Krypton, Neon, and Xenon
In the history of inert gases, many problems stuck in starting. Out of many problems and their several reasons ; one of them was that scientists were dealing with very small amounts of Argon and Helium. To isolate them from air, one had to chemically remove oxygen, nitrogen, hydrogen, and carbon dioxide. All inert gases constitute a negligible part of the earth’s atmosphere.
Detection and isolation of so minor traces was difficult specially when Helium and Argon were known.
Another reason was chemical inactivity of Argon and Helium. Even the most active reagents (for instance, fluorine) were powerless to combine with these gases and isolate them . Chemists had no way of studying inert gases and only physical methods could bring some results. Therefore better physical methods were required and they were being developed during this discovery period.
Scientists developed experimental techniques for analysing small amounts of gases, perfected spectroscopes and devices for determining gas densities.
Finally, an event took place that was of extreme importance for the history of inert gases. Two engineers, U. Hampson from England and G. Linde from Germany, invented and effective process for liquefaction of gases. Hampson built an apparatus that produced on liter of liquid air per hour. The success gave an impetus to the creative though of scientists.
In early 1898 M. Travers, Ramsay’s assistant, began to design a refrigerating apparatus for preparing large amounts of liquid argon. Since atmospheric gases liquefy at different temperatures, they can easily be separated from one another. The discoveries of argon and helium are remarkable also in that they set the chemists thinking not only about the nature of chemical inertness ( the phenomenon was understood only about a quarter of a century later) but about the periodic law and periodic system which were under a serious threat. Three most important characteristics of argon and helium (atomic masses, zero valence, monatomic molecule) put both gases outside the system. That is why Mendeleev was so readily attracted by the convenient thought about N3.
History has a striking power of prediction. Argon had not been properly discovered yet, when on May 24, 1894, Ramsay wrote a letter to Rayleigh in which he asked whether it had ever occurred to him that there was indeed a place in the periodic table for gaseous elements. For instance:
Li Be B C N O F X X X
Mn Fe Co Ni
? Rd Ru Pd…
Ramsay assumed that the system’s small period could contain a triad to elements similar to those of iron and platinum metals in the great periods. The discoveries of argon and helium gave rise to an idea that these gases could occupy the places of two Xs in Ramsay’s graph. The atomic masses of these elements, however (4 and 40, respectively), proved to be too different for He and Ar to be placed in the same period. Gradually, the idea about new triads was relegated to the background and Ramsay proposed to place inert gases at the end of each period. In this case one could even expect the discovery of an element with the atomic mass 20, an intermediate between helium and argon. Ramsay’s report at the session of British Association in Toronto in August 1897, was devoted just to this element. The report was entitled “Undiscovered Gas”. Ramsay wanted to describe interesting properties of the gas but though it unwise not to mention its most remarkable property: the gas had not been discovered yet.
And here again we see the same certainty which permeated Ramsay’s letter to his wife on the eve of argon’s discovery. But not it was not audacity of a romantic but conviction multiplied by experience. The undiscovered gas turned out to be neon. Owing to a whim of fate (a frequent thing in science) the discovery was preceded by another event. The new gas could, obviously, be discovered by gradual evaporation of liquid air and by analysis of the resulting fractions, the ones lighter than argon being especially interesting. On May 24, 1898, Ramsay and Travers received a Dewar flask with liquid air. Unfortunately (or, rather, fortunately) the amount of air was too small to search for argon’s predecessor and the scientists decided to use the material for perfecting the procedure of liquid air fractionation. Having done so, Ramsay and Travers discovered by the end of the day that the fraction remained was the heaviest one. For a week the fraction remained neglected until on May 31 Ramsay decided to investigate it. The gas was scrubbed from possible impurities of nitrogen and oxygen and subjected to spectral analysis. Ramsay and Travers were dumbfounded when they saw a bright yellow line which could belong neither to helium nor sodium. Ramsay wrote down in this diary: “May 31. A new gas. Krypton.” Recall that this name was previously given to undiscovered helium. Now the name found its place in the history of inert gases. Krypton, however, was not the gas about which Ramsay made a report. Its density and atomic mass were higher than the predicted ones.
The discovery of neon promptly followed. Ramsay and Travers selected light fractions formed on the distillation of air and discovered a new inert gas in one of them. Ramsay later recollected that the name “neon” (from the Greek neos for “new”) had been proposed by Ramsay’s twelve-year-old son. In this case the experiment was performed by Travers alone since Ramsay was away. It was on the 7th of June. Then a whole week was required to confirm the result, obtain greater amounts of neon, and determine its density. Neon, as had been expected, turned out to be an intermediate between helium and argon although t had not yet been isolated as a pure gas. The problem of complete separation of neon and argon was solved later.
Still another inert gas was to be discovered by Ramsay and Travers. The scientists, however, did not feel as certain as in the case of neon. One day in July, 1898, the colleagues were busy with distilling liquid air and separating it into fractions. By midnight they collected more than 50 fractions discovering krypton in the last of them (No. 56). After that upon heating the apparatus one more fraction was collected (No. 57) consisting, mainly, of carbon dioxide traces. Ramsay and Travers argued about the expediency of studying it and at last decided to proceed with the of experiment. Next morning the scientists observed the spectrum of fraction No. 57, which turned out to be highly unusual. Ramsay and Travers concluded that it could be attributed to a new gas. pure xenon, however was prepared only in the middle of 1900. The name “xenon” originates from the Greek xenos, which means “stranger”