The discovery of inert gases ranks among the four great scientific events of the end of the 19th century that led to revolutionary changes in natural sciences, the other three being the discovery of X-rays by Roentgen, discovery of radioactivity, and the discovery of electron.
This prominence given by scientists to inert gases has many reasons.The history of their discovery is colourful and exciting. Helium, the mysterious solar element, was discovered on the earth and this fact alone illustrates how inventive and penetrating man’s mind became in his striving for deeper and better understanding of nature.
No less mysterious argon sowed confusion among scientists. Its chemical inertness made it impossible to be classified as a chemical element in the ordinary sense of the term since it revealed no chemical properties. There was nothing left for the researchers but to grow accustomed to the idea that there can be elements unable to enter into chemical reactions. The idea proved extremely fruitful. The discovery of inert gases contributed to development of the zero valence concept. Moreover, forming an independent zero group they added harmony to the periodic system. Almost twenty five years after their discovery the inert gases helped N. Bohr to develop his theory of the electron shells of atoms. This theory, in its turn, explained the chemical inactivity of the inert gases and their atomic structure became the basis of the concepts of ionic and covalent bonds. Thus, the discovery of inert gases contributed greatly to the development of theoretical chemistry.
In the early 60’s they surprised the scientific community once more. Scientists showed that Xenon (mainly) and krypton can form chemical compounds. Now more than 150 such compounds are known. Such late “debunking” of the myth about the complete chemical inactivity of inert gases is a paradoxical and interesting feature in their history.
Inert gases are among the rarest stable elements on the earth. Here are the data given by Ramsay: there is one part by volume of helium per 245 000 parts of atmospheric air, one of neon per 81 000 000, and one of argon per 106, one of krypton per 20 000 000 and one of xenon per 170 000 000. Since then these figures have remained almost unchanged. Ramsay said that xenon content in air is less than that of gold is sea water. This alone shows how excruciatingly difficult was the discovery of inert gases.
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”
If you saw the statement “Inert gases were discovered by H. Cavendish in 1785” you would treat it as a joke. But no matter how paradoxical it seems, it is essentially true. Only the word “discovered” is misused here. One would be equally justified in declaring that hydrogen was discovered by R. Boyle in 1660 or by M.V Lomonosov in 1745. In his experiments Cavendish only observed “something” whose nature became clear on hundred years later. In one of his laboratory records Cavendish wrote that, passing an electric spark through a mixture of nitrogen with an excess of oxygen, he obtained a small amount of residue, no more than 1/125 the initial volume of the mixture. This mysterious gas bubble remained unchanged under the subsequent action of the electric discharge. It is clear now that it contained a mixture of inert gases, the fact which Cavendish could neither understand nor explain.
The famous English physicist’s experiment was described in 1849 by his biographer H. Wilson in the book Life of Herny Cavendish. In the early 80’s of the 19th century Ramsay studied the reaction of gaseous nitrogen with hydrogen and oxygen in the presence of a platinum catalyst. Nothing came out of these experiments and Ramsay did not even publish his results. As he recalled later, he had just read the book by Wilson and wrote “Pay attention” against the description of Cavendish’ experiment. He even asked his assistant C. Williams to repeat the experiment but we do not know the result of the attempt. Most likely, nothing came out of it. The episode, however, turned out to be unforgettable for Ramsay (his “hidden memory”, as he called it) and played a certain role in the prehistory of argon’s discovery. At first, the English physicist J. Rayleigh was the main character in it and the need for a further development of the atomic and molecular theory was its historic background. It was essential to specify the atomic masses of the elements for the development of the theory. Numerous experiments showed that in the majority of cases the atomic masses were not integers. Meanwhile, as early as 1815–1816 the English physician W. Prout advanced a hypothesis, a landmark in the history of natural sciences, that atoms of all chemical elements consist of hydrogen atoms; thus, atomic masses had to be integers. Therefore, either Prout was wrong, or the atomic masses were determined incorrectly.
To remove the discrepancy, new studies of the composition and nature of the gases were required. Rayleigh thought it necessary to determine, first of all, the densities of the main atmospheric gases, nitrogen and oxygen, since their atomic masses could then be calculated on the basis of the density values.
Rayleigh published a short article in the influential English journal Nature on September 29, 1892. It might seem that the article was about a mere trifle; the density of nitrogen separated from atmospheric air differed from that of nitrogen obtained by passing a mixture of air and ammonia over a red-hot copper wire. The difference was very small, only 0.001, but it could not be explained by an experimental error. Atmospheric nitrogen was heavier. Thus, a mystery appeared which was described as “an anomalously high density of atmospheric nitrogen”. Nitrogen obtained by any other chemical techniques was always lighter by the same value.
What was the cause of the discrepancy? Ramsay became interested in the problem. On April 19, 1894, he met with Rayleigh and discussed the situation. Each of them, however, remained firm in his previous conviction. Ramsay believed that atmospheric nitrogen contained an admixture of a heavier gas and Rayleigh, on the contrary, felt that an admixture of a lighter gas in “chemical” nitrogen was responsible for the discrepancy.
Rayleigh’s view seemed more attractive. The composition of atmosphere had been thoroughly studied for more than a hundred years and it was hardly possible that some components of the air could have remained undetected. It is just the time to remember Cavendish’s experiment and for Ramsay’s “hidden memory” to work. On April 29, Ramsay sent a letter to his wife in which he wrote that nitrogen, probably, contained some inert gas which had escaped their attention; Williams is combining nitrogen with magnesium and is trying to establish what remains after the reaction. “We can discover a new element.”
The latter breathes confidence: an unknown gas is a new element which, like nitrogen, is inactive, i.e., it hardly enters into chemical reactions. To separate the “stranger” from nitrogen, Ramsay tried to bond nitrogen chemically and used the reaction of nitrogen with red-hot magnesium shaving (3Mg+N2 = Mg3N2); this is the only example when chemistry played a role in the discovery of inert gases. Entering into polemics with himself Ramsay, however, assumed another possibility: the unknown gas is not a new element but an allotropic variety of nitrogen whose molecular consists of three atoms (N3) like oxygen (O2–molecular oxygen and O3–ozone). The absorption of nitrogen with magnesium must be accompanied with the decomposition of the N2 molecule into atoms; the single N atom could then be added to N2 forming N3. Such was Ramsay’s thinking and later the assumption about the existence of N3 became a trump card in the hands of argon’s opponents. Fruitless attempts to separate an ozone-like nitrogen continued for more than two months but by the 3rd of August Ramsay had 100 cm3 of a gas which was nitrogen with a density of 19.086.
The scientist wrote about his success to Crookes and Rayleigh. He send an ampoule with the gas Crookes for spectroscopic investigations; Rayleigh himself collected a small amount of the new gas. In the middle of August Ramsay and Rayleigh met at a scientific session and made a joint report. They described the spectrum of the gas and underlined its chemical inactivity. Many scientists listened to the report with interest but were surprised: how could it be that air contained a new component? The eminent physisist O. Lodge even asked: “Didn’t you, gentlemen, discover the name of the new gas as well?
The difficulty about the name was settled in early November when Ramsay suggested to Rayleigh to name it argon (from the Greek for “inactive”) taking into account its exceptional chemical inactivity and to assign the symbol A to it (which later became Ar.) On November 30, the president of the Royal Society Lord Kelvin (W. Thomson who in 1871 was the first to use the name “helium”) Publicly described the discovery of a new constituent of the atmosphere as the outstanding scientific event of the year. The nature of the constituent, however, was unclear. Was it a chemical element? Such authorities as D. I. Mendeleev and J. Dewar, the inventor of the flask for storage of liquid air, believed that argon was N3. The absolute chemical inactivity of argon was a new property previously unknown to chemists and, therefore, it was difficult to study the gas (in particular, to determine its atomic mass). In addition, it became clear that argon, unlike all known elemental gases, is monatomic, i.e. its molecule consists of one atom. At a session of the Russian Chemical Society on March 14, 1895, Mendeleev declared: argon’s atomic mass of 40 does not fit the periodic system, hence, argon is condensed nitrogen N3.
Much time had passed before the many problems presented by the discovery of argon were solved. A certain role was played here by the discovery of helium, which also turned out to be an inert and monatomic gas. The argon-helium pair allowed an assumption to be made that the existence of such gases is a regularity rather than a mere chance and one could expect the discovery of new representatives of this family. However, they were not discovered until three years passed. In the meantime scientists thoroughly studied the properties of helium and argon, made precise determination of their atomic masses, and put forward ideas about the location of both elements in the periodic table.