Fluorine | Uses, Properties, & Facts - Britannica

Physical and chemical properties

At room temperature fluorine is a faintly yellow gas with an irritating odour. Inhalation of the gas is dangerous. Upon cooling fluorine becomes a yellow liquid. There is only one stable isotope of the element, fluorine-19.

Because fluorine is the most electronegative of the elements, atomic groupings rich in fluorine are often negatively charged. Methyl iodide (CH3I) and trifluoroiodomethane (CF3I) have different charge distributions as shown in the following formulas, in which the Greek symbol δ indicates a partial charge:

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The first ionization energy of fluorine is very high (402 kilocalories per mole), giving a standard heat formation for the F+ cation of 420 kilocalories per mole.

The small size of the fluorine atom makes it possible to pack a relatively large number of fluorine atoms or ions around a given coordination centre (central atom) where it forms many stable complexes—for example, hexafluorosilicate (SiF6)2− and hexafluoroaluminate (AlF6)3−. Fluorine is the most powerfully oxidizing element. No other substance, therefore, is able to oxidize the fluoride anion to the free element, and for this reason the element is not found in the free state in nature. For more than 150 years, all chemical methods had failed to produce the element, success having been achieved only by the use of electrolytic methods. However, in 1986 American chemist Karl O. Christe reported the first chemical preparation of fluorine, where “chemical preparation” means a method that does not use techniques such as electrolysis, photolysis, and discharge or use fluorine itself in the synthesis of any of the starting materials. He used K2MnF6 and antimony pentafluoride (SbF5), both of which can be easily prepared from HF solutions.

The high oxidizing power of fluorine allows the element to produce the highest oxidation numbers possible in other elements, and many high oxidation state fluorides of elements are known for which there are no other corresponding halides—e.g., silver difluoride (AgF2), cobalt trifluoride (CoF3), rhenium heptafluoride (ReF7), bromine pentafluoride (BrF5), and iodine heptafluoride (IF7).

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Fluorine (F2), composed of two fluorine atoms, combines with all other elements except helium and neon to form ionic or covalent fluorides. Some metals, such as nickel, are quickly covered by a fluoride layer, which prevents further attack of the metal by the element. Certain dry metals, such as mild steel, copper, aluminum, or Monel (a 66 percent nickel, 31.5 percent copper alloy), are not attacked by fluorine at ordinary temperatures. For work with fluorine at temperatures up to 600 °C (1,100 °F), Monel is suitable; sintered alumina is resistant up to 700 °C (1,300 °F). When lubricants are required, fluorocarbon oils are most suitable. Fluorine reacts violently with organic matter (such as rubber, wood, and cloth), and controlled fluorination of organic compounds by the action of elemental fluorine is only possible if special precautions are taken.