John Emsley, University of Cambridge, takes you on a tour of the periodic table. In this issue: It's lazy, it's hard working, it's colourless, it's colourful - it's argon
Argon's name comes from the Greek word argos, meaning lazy and indeed for more than 100 years after its discovery chemists were unable to get it to combine with any other element. Then in 2000, chemists at the University of Helsinki, led by Markku Räsänen, announced the first argon compound - argon fluorohydride (HArF) - which they made by condensing a mixture of argon and hydrogen fluoride onto caesium iodide at -265°C and exposing it to uv light. On warming above -246°C HArF reverted back to Ar and HF. No other process has induced argon to react, including high temperatures, electrical discharges, or ionising radiation.
A chance discovery
There are 50 trillion tonnes of argon swirling around in the Earth's atmosphere, which has slowly built-up over billions of years. Almost all of this Ar has come from the decay of the radioactive isotope potassium-40, which has a half-life of 12.7 billion years. Although argon makes up 0.93 per cent of the atmosphere, the gas evaded discovery until 1894 when the physicist Lord Rayleigh (1842-1919) and the chemist William Ramsay (1852-1916) identified it. In 1904 Rayleigh won the Nobel prize in physics and Ramsay won the Nobel prize in chemistry for their work on the discovery of argon.
The story of its discovery began when Rayleigh found that the nitrogen extracted from the air had a higher density (1.257 g dm-3) than nitrogen made by decomposing ammonia (1.251 g dm-3). The difference was small but real. Ramsay wrote to Rayleigh suggesting that he would look for a heavier gas in the nitrogen obtained from air, while Rayleigh should look for a lighter gas from ammonia. Ramsay removed all the nitrogen from his sample by repeatedly passing it over heated magnesium to form magnesium nitride. He was left with 1 per cent which would not react and which was denser than nitrogen (1.784 g dm-3). The atomic spectrum of this '1 per cent' showed new red and green lines, confirming that it was a new element.
In fact, argon had been isolated earlier in 1785 in Clapham, South London, by Henry Cavendish. He passed electric sparks through air and absorbed the gases which formed, but he was puzzled that there remained an unreactive 1 per cent. Cavendish did not realise that he had stumbled on a new gaseous element.
Most argon is used in the production of steel where it is blown through the molten iron, along with oxygen. The argon does the stirring while the oxygen removes carbon as carbon dioxide. Argon is also used to exclude air to prevent oxidation of hot metals, in welding aluminium and in the production of titanium. Atomic energy fuel elements are protected with an argon atmosphere during refining and reprocessing, and the ultra-fine metal powders needed to make alloys are produced by directing a jet of liquid argon at a jet of the molten metal.
Some smelters prevent toxic metal dusts from escaping into the environment by venting them through an argon plasma torch. In this, argon atoms are electrically charged to reach temperatures of 10,000°C and the toxic dust particles passing through are changed into to a blob of molten scrap.
For a gas that is chemically lazy argon has proved to be eminently employable. Illuminated signs glow blue if they contain argon and bright blue if a little mercury vapour is also present. Double glazing is even more efficient if the gap between the two panes of glass is filled with argon rather than just air because argon is a poorer conductor of heat. Thermal conductivity of argon at room temperature (300 K) is 17.72mW m-1 K-1 (milliWatts per metre per degree) whereas for air it is 26mW m-1 K-1. For the same reason argon is used to inflate diving suits. Old documents and other artefacts that are susceptible to oxidation can be protected by storing them in an atmosphere of argon. Blue argon lasers are used in surgery to weld arteries, destroy tumours and correct eye defects.
The most exotic use of argon is in the tyres of luxury cars. Not only does the gas protect the rubber from attack by oxygen, but it ensures less tyre noise when the car is moving at speed.
Argon (Ar) is one of the noble gases of Group 18 of the Periodic Table and exists only as inert atoms. Globally 750,000 tonnes per year are extracted from liquid air. Atomic number: 18; atomic mass: 39.948; melting point: -189°C; boiling point: -186°C.