ELEMENTS OF CHANCE

To stumble upon one gas may seem lucky, but to find five is nothing short of genius. Hugh Aldersey-Williams celebrates the centenary of a research project which has shaped 20th-century life
Click to follow
The Independent Culture
IMAGINE finding the proverbial masterpiece in the attic. You take it to be examined and are told that it is an original, it is indeed a masterpiece, and, what's more, by a painter completely unknown to the art world. Naturally, you search through the attic once more, with greater purpose now; you find another painting, and then several more - the complete oeuvre, in fact, of a great master whom nobody knew existed.

This is pretty much what happened to Sir William Ramsay, professor of chemistry at University College, London. Ramsay discovered five new chemical elements during the 1890s, three of them exactly one hundred years ago this summer. He was given the Nobel Prize for Chemistry in 1904.

This new group of elements had a strong family resemblance: all are gases, all are colourless and odourless, all remarkably unreactive. They were quickly dubbed "inert gases", and most chemists found them boring. Today, however, it is their very laziness that gives them such a wide range of applications. And when they are made to react, their compounds are so frail that they provide scientists with a unique window on chemical reactivity (just as you might learn a lot more about cooking by making a delicate omelette than by frying an egg).

Ramsay made his first discovery in 1894 with Lord Rayleigh, professor at the Cavendish Laboratory in Cambridge, who had found that nitrogen obtained by chemical means was lighter than nitrogen extracted from the air. Ramsay solved this puzzle by burning shavings of magnesium in atmospheric nitrogen: most of the gas combined with the reactive metal, but some was left over, and its spectrum (the unique signature of light wavelengths emitted when heated to a glow) did not correspond with any known gas.

Ramsay and Rayleigh announced the discovery of a new element, which they named argon. Identified solely on the basis of its spectrum, however, argon's existence was doubted by many scientists; its atomic weight - the basic measure of an element - could not be determined in the usual fashion (by measuring amounts consumed in various chemical reactions) because of its unyielding inertness.

The periodic table, arranged by the Russian chemist Dmitri Mendeleev, had brought some order to the miscellany of chemical elements known by 1869. Ramsay became excited that argon might be one of a number of gaseous new elements. He also became the man to see if you'd found a strange gas. In 1895, he was notified by an American geochemist that he'd obtained an inert gas by heating a particular mineral sample.

Could this be argon too? Ramsay repeated the experiment and examined the spectrum of the gas that came off. It was not argon. This time, the spectral lines corresponded to those observed by a French astronomer, Pierre Janssen, during the 1868 solar eclipse. The chemist Edward Frankland proposed then that the lines could only be emissions from a new element, which he boldly named helium. Now, after 27 years of doubt, helium's existence was finally confirmed.

Since argon is abundant in the air, Ramsay and his assistant Morris Travers, reckoned that other new gases might also be all around us, just waiting to be detected. They spent the next years trying to obtain further new elements from minerals, but without success: the day when the new element would arrive became a laboratory joke. In May 1898, they tried a new tack: they boiled off a gallon of liquefied air until a residue remained. Spectral analysis of this residue once again revealed new lines: this gas they called krypton.

Thus encouraged, Ramsay and Travers scaled up their experiment by a factor of a thousand, starting not with liquefied air but with liquid argon - although supposedly pure, perhaps the argon was hiding traces of similar new elements.

Despite the jibes of rivals and sceptics, Ramsay was confident of success. "It is unlikely that anyone will overtake us; for it is no easy work to make 18 quarts of pure argon," he wrote. In a series of careful evaporations, they found that a light gas boiled off ahead of the argon. In June, Ramsay announced the discovery of this new gas, neon. Then, when the argon had boiled away, there was a residue containing not one, but two further gases. Krypton was one; the other was yet another new element, xenon, whose discovery Ramsay announced at the annual meeting of the British Association for the Advancement of Science.

The next two years were spent purifying samples of the elements and measuring their physical properties to prove their existence as elements once and for all. It wasn't until 1900 that the sceptics, who included Mendeleev himself, were convinced. The last and heaviest element in the family was also discovered that year: a German scientist, Friedrich Dorn, found that radium emitted a gas, later called radon, as one of the byproducts of its radioactive decay. Ramsay confirmed that this was yet another inert gas.

That label - inert gases - discouraged research into the new elements' chemistry for decades. Indeed, it was not until 1962 that a Canadian chemist, Neil Bartlett, dispelled the myth of the gases' inertness. He managed to combine xenon with fluorine, the most reactive element in the periodic table. During the course of the next decade, chemists were able to make many compounds of xenon with fluorine as well as oxygen. Radon compounds were made too. The first fluoride of krypton and chloride of xenon were reported in 1967, and new compounds continue to be announced. Despite, or often because of, their reluctance to react, the gases (now termed "noble", just as the relatively unreactive silver, gold and platinum are noble metals) have widespread use.

There will be a little of most of them in your home. Their reactive compounds have many uses too, especially in lasers where they are produced by electrical excitation before decomposing to emit laser light. After a slow start, the chemistry of the noble gases is now an area of intense research.

NEON

Etymology: Greek for new.

Uses: Thanks to its red glow in an electrical discharge, neon was in use in illuminated signs as early as 1910, shortly after liquefaction had become commercially viable. This made it economic to condense air to a liquid and then separate its constituent gases by boiling them off one after another. These could then be re-condensed into a liquid for easy storage and transported in pressurised cylinders. Car brake lights that use a neon discharge illuminate one fifth of a second sooner than conventional bulbs, thus providing an extra car's length of stopping distance at 50mph.

Research interest: The heavier noble gases form clusters like conventional solids, whereas lightweight helium does not even solidify at absolute zero. Marius Lewerenz at the Max Planck Institute at Gottingen has examined the structure and stability of small helium clusters, which he calls "extremely floppy". Neon is somewhere in-between. Recent studies at the DESY Synchrotron accelerator in Hamburg have shown that clusters of up to 300 neon atoms are "liquid-like".

Anything else I should know? Not all "neon" lighting contains neon. Other colours are produced by other elements or combinations - a mercury vapour discharge is greenish, helium is cream-coloured, xenon is blue and so on.

KRYPTON

Etymology: Greek for secret.

Uses: Krypton is used in the fluorescent lamps of lighthouses. Krypton ion lasers are used in eye surgery. Other lasers are filled with a mixture of fluorine and either xenon or krypton; the unstable noble gas fluoride is formed in the electrical discharge and then emits laser light.

Research: Scientists at the University of Paris XI reported this year that under a pressure of several atmospheres, it is possible to push atoms of krypton or xenon into hydrophobic sites in proteins. These sites are important in the study of biochemical reactions: measurements of the nested noble gas atom reveal the characteristics of these sites, while other substances that might also fit into the sites would become fully bonded, so messing up the picture obtained.

In 1994, Dr Martin Saunders at Yale University made what he claimed to be the first compounds of helium and neon. What he did was a bit of a cheat: these "compounds" have no chemical bonds with the noble gas atom, but are trapped in the cage-like structure of molecules of buckminsterfullerene (the 60-atom form of carbon). At extremely high pressures, some of the cage molecules open to admit an atom of the noble gas. This year, Saunders also succeeded in isolating a pure buckminsterfullerene/krypton compound.

HELIUM

Etymology: Helios is Greek for the sun, where this gas was first detected.

Uses: These revolve around helium's unique physical properties - its lightness and the fact that it becomes liquid at only four degrees above absolute zero. Helium is used in airships instead of the flammable hydrogen and in weather (and party) balloons. In the helium-oxygen breathing mixture used by divers, helium averts "the bends" (decompression sickness, caused by nitrogen dissolving in the blood under pressurised conditions and then coming out of solution to create gas bubbles that block the blood vessels). Helium is much less soluble. The same mixture is used in medicine as a more fluid substitute for air in the treatment of patients with severe respiratory problems. Its fluidity makes helium helpful for the detection of leaks. Liquid helium is also used to maintain the very low temperatures necessary for metals to become superconducting (to carry electricity without resistance) in the electromagnets of MRI scanners.

Research interest: Helium is of interest both to theoretical physicists and chemists as the next simplest element after hydrogen. The quantum mechanics of helium - with its two electrons in orbit around a nucleus of two protons and two neutrons - are far more complex than in the hydrogen atom, with its single electron orbiting a single proton. The two-atom helium molecule likewise provides something of an extreme example among molecules, given its great fragility. The bond between the two helium atoms is so long and so weak that it barely holds together; when it breaks, at just one hundredth of a degree above absolute zero, it does so without the build-up of vibrations necessary to break full-strength bonds. So the helium molecule is, in effect, a time-frozen example of a molecule on the brink of dissociation.

Anything else I should know? Breathing a helium mixture makes you sound like Mickey Mouse. This is because the vocal cords vibrate more rapidly and sound travels faster in the low density. It can be dangerous, as the helium rapidly displaces other gases in the lungs, leading to oxygen starvation.

XENON

Etymology: Greek for stranger.

Uses: An electrical discharge in xenon produces a bright white light which is used in theatre lighting, strobes and photographic flash bulbs, and the latest generation of car headlamps.

Research interest: Fluorinated xenon compounds are becoming useful in chemical synthesis, permitting chemists greater control in the addition of fluorine to organic molecules (fluorinated organic compounds are often useful - CFCs and Teflon are both fluorinated) than is possible using the corrosively reactive element fluorine itself.

Xenon is also gaining ground in anaesthesia. The traditional nitrous oxide (laughing gas) has now been recognised as a greenhouse gas. It can also interfere in the action of enzymes and other metabolic processes and is unsuitable for use during certain operations. "Xenon is not involved in the metabolism, so the risk of dangerous compounds being formed is eliminated. And there is also the potential for rapid recovery without headaches," according to Howard Fee, professor of anaesthetics at Queen's University Belfast.

Xenon is an expensive substitute, however, and needs special equipment to make sure it is all recovered after use. Professor Fee is also interested in the use of xenon as a sedative. Unlike conventional sedatives, it works without reducing the patient's blood pressure. This could be especially valuable in treating critically ill patients.

Anything else I should know? There is an impending shortage of xenon, because Nasa plans to use the gas in ion engines to steer satellites. Xenon used in this way will be lost in space.

RADON

Etymology: From the Latin for ray.

Uses: No general uses.

Research interest: The chief focus here is on immediate practical countermeasures against this radioactive gas. Radon seeps out of uranium-rich rock and is the second most significant known cause of lung cancer; it "interacts in a synergistic way" with tobacco smoke, according to the National Radiological Protection Board. In Britain, Cornwall and Devon are worst affected. In some parts of Eastern Europe, radon is prevalent in spa waters. Elsewhere, a sudden surge in radon levels can indicate an imminent earthquake or volcanic eruption. Researchers, especially in places where these minerals occur, are working on the development of materials that block the passage of the gas - not easy, since, like all noble gases, its single atoms can fit through gaps in the molecular structure of many materials.

Anything else I should know? The radon hazard was first recognised during the 1980s in bizarre circumstances: Stanley Watras, a construction worker at an American nuclear plant, triggered alarms indicating a high level of radiation. He did this not when he left the plant - which would have suggested pilfering radioactive material - but in the morning when he arrived for work. It turned out that his house contained high levels of radon, being built on uranium- rich bedrock.

ARGON

Etymology: Greek for idle.

Uses: Argon is the most abundant noble gas, and makes up 0.8 per cent of the atmosphere. It is therefore used when cost or quantity are important factors. The welding of metals, especially the more reactive ones such as aluminium, requires an atmosphere free of oxygen to produce a clean joint. Pure nitrogen is adequate in some circumstances, but nitrides and bubbles of nitrogen can weaken the joint. Argon is better.

Argon is the gas inside ordinary light-bulbs, since it does not react with the tungsten filament even at white heat, and also in the cavity between panes of glass in superior double-glazing, where it is a better insulator than air. It is also used in a few slaughterhouses to stun chickens without stress, - making for more tender meat.

Research interest: Argon forms many well-defined clusters - including an especially stable cluster of 13 atoms arranged in an icosahedron. Such clusters are studied by placing them alongside an organic molecule such as benzene, where they are stabilised by that molecule's electron clouds.

Ionised argon clusters provide a simple model for the study of the complex nature of chemical reactions that take place in solutions. The electrical charge does not spread evenly throughout the cluster, but remains localised on two or three argon atoms. The system thus resembles the ion of a dissolved salt with solvent molecules clustered around it, according to Professor Peter Knowles at the University of Birmingham.

Anything else I should know? Earthly chemists made little of the first of the inert gases. The Martians, on the other hand, had more luck - HG Wells armed them with an argon compound with which to poison London in The War of the Worlds.

Comments