The Discovery of the Other Inert Gases
During the next three years four other inert gases were discovered. In 1868 in India French astronomer Pierre-Jules-César Janssen (1824–1907) observed a total eclipse of the sun and made the first spectroscopic study of its chromosphere [28]. English astronomer Sir Norman Lockyer (1836–1920) found that a new yellow spectral line (D3) noted by Janssen did not belong to any known element but to a hypothetical new element, which he named “helium” (Greek, sun) [29]. (The suffix “ium,” characteristic of the names of metals, apparently indicated that Lockyer thought that the element was a metal.) For a quarter of a century scientists believed that although it might possibly exist on the sun, it had never been found on the earth. In fact, most spectroscopists doubted its existence, and some even ridiculed it [29].
In 1888–1890 American chemist William F. Hillebrand (1853–1925) treated the mineral uraninite with acid and noted the evolution of a gas that he thought was nitrogen [30]. Ramsay disagreed with Hillebrand’s results and repeated the experiment with a similar uranium mineral, cleveite [31, 32]. He obtained nitrogen but also argon and another gas with different spectral lines. He sent samples of the unknown gas to Sir Norman Lockyer and Sir William Crookes. By March 1895 the new gas was shown to be identical with Lockyer’s solar element—helium [27, 33]. At about the same time Swedish chemist Per Theodor Cleve (1840–1905) and his student Nils Abraham Langlet (1868–1936), independently of Ramsay, also discovered helium in cleveite. Ramsay had announced his discovery earlier, but the Swedes’ gas was purer than Ramsay’s, and they obtained a much better value for its atomic weight [34].
Because the atomic weights of helium and argon were found to be ca. 4 and 40, respectively, Ramsay believed that they might belong to a new group of the periodic system and consequently an intermediate member of the group with an atomic weight of ca. 20 might exist [35]. After abandoning their attempts to discover new gases by heating rare minerals, Ramsay and Travers decided to fractionate liquid air. They removed oxygen and nitrogen by reaction with red-hot copper and magnesium, respectively [36, 37]. On May 31, 1898 they examined the spectra of the inert residual gas and observed a bright yellow line with a greener tint than that of helium and a brilliant green line that did not coincide with any line of hydrogen, helium, argon, or mercury [21, pp 90–91]. They announced their discovery on June 6, 1898 and named the new gas krypton (Greek, hidden) [17, pp 251–255].
However, krypton was not the element intermediate between helium and argon that Ramsay and Travers were seeking but a denser one. Therefore, they continued their search for more than two years for the lighter gas by liquefying and solidifying their argon by surrounding three liters of it with liquid air boiling under reduced pressure, allowing the argon to volatilize, and collecting the more volatile portion, which distilled off first. According to Ramsay’s laboratory notebook, the lightest fraction
gave magnificent spectrum with many lines in red, a number of faint green, and some in violet. The yellow line is fairly bright, and persists at very high vacuum, even phosphorescence [21, pp 95–97].
Willie, Ramsay’s 13-year-old son, asked his father, “What are you going to call the new gas? I should like to call it novum.” (Apparently, at that time, teenagers were fluent in Latin. What a contrast with today!) Ramsay agreed but chose a similar, but better sounding name “neon” (Greek, new) for the gas discovered in June 1898.
By using a new liquid air machine provided by British chemist and industrialist Ludwig Mond (1839–1909), Ramsay and Travers were able to prepare larger amounts of neon and krypton. By repeated fractionation of krypton, on July 12, 1898 they isolated an even heavier inert gas that exhibited a bright blue glow in a vacuum tube. They named it xenon (Greek, stranger) [17, pp 251–255].
The last inert gas to be discovered was first called radium emanation, emanon (no name spelled backwards!), or niton. Pierre (1859–1906) and Marie Curie (1867–1934) noted that when air contacts radium compounds, it becomes radioactive. In 1900 Friedrich Ernst Dorn, Professor of Physics at the Universität Halle, explained this fact by showing that one of the disintegration products of radium was an inert gas, now known as radon [38–40]. In 1903, together with future (1921) Nobel chemistry laureate Frederick Soddy (1877–1956), Ramsay detected the presence of helium in the emanations of radium. In 1910 Ramsay and Robert Whytlaw Gray (1877–1958) determined its density and showed that it is the densest gas known [41].
We now fast-forward more than a half century. As chemical educators, we visualize ourselves as open-minded scientists uninfluenced by authority, who pride ourselves on viewing our scientific beliefs not as absolute truths but as tentative hypotheses that we are prepared to modify or abandon in the light of new discoveries. However, in July 1962, while attending a conference on Advances in the Chemistry of Coordination Compounds held at Ohio State University in Columbus, my open-mindedness was put to the test—and I flunked! [42]. Someone interrupted one of the lectures and announced that Neil Bartlett, a young (born 1932) and comparatively unknown lecturer at the University of British Columbia at Vancouver, had prepared a compound of an inert gas—xenon hexafluoroplatinate(V), XePtF6 [43, 44].
For chemists of my generation the inertness of the inert gases was a watchword that we diligently and regularly taught to students in our introductory chemistry courses. A common witticism among us was that a book on The Chemistry of the Inert Gases would be a volume with blank pages. Therefore, faced with the news that one of chemistry’s most cherished assumptions had been broken, I chose to assume that the announcement was a joke. After all, chemical educators, especially when away from their academic home grounds, have been known to delight in foisting all sorts of pranks on their unsuspecting colleagues. But the joke was on me when I found a detailed report about Bartlett’s discovery in Chemical & Engineering News on my return from the conference. This discovery forced every teacher or textbook author of introductory chemistry to revise his or her previous treatment of atomic structure—a humbling experience that shows how far short we fall from our ideals.
In 1962 several inert gas fluorides were isolated [45–48], and soon many other inert gas compounds were prepared. Since then, inert gases are known as “noble gases,” and many books, numerous reviews, a book-length bibliography, and an entire volume of Gmelins Handbuch der anorganischen Chemie have been devoted to the chemistry of a group of elements that not too long ago were universally thought to have no chemistry at all. Surely, a cautionary tale for all of us!
