Fluorine (F) is a chemical element with atomic number 9. The only naturally occurring fluorine isotope is ¹⁹Fl, with 9 protons and 10 neutrons.

Fluorine belongs to a group of elements called halogens, and is a highly reactive element. It reacts with all other chemical elements, except for some of the noble gases.

Fluorine, in the form of fluoride ions (F⁻), can prevent and reduce tooth cavities, and is therefore commonly found in toothpastes, and is in some places also added in small amounts into the drinking water.