Fluorine, with atomic number 9, is the most reactive element.
It is the lightest halogen and exists as a pale-yellow diatomic gas, F₂, under ordinary conditions. Fluorine atoms strongly attract electrons, giving the element the highest electronegativity of any element and helping explain its extraordinary reactivity. It reacts with nearly every other element, including materials such as glass and many metals.
The name comes from fluorite, a mineral historically added to metal ores to help them melt. Chemists recognized that fluorite contained an unknown element, but isolating fluorine was exceptionally dangerous because both fluorine and its compounds can attack tissue and equipment. French chemist Henri Moissan finally isolated it in 1886 using electrolysis, an achievement that earned him the 1906 Nobel Prize in Chemistry.
A common mix-up is chlorine, another highly reactive halogen. Chlorine is widely encountered in disinfectants and bleach, but halogen reactivity decreases down the group, making fluorine more reactive than chlorine, bromine, or iodine.