Fluorine is a chemical element with symbol F and atomic number 9. Fluorine is the lightest of the halogens, a group of compounds including chloride, bromine, iodine and astatine. Under standard conditions, it exists as a highly toxic pale yellow diatomic gas. Fluorine is the most electronegative element, making it extremely reactive. It reacts with nearly all other elements, except for helium and neon. Fluorine is a vital element in the nuclear energy industry, has been used in commercial applications, such as refrigeration, and was used to make chemical weapons. Fluorine-containing drugs are used in medicine as anesthetics, antibiotics, anti-cancer and anti-inflammatory agents, psychopharmaceuticals, and in many other applications.