William Ramsay’s Noble Quest

This magnificent nighttime view of the Montreal skyline shows the glow of neon lights reflected in the waterfront. The glowing colours are produced by passing an electric current through a sealed glass tube of a rarefied gas such as neon, hence the name. The gas is not necessarily neon, however. A range of gases are used to produce light of different colours.

Montreal Skyline. Credit: Matthieu Levésque.

Neon and the other noble gases that are often used for fluorescent lighting were unknown to Mendeleyev. This is the story of their discovery. 

Group VIII

When Mendeleyev compiled his Periodic Table he classified the elements according to their chemical properties and slotted each element into one of eight columns or Groups, where those in each Group had similar properties. This structure remains the basis for the Periodic Table even today. 

The Periodic Table (1871)

Unlike the modern Periodic Table, Mendeleyev’s Group VIII included various metals that did not seem to fit in any of the other seven columns. In today’s Periodic Table, they are still found together as three rows of four elements forming the right half of the central block known as transition metals, shown in pink below. These elements include the corrosion resistant noble metals. This was originally an ancient alchemical term for silver (Ag) and gold (Au) that reflected their suitability for coinage and jewellery. But even today, the markedly unreactive elements that form the bottom two quartets of Mendeleyev’s Group VIII are known as noble metals.

The modern Periodic Table of the Elements

Our knowledge of the true Group VIII elements arrived towards the end of the nineteenth century and one man, the Scottish chemist William Ramsay (1852-1916), played a key role in the discovery of each of them. Their identification as new elements relied on spectroscopy, which might be described as chemical fingerprinting. This was necessary to distinguish and classify these otherwise rather featureless, unreactive gases. Each element emits light at characteristic wavelengths when its electrons are excited by heating or by the passage of an electric current. This is why fluorescent tubes shine with a characteristic colour that depends on the gas they contain.

Lord Rayleigh

Lord Rayleigh’s Puzzle

Lord Rayleigh (John William Strutt, 1842-1919) was the second Cavendish Professor of Physics at the University of Cambridge, but in 1884 he left Cambridge for his baronial estate in Essex where he continued to work in his private laboratory. Rayleigh had a wide range of interests and in the early 1890s he made a perplexing discovery while studying the composition of air. It seemed that nitrogen in the air was slightly denser than nitrogen derived from chemical compounds. The difference was just 0.5%, but he had checked his results cautiously and meticulously over a two-year period and he was in no doubt that his findings were correct. In 1892 he reported his results in the journal Nature, still in search of an answer. He opened with an invitation:

I am much puzzled by some recent results as to the density of nitrogen, and shall be obliged if any of your readers can offer suggestions as to the cause.

There was no immediate response. 

Ramsay was Professor of Chemistry at University College, London, and when the issue was brought to his attention he could see where the answer might lie. Ramsay realised that the anomalous density measurements might be due to the presence of an additional unknown gas in the atmospheric ‘nitrogen’ sample.

Argon

Henry Cavendish

At around this time Rayleigh found out that the same discovery had been made a century earlier by Henry Cavendish (1731-1810), a painfully shy, but extremely rich, scientist whose remarkable research remained largely unpublished in his notebooks until long after his death. Rayleigh’s own professorship at the Cavendish laboratory in Cambridge was named in his honour and established through an endowment by his relative William Cavendish, 7th Duke of Devonshire. In 1795 Cavendish had undertaken a long laborious experiment in which he removed oxygen, nitrogen, carbon dioxide and water vapour from the air. He found that after removing the known atmospheric gases a small bubble of gas remained that he was unable to identify and suggested that it was a new unreactive constituent of air.

Rayleigh joined forces with Ramsay to establish the identity of this curious atmospheric component. Ramsay took a quantity of Rayleigh’s atmospheric ‘nitrogen’ sample and passed it over red-hot magnesium. This removed the nitrogen, which bonded with the magnesium to form a solid grey powder—magnesium nitride. What remained was a small volume of gas that did not partake in the reaction. Ramsay described this gas as: an astonishingly indifferent body, as it would not even react with the extremely reactive gas fluorine. 

Ramsay’s colleague William Crookes (1832-1919) analysed the spectrum of the new gas and found that it displayed a hitherto unknown series of emission lines, so it was indeed a new element. Rayleigh and Ramsay named the new gas argon, from the Greek word argós meaning idle or lazy. It forms around one percent of the Earth’s atmosphere.

Helium

Following his announcement of the discovery of argon, in February 1895 Ramsay was contacted by the mineralogist Henry Miers who told him of an intriguing property of a uranium-containing mineral called cleveite. According to Miers, an American chemist William Hillebrand had found that cleveite released an unreactive gas when heated with sulphuric acid. Hillebrand presumed this gas was nitrogen, but Miers was not so sure. Ramsay decided to investigate for himself. He bought a sample of cleveite for three shillings and sixpence and obtained a sample of the unreactive gas. Chemical analysis ruled out all known gases except argon. But when the spectrum of the gas was analysed the mystery only deepened—this was not argon it was a second inert gas.

Helium was originally thought to be an element found in the sun, but not on Earth.

Within a week, Crookes had found the solution and it was quite incredible. Crookes identified the gas as helium. But helium had never been seen on Earth before. Its existence was deduced from the spectrum of the sun. It was first observed in 1868 by the French astronomer Jules Janssen during a total eclipse in Guntur, India, and later the same year by English astronomer Norman Lockyer who suggested it must be a constituent of the sun that was unknown on Earth. Lockyer and his collaborator Edward Frankland named the element helium after helios, Greek for sun. But without a physical sample in the laboratory their identification would always be open to dispute. Indeed, they assumed that helium must be some unknown kind of metal, which is why it has the suffix ‘ium’ usually reserved for metals. 

Neon, Krypton and Xenon

Now that Ramsay had proved that helium was actually an inert gas that certainly did exist on Earth, he realised there might be other inert gases in the atmosphere awaiting discovery. Ramsay’s strategy for tracking them down was to liquefy a quantity of air by cooling it to below minus two hundred degrees Centigrade, then to heat the liquid air gradually, raising its temperature and distilling off its various components. Although argon is fairly abundant, the other inert gases make up much smaller proportions of the air. (Xenon forms just one part in ten million of air.) Nevertheless, by 1898 Ramsay and his assistant Morris Travers had succeeded in isolating three more inert gases in sufficient quantities to confirm their chemical inactivity and photograph their spectra, thereby establishing each as a new element. Like argon they were given names derived from Greek. Neon from neos meaning new, krypton from kruptos meaning hidden and xenon from xenos meaning strange. Collectively the inert gases: helium, argon, neon, krypton and xenon are known as noble gases by analogy with the unreactive noble metals.

Noble Gases

William Ramsay personifying chemistry in Vanity Fair magazine in 1908.

Ramsay had not just discovered one or two new elements, he had added an entire new Group to the Periodic Table. He was knighted in 1902 and two years later Sir William Ramsay became the first Briton to receive a Nobel Prize for Chemistry. Rayleigh received the Nobel Prize for Physics in the same year for his many contributions to physics including his role in the discovery of argon.

The image on the right from Vanity Fair magazine shows Ramsay in front of the Periodic Table where he is indicating Group VIII, the noble gases that he had discovered. Helium (He) heads the group, but is cut off in the picture. Beneath helium are neon (Ne), argon (Ar), krypton (Kr) and xenon (Xe). (In the picture argon and xenon are identified by their old symbols ‘A’ and ‘X’.) This Periodic Table was, in fact, already out of date. Ramsay had identified another noble gas radon (Rn) that is now found beneath xenon in the table. I will have more to say about radon in the next blog post.

Further Information