Molecule of the Month: The world's loneliest gas: Argon refuses to react. That's why industry loves it, says John Emsley

One hundred years after the discovery of argon, the inert gas used in fluorescent lighting, the British Association for the Advancement of Science heard from Professor Frank Wilkinson of Loughborough this month that the blue beam of argon lasers, which can probe molecular states lasting only one- trillionth of a second, is being used to weld arteries, treat tumors and study the form of oxygen that is thought to cause skin cancer.

It was at the BA annual meeting in Oxford that Lord Rayleigh and Professor William Ramsay announced the discovery of argon. A few months later, in January 1895, they entered their results in a competition for 'some new . . . discovery about atmospheric air', organised by the Smithsonian Institution in Washington, and won the dollars 10,000 prize, worth about pounds 100,000 today. Lucky argon. But it was second time lucky.

The possibility of its existence was first revealed in 1766 in Clapham, London, by a wealthy eccentric, Henry Cavendish. When he was investigating the chemistry of the atmosphere, he passed electric sparks through a sample of air and absorbed the gases formed. He was puzzled that 1 per cent of their volume would not combine chemically, but he did not realise he had stumbled on a new element. For more than a century his observations were neither understood - nor forgotten.

The actual discovery of argon began with the puzzle: why did the density of nitrogen depend on how it was obtained? Nitrogen extracted from the air had a density of 1.257g per litre, whereas that from decomposing ammonia had a density of 1.251g. Rayleigh and Ramsay knew that either atmospheric nitrogen must contain a heavier gas, or chemically derived nitrogen contained a lighter gas.

They believed that the answer lay in the nitrogen from the air, so Ramsay passed a sample of this overheated magnesium, which reacts to form a solid, magnesium nitride. Like Cavendish, he was left with about 1 per cent of the volume that would not react, and was 30 per cent denser than nitrogen. When they examined its spectrum, they observed new lines that could be explained only by a new element - and they named it argon from the Greek argos, meaning idle.

Argon is now an important industrial gas, and hundreds of plants around the world extract it from liquid air. Earlier this year, at Eggborough, North Yorkshire, MG Gas Products opened a pounds 20m plant controlled by computer and manned by only six technical staff. It processes 375 tonnes of air a day, separating it into oxygen, nitrogen and argon, which are shipped out as liquids. Tony Bonnett, of MG Gas Products, says argon is particularly important for the metals industry. Most is used in the purification of steel, where it is blown through the molten metal. It is also used to prevent oxidation of hot metals such as aluminium, and when welding titanium.

The alloys from which high-grade tools are made require metal powders, and these are produced by directing a jet of liquid argon, at minus 190C, at a jet of the molten metal. The result is an ultrafine powder with a clean surface.

Some smelters prevent toxic metal dusts escaping to the environment by venting them through an argon plasma torch. Here, argon atoms are electrically charged to reach temperatures of 10,000C, and the dust particles are turned into a blob of molten scrap.

Some consumer products contain argon. It is used to fill the gap between the panes of sealed double glazing, because it is a poorer conductor than air. Inside light bulbs, it dissipates the heat of the filament while not reacting with it. Illuminated signs glow blue if they contain argon, and bright blue if they contain a little mercury vapour.

But the most exotic use of argon is in the tyres of luxury cars: not only does it protect the rubber from attack by oxygen, but also it ensures less tyre noise at high speeds.

Many of these uses rely on argon's chemical inertness - so far nothing has been found to induce it to react with any other material, no matter how high the temperature to which it is heated, nor how strong the electrical charge passed through it. So argon gas consists entirely of single argon atoms. Even compounds containing argon - the so-called argon clathrates - hold it only as trapped atoms in the lattice of a larger molecule.

Several trillion tonnes of argon are swirling around in the world's atmosphere, where the gas has slowly built up over billions of years. It is a decay product of the radioactive isotope potassium-40, which has a half-life of 12.7 billion years, and transforms to argon-40.

It is possible to date minerals by measuring the ratio of potassium to argon that they contain. Argon-40 accounts for 99.6 per cent of the argon in the atmosphere, the remainder being mainly the lighter isotopes argon-36 and argon-38.

The writer is author of 'The Consumer's Good Chemical Guide', published by W H Freeman, pounds 18.99.

(Photograph omitted)