In 1892 Lord Rayleigh published a short note in Nature about a discrepancy he could not explain. He had prepared nitrogen two ways. Nitrogen extracted from air, with the oxygen and the water and the carbon dioxide removed, came out at 1.2572 grams per litre. Nitrogen produced chemically, by decomposing ammonia, came out at 1.2505.
The difference was about half a percent. It was also about a thousand times larger than his experimental error, and it would not go away no matter how often he repeated it.
Rayleigh was a physicist and treated it as a measurement problem. William Ramsay, professor of chemistry at University College London and born in Glasgow on 2 October 1852, treated it as a chemistry problem, and asked a different question: what if the nitrogen from the air is not pure nitrogen?
Taking the Nitrogen Away
Ramsay’s method was direct to the point of brutality. He took atmospheric nitrogen and passed it repeatedly over magnesium heated to red heat, which greedily forms magnesium nitride. Run the gas over hot magnesium long enough and the nitrogen is consumed.
Something was left. Roughly one part in eighty of the original volume remained after the nitrogen was gone, and it was denser than nitrogen, and it did nothing at all. It would not combine with magnesium, or oxygen, or hydrogen, or chlorine, or sodium, or anything else he tried on it.
The decisive evidence was not chemical, it was optical. Put the gas in a discharge tube and it produced a spectrum of lines that matched nothing in the catalogues. This was the same technique that had already found an element in the Sun almost thirty years before anyone found it on Earth, and it meant that whatever the residue was, it was new.
Ramsay and Rayleigh announced it in January 1895 and named it argon, from the Greek for idle or lazy. It makes up just under one percent of the atmosphere. It had been sitting in every flask of air anyone had ever measured, and for a century nobody had noticed because it never reacted with anything and therefore never showed up in an analysis that depended on reactions.
The Awkward Problem of Where to Put It
Then the real difficulty began, and it was not experimental. Argon’s atomic weight came out near 40, which put it uncomfortably close to calcium, and its valence was zero, which was not a number the periodic table had a column for.
Mendeleev’s table had made its reputation by predicting elements from the gaps in it, which is precisely why an element that fitted no gap was such a problem. Mendeleev’s own response was to deny that argon was an element at all. He proposed that it was a triatomic form of nitrogen, N3, analogous to ozone, and he was not being unreasonable: an element with no chemistry is very nearly a contradiction in terms, since chemistry is how elements are identified.
The argument was settled by finding more of them.
Five Elements in Four Years
In March 1895 Ramsay heard that the mineral cleveite gave off gas when treated with acid. He collected it, ran it through a discharge tube, and saw a brilliant yellow line. He sent the tube to William Crookes for a proper measurement. It was the line Janssen and Lockyer had recorded in the solar spectrum during the eclipse of 1868 and named helium, after the Sun, on the assumption that it was an element that existed only there.
Helium on Earth, and inert. Now there were two.
Ramsay reasoned that if the table needed a whole new column, the column should have more members, and he went looking for them where they had to be, in the air itself. With Morris Travers he built apparatus to liquefy air and distil it in fractions, which was at the limit of what was technically possible in 1898. In May and June of that year the fractions produced three more spectra nobody had seen. They named them krypton, the hidden one, neon, the new one, and xenon, the stranger.
Twelve years later, working with Robert Whytlaw Gray, Ramsay isolated the last of the family, the radioactive gas that comes off radium, and named it radon. That one connected the new column directly to the work on radioactivity going on in Paris at the same time, since radon is what radium becomes.
The table now had its Group 0, sitting at the far right, and it worked. The zero valence was not an anomaly to be explained away, it was the consequence of a filled outer electron shell, which is the reason the modern table is drawn the way it is. Ramsay had no electrons to reason with in 1898. He had densities and spectra and an argument from symmetry.
He took the 1904 Nobel Prize in Chemistry. Rayleigh took the Physics prize the same year, which is a neat piece of committee work: one discrepancy, two disciplines, two prizes.
What the Lazy Elements Turned Out to Be Good For
The elements that refused to do anything became useful precisely because they refuse to do anything.
- Argon fills the space inside welding shrouds and light bulbs and the display cases of archives, because a gas that will not react is a gas that will not oxidise what it surrounds. Museums store manuscripts in it
- Helium cools the superconducting magnets in every MRI scanner and every particle accelerator, because it stays liquid down to a degree above absolute zero and will not freeze
- Neon produced the sign, which is worth noting because the colour is not a dye or a coating. It is the emission spectrum of the element itself, the same red orange lines Ramsay saw in the tube in 1898
- Xenon is an anaesthetic and an ion propellant, and krypton defined the metre from 1960 to 1983, when the length of the standard was fixed by the wavelength of one of its orange lines
The Half Percent
The part of this story worth keeping is the beginning rather than the end. Five elements, a new column of the periodic table and the eventual explanation of electron shells all came out of a disagreement of 0.0067 grams per litre between two samples of what everyone agreed was the same gas.
Rayleigh could have called it experimental error and moved on, and most people would have. Lavoisier had built modern chemistry a century earlier on exactly this habit, weighing things before and after with obsessive care and refusing to accept a discrepancy as noise.
Ramsay’s contribution was to assume that the anomaly was not an error in the measurement but a fact about the world, and then to design the one experiment that could tell the difference. The air of every laboratory in Europe had been one percent unexplained for a hundred years.
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