Fluorine (chemical symbol - F, obsolete. fluor; from lat. Fluorum) is a chemical element of the 17th group (according to the outdated classification - the seventh group of the main subgroup, VIIA) of the second period of the periodic system of chemical elements of D. I. Mendeleev, with atomic number 9.
It is the most chemically active nonmetal and the strongest oxidizer. Fluorine belongs to the halogen group and is the lightest element of this group.
As a simple substance (under normal conditions), fluorine is a diatomic gas (formula - F2) of pale yellow color with a pungent odor resembling ozone or chlorine. At cryogenic temperatures, it is a yellow liquid, a colorless gas (in thick layers — greenish-yellow, n.o.)