It is the most reactive of all elements and combines, often violently, with nearly all other elements. Even a compound such as water will burn with a bright flame in an atmosphere of fluorine gas. Many of the element's compounds are toxic and can cause deep, severe burns on contact. Yet fluorine exists in many harmless compounds as well. Most of it occurs in the mineral fluorspar, which is found throughout the world. In the United States, Illinois produces more than half the nation's fluorspar. Fluorine is also a constituent of the mineral cryolite, which is...