Fluorine

Category: halogen Atomic number: 9 Colour: pale yellow Melting point: −220°C (−363°F) Boiling point: −188°C (−307°F) First isolated: 1886

Fluorine is part of group 17 of the periodic table, the halogens, which also include chlorine, bromine, iodine and astatine. Halogen means ‘salt-producing’ – this is because halogens react with metals to give a range of salts, including calcium fluoride, sodium chloride (table salt) and silver bromide. They also share the properties of being highly reactive and potentially lethal. Fluorine is particularly dangerous in its pure form – if you breathe a concentration of air containing just 0.1 per cent fluorine it will be fatal within minutes, and if a stream of the gas is aimed at solids such as bricks or glass, they burst instantaneously into flames.

There are, however, safer compounds that contain fluorine. The mineral fluorspar (calcium fluoride) was in use by the 1520s as a ‘flux’ in furnaces – the way it melted and flowed when heated made the metal easier to work with. Alchemists of the period knew that fluorspar and fluorides in general contained an unknown substance, but they were unable to isolate it. (Or, if anyone did manage to, they probably died in the process and were unable to tell the tale!)

In 1860, the English scientist George Gore came close to isolating the gas: he ran a current of electricity through hydrofluoric acid and may have safely produced a quantity of fluorine, but he couldn’t prove it. It was not until 1886 that the French chemist Henri Moissan succeeded in using electrolysis to isolate fluorine (without dying in the process), a feat for which he was eventually rewarded with a Nobel Prize.

We do encounter fluorine on a regular basis in the stable form of fluorides – these substances are essential for humans and are added to water in many areas, following research that discovered that places with natural levels of fluoride in the water have lower levels of tooth decay. This practice is controversial, but fluoride is also in regular use in toothpaste – when applied to teeth, it forms tiny crystals that help make them more resistant to acids and to tooth decay.

Another common compound is polytetrafluoroethylene. This is a daunting name for the substance that you probably know better by the trademarked name Teflon. It was discovered in 1938 by Roy Plunkett in the DuPont laboratories, where he was researching new types of refrigerant (cooling gas). After storing the gas polytetrafluoroethylene in cylinders, he found that it left behind a residue of white powder.

The substance turned out to be a plastic that was heat-resistant, chemically inert and extraordinarily flexible at very low temperatures – this quality led to it being used in space exploration, while the fact that nothing would stick to it resulted in its widespread use in non-stick pots and pans. Teflon is also used in ‘breathable clothing’, which keeps the rain out while allowing water vapour to escape, making it ideal for anyone who exercises or works in rainy conditions.