Fluorine – facts and properties

Position in the periodic table

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Occurrence and isolation

Fluorine is fairly common in the Earth's crust, more so than chlorine, but it is always found as the fluoride ion, never as the free element. The principal mineral is fluorite, or fluorspar, which is calcium fluoride; it forms cubic crystals in many colours and is mined on a large scale in China, Mexico, South Africa and Mongolia. Cryolite, a sodium aluminium fluoride once mined in Greenland for aluminium smelting, is now almost exhausted. Fluorapatite is a component of phosphate rock, and fluorine compounds are recovered as a by-product of fertiliser manufacture. Seawater, bones and tooth enamel all contain fluoride.

The name comes from the Latin fluere, to flow, because fluorspar has been used for centuries as a flux that helps metal ores melt. The year 1670 is often given as the date of discovery, when glass was first etched with fluorspar and acid, a reaction that produces hydrogen fluoride. Isolating the element was another matter. Several chemists were poisoned or killed by hydrogen fluoride in the attempt, and it was Henri Moissan who finally succeeded in 1886 by electrolysing potassium fluoride dissolved in anhydrous hydrogen fluoride at low temperature, work that earned him a Nobel Prize. Fluorine is still made essentially by Moissan's method, in cells of steel or Monel alloy, because few materials withstand the gas. The hydrogen fluoride needed as a starting material is produced from fluorspar and sulfuric acid.

Properties and reactions

Fluorine has the electron configuration [He] 2s² 2p⁵: seven valence electrons, one short of a filled shell. It occupies period 2 and group 17 as the lightest halogen. Its electronegativity of 3.98 is the highest of any element and defines the top of the Pauling scale. In every compound fluorine has the oxidation state minus one; it is never positive. With an atomic radius of 42 pm the atom is very small, and the fluoride ion is correspondingly compact.

The element forms the diatomic molecule F₂, a pale yellow gas with a pungent smell and a density of 1.70 g/L. It liquefies at -188.12 °C and solidifies at -219.62 °C. The bond between the two fluorine atoms is surprisingly weak, because the lone pairs on such small atoms repel one another strongly. This weak bond, together with the exceptional strength of the bonds fluorine forms to other elements, makes it the most powerful oxidising agent among the elements. Fluorine combines with almost every element, often at room temperature and with a flame: hydrogen explodes with it even in the dark, sulfur, phosphorus and many metals ignite, water is decomposed into hydrogen fluoride and oxygen, and even glass, sand and asbestos burn in fluorine. The noble gas xenon forms stable fluorides. A few metals, among them nickel, copper and steel, resist attack because a dense fluoride layer forms on their surface. Hydrogen fluoride dissolves in water to give hydrofluoric acid, a moderately weak acid that nevertheless etches glass and whose vapours are highly toxic.

Uses in everyday life and technology

Most fluorspar is never turned into elemental fluorine; it is converted to hydrogen fluoride and from there into fluorine compounds. Fluorinated hydrocarbons serve as refrigerants in fridges and air conditioners. The older chlorofluorocarbons, the CFCs, destroyed stratospheric ozone and have been banned; their replacements are now being phased down in turn because they are potent greenhouse gases. Polytetrafluoroethylene, better known as Teflon, coats frying pans, seals pipe threads and forms the membranes of breathable waterproof jackets. Fluoropolymers are so durable because the carbon-fluorine bond is one of the strongest in organic chemistry.

In aluminium smelting, cryolite lowers the melting point of aluminium oxide. Elemental fluorine is needed chiefly to make uranium hexafluoride, the gas used to enrich uranium for nuclear power stations, and sulfur hexafluoride, an insulating gas for high-voltage switchgear. Fluoride in toothpaste, mouthwash and some table salt hardens tooth enamel: fluoride ions are built into the mineral of the tooth and make it more resistant to acid. A large share of modern medicines, including antidepressants, antibiotics and anaesthetics, contain fluorine atoms that improve their stability and effect in the body. Hydrofluoric acid etches glass, cleans silicon wafers and catalyses reactions in oil refining. Per- and polyfluoroalkyl substances, the PFAS, among them fluorinated surfactants, make paper, textiles and firefighting foam repel water and grease, but they barely break down in the environment at all and are now being restricted.

Molecules containing this element

• Hydrogen fluoride HFView in 3D
• Sodium fluoride NaFView in 3D
• Potassium fluoride KFView in 3D

Quick question

Which of these elements has the highest electronegativity?

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