in your element Peculiar protactinium Richard Wilson relates how the rare, highly radioactive, highly toxic element protactinium puzzled chemists for a long time, and was discovered and named twice from two different isotopes before finding its place in fundamental research. he first glimpse of element 91 residues that contained the protactinium came in 1900, thirty years after they succeeded in isolating. TDmitri Mendeleev predicted The 1960s and 70s saw our knowledge its existence, when William Crookes, of protactinium’s chemistry expand vastly the discoverer of thallium, put down his to include its oxides, halides, coordination studies with cathode rays and spectroscopy complexes and organometallic complexes to investigate Becquerel’s newly reported such as protactinocene. A declining interest phenomenon — radioactivity. in a thorium fuel cycle, however, and the Crookes separated a highly radioactive abandonment of research on its chemistry material from his uranium salts that was able by all but a few by the mid-1980s, sent to expose a photographic plate, yet could protactinium back into relative obscurity not be detected by a spectroscope1, leaving despite its importance in the actinide series. him unable to identify the material as a new Protactinium’s significance stems from IRINA ©SHUTTERSTOCKPHOTO.COM/TISCHENKO element. It was in 1913, based on the work recognition of uranium as a strategic resource the fact that it is the first actinide to possess of Rutherford and Soddy on the nature of for weapons, inspired the investigation of a 5f electron, with intermediate properties radioactive decay and isotopes, that Fajans a thorium-based nuclear fuel cycle, which between those of thorium and uranium. and Göhring identified element 91. They involves protactinium as a major player. Its ground-state electron configuration named it brevium because the isotope they This newly identified use for protactinium [Rn]5f 26d7s2 is only marginally more stable had encountered (234mPa) had a brief half-life resulted in the separation of approximately than the [Rn]5f6d27s2 configuration, a result of only one minute2. 100 grams of it from 60 tonnes of uranium- of its nearly degenerate 5f and 6d orbitals. Meanwhile, a hunt was on for the element refining sludges. The availability of this stock Its unique electronic structure makes whose decay would generate actinium, for research in the 1950s led H. W. Kirby, an protactinium a critical stepping stone for which Fajans’s isotope was not. Soddy American pioneer in protactinium research, understanding the electronic structure and hypothesized that this element must be an to speculate that the 1960s would see much bonding of the 5f elements. 4 alpha particle (2He) emitter of the group v of the “mystery and witchcraft” fade from the This becomes increasingly evident elements positioned under tantalum in the chemistry of this element5. as computational chemists continue to periodic table, and thus named eka-tantalum The mystery and witchcraft mostly came demonstrate the important interplay of in those days. In March of 1918, Lise Meitner from protactinium’s confusing chemical the 5f, 6d and even the core 6p electrons, and the conscripted Otto Hahn beat Soddy properties, not quite truly actinide yet when describing the electronic structure in the hunt for eka-tantalum and discovered not quite transition metal. This confusion and bonding behaviour of the actinides, the isotope 231Pa, code-named “abrakadabra” persisted until the actinide elements were suggesting that protactinium still has more in their correspondence, which indeed identified as a discrete periodic series. contributions to make to chemistry. Although converted into actinium through alpha Element 91 shares a pentavalent oxidation laboratory work with protactinium has been decay. Their suggested name protoactinium state with the transition metals niobium largely abandoned, its future contributions — meaning ‘parent of actinium’ — and tantalum, and possesses a tetravalent to chemistry may well come from where superseded Fajans’s brevium because of the oxidation state accessible under reducing Meitner and Hahn first found it, in silico. ❐ longer half-life (roughly 32,000 years) of conditions, in a similar manner to the other their isotope 231Pa (refs 3,4). In 1949, IUPAC tetravalent actinides. However, unlike the RICHARD E. WILSON is in the Heavy would simplify it to protactinium, Pa. other pentavalent actinides, protactinium Elements Group, Chemical Sciences and Little chemical research would be exists as a mono-oxo ion rather than the Engineering Division, Argonne National conducted with protactinium for the following di-oxo ones archetypical of uranium, Laboratory, 9700 S. Cass Avenue, Argonne, forty years — a consequence of its rarity, neptunium, plutonium and americium, Ilinois 60439, USA. e-mail: [email protected] difficulty of isolation, radiotoxicity and lack which makes it a truly unique actinide. But References of commercial application. In the 1950s, the Pa(v)’s most lamented property is perhaps 1. Crookes, W. Proc. R. Soc. London 66, 409–423 (1900) development of nuclear power, along with the its insolubility, which causes protactinium to 2. Fajans, K. & Göhring, O. Naturwissenschaften 14, 339 (1913). adsorb to glass surfaces. It proved fortunate 3. Hahn, O. & Meitner, L. Phys. Z. 19, 208 (1918). 4. Fajans, K. & Morris, D. F. K. Nature. 244, 137–138 (1973). for Meitner and Hahn, however, because it 5. Kirby, H. W. The Radiochemistry of Protactinium (National was the silica fraction of their pitchblende Academy of Sciences – National Research Council, 1959). Ra Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Rf Db Sg Bh Hs 586 NATURE CHEMISTRY | VOL 4 | JULY 2012 | www.nature.com/naturechemistry © 2012 Macmillan Publishers Limited. All rights reserved.