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Mendeleev's periodic table had no place for gases that refuse to react, because none were known. A whole group of them was found not by a dramatic experiment but by taking a very small number seriously. This post follows William Ramsay from that number to five new elements, checks the maths behind it, and links it to the kinetic theory you need for JEE.
Who was William Ramsay?
The Nobel Prize biography says Ramsay was born in Glasgow on 2 October 1852, earned his doctorate in Tübingen with a thesis on orthotoluic acid, and became Professor of Chemistry at University College, Bristol, in 1880, then held the Chair of Inorganic Chemistry at University College London from 1887 until 1913. He won the 1904 Nobel Prize in Chemistry "in recognition of his services in the discovery of the inert gaseous elements in air, and his determination of their place in the periodic system", according to the Nobel facts page. Lord Rayleigh received the physics prize the same year for the shared work. Ramsay died in 1916.
Why was nitrogen from air heavier?
Rayleigh told the story in his Nobel lecture. He measured the density of nitrogen made in two ways: by removing oxygen from air, and by a method that took part of its nitrogen from ammonia. "To my surprise and disgust," he said, the two densities "differed by a thousandth part", far beyond his experimental error.
Instead of averaging the two, he made the gap bigger. Using pure oxygen instead of air in the ammonia method, so that all the nitrogen came from ammonia, the ammonia nitrogen became "1/200 part lighter than that from air". His advice is worth copying into your notebook: "It is a good rule in experimental work to seek to magnify a discrepancy when it first presents itself, rather than to follow the natural instinct of trying to get quit of it."
Ramsay supplied the explanation. In his own Nobel lecture he recalled reading Henry Cavendish's eighteenth-century account of sparking nitrogen with oxygen, which left a small residue of "not more than 1/125th of the whole". Opposite that line, in his copy, he had written "look into this". In 1894 he suggested to Rayleigh that air contained an unknown heavier gas. Working at first separately, then together, they removed nitrogen from air with red-hot magnesium and with electric sparks, and isolated a gas making up about one per cent of the atmosphere. They named it argon and announced it in August 1894; the full account, Argon, a new constituent of the atmosphere, appeared in 1895.
Does a little argon really explain the 1/200?
Let us check. In his lecture Ramsay quotes measurements by Kellas: argon is 0.01186 parts by volume of "atmospheric nitrogen", the gas left when oxygen is removed from air. For ideal gases at the same temperature and pressure, density is proportional to molar mass, so we compare mean molar masses.
Pure N2: M = 28.01 g/mol.
Atmospheric nitrogen: M = (1 – 0.01186) × 28.01 + 0.01186 × 39.95 = 27.68 + 0.47 = 28.15 g/mol.
Ratio = 28.15 / 28.01 = 1.005, so atmospheric nitrogen is about 0.5% denser.
And 1/200 is 0.5%. A gas at just over one per cent explains Rayleigh's gap almost exactly. The common slip is to use mass fractions instead of mole (volume) fractions when averaging molar mass. For gas mixtures, volume fraction equals mole fraction, so multiply by those.
How did the specific heat ratio prove argon is monatomic?
Rayleigh's lecture also gives argon's ratio of specific heats, Cp/Cv, as 1.67, "the highest possible". The kinetic theory you study for JEE tells you why. A monatomic ideal gas has only translational energy, so Cv = (3/2)R and Cp = Cv + R = (5/2)R. That gives γ = 5/3 = 1.67. A diatomic gas like nitrogen near room temperature has γ = 7/5 = 1.40. So a γ of 1.67 meant argon's molecules were single atoms. Ramsay later used the same test on helium.
How were helium, neon, krypton and xenon found?
According to Ramsay's lecture and the Nobel biography:
Helium, 1895. Searching minerals for a source of argon, Ramsay examined the gas given off by a uranium mineral and saw a brilliant yellow spectral line. It matched a line first seen in the Sun's spectrum by Pierre Janssen during a solar eclipse that an expedition had travelled to India to observe in 1868.
A prediction. Helium and argon looked like members of one family. In 1897 Ramsay predicted, from the periodic table, an undiscovered element with atomic weight about 20.
Krypton, neon, xenon, 1898. With his assistant Morris Travers and liquid air from William Hampson's new machine, Ramsay examined the last traces of evaporating liquid air and the lightest fractions of liquefied argon. Krypton ("hidden") was reported to the Royal Society on 3 June 1898, neon ("new") on 13 June, and xenon ("stranger") in September.
These gases really are hidden. NASA's Earth fact sheet lists neon at 18.18 parts per million of dry air and krypton at 1.14 parts per million, compared with argon at 9340.
Ramsay also made a mistake worth knowing. He and Travers announced a gas they called "metargon", which, he admitted in his lecture, "turned out to be argon in the main", contaminated with carbon monoxide from impure phosphorus.
What can a student take from Ramsay's way of working?
Chase small gaps. A difference of 0.5% led to a new group of the periodic table. When your answer is slightly off, find out why before moving on.
Write in the margins. "Look into this" in a library book paid off years later. Keep a list of things you do not yet understand.
Let theory tell you where to look. The periodic table predicted neon before anyone saw it.
Own your errors. Metargon did not make Ramsay less of a scientist.
If gas laws feel abstract, the Fermi Simulation Lab has interactive simulations for ideas that are easier to see than read. For the exceptions in group 18 and elsewhere, see our guide to periodic trends and their exceptions.
Tonight, redo the density check above for air with 0.934% argon by volume, and write one sentence on what Rayleigh's rule means for your next mock test.
