in your element Argon out of thin air Markku Räsänen remembers making a neutral compound featuring argon, and ponders on the reactivity of this inert element. he discovery of the noble gases shows improper handling of the reactive precursor how innovative researchers have HF, the first signals of this new species were Tbeen when it came to separating and obtained in my group on 21 December identifying minor amounts of components 1999 — a time of the year conducive to from mixtures using relatively simple tools. experimentation, with no interfering students First indications of an inert component present in the lab. Conclusive experimental in air appeared in 1785 through results were obtained from the vibrational Henry Cavendish’s experimental studies, spectral effects of H/D and36 Ar/40Ar isotopic when he found about 1% of an unreactive substitutions3. A neutral molecule containing component. He could not, however, identify argon — I couldn’t have wished for a better the unreactive species, which turned out to Christmas present. be argon, and the names connected with its A number of stable argon compounds discovery are Lord Rayleigh and Sir William have been computationally predicted, and Ramsay (pictured). In 1892, Rayleigh and are waiting for experimental verification. Ramsay’s exceptionally skilful eudiometric Recent extensions of this chemistry measurements, after chemical separation of include, for example, the preparation and the constituents, revealed that the density / ALAMY © THE PRINT COLLECTOR characterization of ArBeS (ref. 4) and ArAuF of nitrogen prepared by removing oxygen, (ref. 5). Our chemical understanding of the carbon dioxide and water from air with chemically. This idleness of argon finds many group 18 elements is changing, as progress heated copper differed from the density of chemical and industrial uses, for example as in noble gas chemistry has slowly started nitrogen prepared from ammonia. a chemically inert atmosphere, in welding, to enter schoolbooks, starting with xenon, The understanding that this meant food storage and insulating windows. which shows extensive bond formation. another element, X, was present, and its Argon isotopes also serve in geological age The future will tell as to what extent identification through spectroscopy — not determinations, and laser technology uses noble gas hydrides and related species exist. available at the time of Cavendish’s studies argon extensively, in the form of Ar+ cations in At present, the number of experimentally — came two years later. Ramsay suggested1 gas lasers and excited ArF in exciplex lasers. characterized noble gas hydrides amounts to Lord Rayleigh: “Seeing that X is very Owing to the same inertness, it has to about 30, and it is easy to design more inactive, what do you think of argon αργον been hard to coerce argon into forming of such metastable molecules. Chemists (idle) for a name?” They had been working neutral compounds. In 1995, noble gas (Ng) want to explore the frontiers of chemistry separately but on very similar studies, and hydrides were discovered that have a general for basic knowledge, and research of novel made a joint announcement. Their findings structure of HNgY, where Ng is Kr or Xe bonds between noble gases and other on noble gases were recognized in 1904 by and Y an electronegative atom or fragment. elements offers a challenging field for this. the Nobel Prize in Physics for Rayleigh and These molecules consist mainly of a covalent The excellent computational tools constantly that in Chemistry for Ramsay. bond between H and Ng and an ionic bond being developed with experimental work Argon didn’t fit in the periodic table between Ng and Y. offers very good possibilities for success. ❐ known at the time, and it was Ramsay who Perhaps surprisingly, as early as 1995, low- suggested including an additional column, level quantum chemical calculations suggested MARKKU RÄSÄNEN is in the Department of group 18. Helium, and other elements the existence of a chemical compound Chemistry, University of Helsinki, PO Box 55 subsequently discovered, were to find their containing an Ar atom2: hydridoargon (A. I.Virtasen aukio 1), FIN-00014 University place with the ‘rare gases’. This name is fluoride, HArF, that was predicted to be of Helsinki, Finland. somewhat misleading, however, with argon preparable at cryogenic conditions. The e-mail: [email protected] being the third most abundant atmospheric synthesis of HArF from H, Ar and F in solid References substance after nitrogen and oxygen (it argon was tried, in a similar manner to that of 1. Klein, M. L. & Venables, J. A. (eds) Rare Gas Solids Vol. 1, p19 makes up about 0.94% of our atmosphere). HXeI from H + Xe + I, but several attempts (Academic Press, 1976). 2. Pettersson, M., Lundell, J. & Räsänen, M. J. Chem. Phys. A preferred term for group 18 is ‘noble gases’, in my group failed. Still, during the following 102, 6423–6431 (1995). in reference to their reluctance to bond several years, more and more extensive 3. Khriachtchev, L., Pettersson, M., Runeberg, N., Lundell, J. computational studies strengthened the belief & Räsänen, M. Nature 406, 874–876 (2000). 4. Wang, Q. & Wang, X. J. Phys. Chem. A 117, 1508–1513 (2013). that HArF can exist. After learning the reasons 5. Wang, X., Andrews, L., Brosi, F. & Riedel, S. Chem. Eur. J. for the failed experiments, mainly due to 19, 1397–1409 (2013). P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br 82 NATURE CHEMISTRY | VOL 6 | JANUARY 2014 | www.nature.com/naturechemistry © 2014 Macmillan Publishers Limited. All rights reserved.