Read more: “The nature of nothingness“
NOBLE gases are so called because, like the nobility, they do nothing. You might also call them rare gases, because they are so rare on Earth as to be nearly non-existent. The one exception is argon, which we inhale as 1 per cent of every breath, though it has no effect on our bodies whatsoever. Helium, neon, argon, krypton, xenon and radioactive radon are odourless, tasteless, practically non-reactive wisps of unconnected atoms. In this material universe, they amount to just about nothing at all.
And yet… it would be hard to make a case that any other group of elements has had a greater impact on our understanding of the universe. For example, Darwin’s theory of evolution needs an Earth many millions of years old in order for it to have had time to work. Yet the Bible placed a limit on Earth’s age at a mere 6000 years. How was this argument resolved? The answer was helium, which is generated in rocks containing uranium and thorium.
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When these elements undergo radioactive decay they release alpha particles, which are really just helium nuclei that easily pick up electrons to create the gas. In 1906, armed with this idea and the rate of production of alpha particles by uranium, thorium and their decay products, Ernest Rutherford and Frederick Soddy dated several rocks at up to 500 million years; the Earth would have to be at least that old. (Later work with lead isotopes pinned down the age to around 4.5 billion years.) Not only did Rutherford and Soddy create the concept of radioactive dating, they also kick-started our modern understanding of the cosmos and its great age.
What if you want to probe the interior of the sun? The answer is to use argon, as physicist Ray Davis did. He focused on solar neutrinos – ghostly particles created by nuclear fusion in the sun’s core – as a way to test models of nuclear reactions in stars. Neutrinos reverse the natural decay of the radioactive isotope argon-37 into chlorine-37. So in 1958, Davis set up a huge vat of cleaning fluid containing chlorine-37 deep in a mine in South Dakota and used a Geiger counter to detect any argon created. Davis’s pioneering work revealed much about not only the sun but also the peculiar nature of neutrinos. It won him the Nobel prize for physics 44 years later.
Xenon, meanwhile, can tell you about the formation of the solar system. Xenon-129 is an isotope produced by the radioactive decay of iodine-129, which is created in quantity only in supernovas and has the relatively short half-life, in cosmological terms, of 16 million years. The discovery of unexpectedly large amounts of xenon-129 in meteorites was the first evidence that the solid bodies of the solar system formed within the surprisingly short time of a hundred million years after a nearby supernova seeded the material that made them. That shocked theorists who thought it could never have happened so quickly.
Even though the noble gases are rare on Earth, they are not rare in the universe as a whole. This tells us that Earth’s atmosphere must have formed after the planet itself. As the Earth formed it was too small to retain gases, which drifted away into the cosmos. The main components of today’s atmosphere – nitrogen, oxygen, water and carbon dioxide – must have been locked away in non-volatile forms. Water was trapped in hydrated minerals, carbon dioxide in carbonates, and so on. Only as the Earth and its gravitational attraction grew did these gases, escaping from volcanic eruptions, create the atmosphere.
So these wisps of nearly nothing reveal much about the Earth and its place in the universe. Yet for me the most fascinating aspect of the noble gases is how they were discovered. By the 1860s more than 50 elements had been found, often revealing themselves when subjected to the actions of other chemicals, heat or even electricity. We now know that the noble gases are, in the main, stubbornly non-reactive because they contain a full outer shell of electrons – a prerequisite for stability. But back in mid-Victorian times their aloofness meant the noble gases had completely eluded detection.
“These wisps of nearly nothing reveal much about the Earth and its place in the universe”
The first hint of their existence appeared in 1868 as a faint line in the spectrum of light from the sun, indicating the presence of an element not known on Earth. This was given the name helium after Helios, the Greek god of the sun. At the time, it raised speculation about elements in the stars being different from those on Earth, but a few years later the same line was found when a uranium mineral called cleveite was heated, and the Earth and sun were once more united.
Nothing happened for a while, until the trail was picked up from a different direction with a different end in view, when the British physicist John William Strutt – himself a noble, Lord Rayleigh – began to wonder why the atomic weights of the elements seem to be nearly whole number multiples of hydrogen. Why whole numbers? And even more puzzling, why only “nearly” whole numbers?
His attitude was, if you don’t understand something, measure it. He spent 10 years making precise measurements of the densities of the gases, from which their atomic weights could be calculated, starting with hydrogen, oxygen and then to nitrogen. No reason to expect a breakthrough here; it was a routine experiment. Rayleigh bubbled air through liquid ammonia, NH3, and then passed it through a tube containing red-hot copper. That stripped the air of its oxygen, which combined with hydrogen from the ammonia, leaving just nitrogen.
Surprise and disgust
Rayleigh did what a good scientist does: he carried out this experiment again and again to check the results. He then repeated it with a difference. Initially, some of his nitrogen would have come from the ammonia he used; this time he got rid of the ammonia so all the nitrogen came from air. “To my surprise and disgust the densities of the two methods differed by a thousandth part,” he wrote.
Nitrogen from air was apparently heavier than that from ammonia by just 0.1 per cent. I would have put it down to experimental error and moved on. But as Rayleigh said, “It is a good rule in experimental work to seek to magnify a discrepancy when it first appears rather than to follow the natural instinct to trying to get quit of it.”
That’s just what he did, this time replacing air with oxygen so that the nitrogen he collected came only from ammonia. He found that the discrepancy was indeed magnified: it was now 0.5 per cent. Something real was happening, but what? He wrote a letter to the journal Nature, asking for help. It began, “I am much puzzled by some recent results as to the density of nitrogen, and shall be obliged if any of your chemical readers can offer suggestions as to the cause.”
First suggestion: nitrogen in air is nothing but nitrogen, while nitrogen in ammonia is chemically combined with hydrogen. So perhaps he had nitrogen in two different chemical states which affected their atomic weights. But how?
No answer. Bad idea. Start again.
Finally, after other suggestions led nowhere, came the idea that a heavier gas might be mixed in with nitrogen from air. This contradicted Occam’s razor, which in colloquial terms means “keep it simple”. Invoking an unknown substance, a cryptic gas heavier than nitrogen, to explain the results had shades of phlogiston and the ether (see “Out of the ether: the changing face of the vacuum”) – illusory substances invented in other contexts as a fig-leaf for our lack of understanding.
But there is an even more hallowed tenet of science: test your ideas, experiment and observe. So in 1894, together with William Ramsay, Rayleigh passed electrical sparks through air augmented with pure oxygen to produce nitrogen oxides. They removed these by dissolving them in a weak alkali solution. Lo and behold, when all the nitrogen and oxygen were gone, a small amount of colourless gas remained, which they named argon. That comes from the ancient Greek for a lazy thing, since the gas wouldn’t react with anything. It showed a pattern of emission lines never seen before, so argon was not only a previously unsuspected component of air, it was an entirely new element.
Ramsay moved on to investigate whether the gas seeping out of uranium-bearing rocks was argon, but in 1895 identified it as helium. Arguing from his understanding of the then-primitive periodic table, he suggested that helium and argon might represent a new family of elements. He went so far as to predict another such element with a mass of 20. He soon discovered it and named it neon. Krypton and xenon followed several years later, and in 1904 both men received a Nobel prize, Rayleigh in physics and Ramsay in chemistry. This is the only time an element or column of elements has been the basis for these two prizes in the same year.
“Ramsay suggested that helium and argon might represent a whole new family of elements”
In 1910, Ramsay collected the full set by producing and characterising radon. This nasty radioactive gas had been noticed before but it was Ramsay who proposed and then demonstrated that it was another noble gas.
The discovery of the noble gases fascinates me because it is about the whole fabric of science and the roots of discovery. Rayleigh was not looking for a new element, he was trying to solve the riddle of nearly whole-number atomic weights. In this he failed: the explanation awaited the discovery of both protons and neutrons. The discovery of argon, which opened the door to the other noble gases, was serendipitous, the result of chance combined with careful experimentation and an open, inquiring mind. Like many other important scientific advances, it happened not as a result of purposeful planning but while trying to understand something else.
So, if you want to succeed in science, keep in mind the advice offered by cosmochemist Michael Lipschutz to his students: “Obey the Biblical injunction: seek and ye shall find. But seek not to find that for which ye seek.”
Role playing for krypton and friends
They may be loners, but the noble gases have many uses. Just think how dull it would be downtown without the red glow of neon lights or the blue-white of krypton. They play more profound roles, too. Superconductivity, for example, was discovered while searching for the coldest temperatures on Earth using liquid helium.
In the second world war, the Allies wanted to know how Hitler’s attempts to build an atomic bomb were going. So they attached a trap beneath a bomber and flew it over suspect German sites in search of xenon-133. This is a fission product of uranium that doesn’t react with anything else and has a half-life of five days, so should hang around long enough to be detected. A positive result would have been definitive, but the negative result they obtained meant that they were looking at the wrong sites, or the experiment was somehow flawed, or – as proved to be the case – Hitler didn’t have the bomb.
Xenon-133 is also valuable in medicine. It is used as a radioactive marker to identify pulmonary embolisms, and xenon gas is an excellent anaesthetic, and is used today in Russia and Germany.
