.............................................................. Chemical investigation of hassium (element 108) Ch. E. Du ¨ llmann*†, W. Bru ¨ chle‡, R. Dressler†, K. Eberhardt§, B. Eichler†, R. Eichler†, H. W. Ga ¨ ggeler*†, T. N. Ginterk, F. Glaus†, K. E. Gregorichk D. C. Hoffmank{, E. Ja ¨ ger‡, D. T. Jost†, U. W. Kirbachk, D. M. Leek, H. Nitschek{, J. B. Patink{, V. Pershina‡, D. Piguet†, Z. Qin#, M. Scha ¨ del‡, B. Schausten‡, E. Schimpf‡, H.-J. Scho ¨tt‡, S. Soverna*†, R. Sudowek, P. Tho ¨ rle§, S. N. Timokhinq, N. Trautmann§, A. Tu ¨ rler**, A. Vahle††, G. Wirth‡, A. B. Yakushevq & P. M. Zielinskik * Departement fu ¨r Chemie und Biochemie, Universita ¨t Bern, CH-3012 Bern, Switzerland † Labor fu ¨r Radio- und Umweltchemie, Paul Scherrer Institut, CH-5232 Villigen, Switzerland ‡ Gesellschaft fu ¨r Schwerionenforschung mbH, D-64291 Darmstadt, Germany § Institut fu ¨r Kernchemie, Universita ¨t Mainz, D-55128 Mainz, Germany k Nuclear Science Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA { Department of Chemistry, University of California, Berkeley, California 94720- 1460, USA # Institute of Modern Physics, Chinese Academy of Sciences, Lanzhou 730000, P.R. China q Flerov Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research, 141980 Dubna, Russia ** Institut fu ¨r Radiochemie, Technische Universita ¨t Mu ¨nchen, D-85748 Garching, Germany †† Research Center Rossendorf e.V., D-01314 Dresden, Germany ............................................................................................................................................................................. The periodic table provides a classification of the chemical properties of the elements. But for the heaviest elements, the transactinides, this role of the periodic table reaches its limits because increasingly strong relativistic effects on the valence electron shells can induce deviations from known trends in chemical properties 1–4 . In the case of the first two transactinides, elements 104 and 105, relativistic effects do indeed influence their chemical properties 5 , whereas elements 106 and 107 both behave as expected from their position within the periodic table 6,7 . Here we report the chemical separation and characteriz- ation of only seven detected atoms of element 108 (hassium, Hs), which were generated as isotopes 269 Hs (refs 8, 9) and 270 Hs (ref. 10) in the fusion reaction between 26 Mg and 248 Cm. The hassium atoms are immediately oxidized to a highly volatile oxide, pre- sumably HsO 4 , for which we determine an enthalpy of adsorp- tion on our detector surface that is comparable to the adsorption enthalpy determined under identical conditions for the osmium oxide OsO 4 . These results provide evidence that the chemical properties of hassium and its lighter homologue osmium are similar, thus confirming that hassium exhibits properties as expected from its position in group 8 of the periodic table. The discovery of Hs was reported in 1984 (ref. 11) with the identification of the nuclide 265 Hs with a half-life of T 1/2 ¼ 1.5 ms (refs 11, 12). In 1996, the much longer-lived isotope 269 Hs, with a half-life of about 10 s, was observed in the a-decay chain of 277 112 (ref. 8). Recently, evidence for the existence of the neighbouring nuclide 270 Hs was found and a half-life of about 4 s was deduced from its measured a-decay energy 10 . The latter two Hs isotopes might be sufficiently long-lived to allow their chemical character- ization. Suitable reactions for their direct production are 248 Cm( 26 Mg, 5,4n) 269,270 Hs, for which formation cross-sections of a few picobarn have been estimated 13 . The periodic table suggests that Hs is a member of group 8 and thus chemically similar to its lighter homologues ruthenium (Ru) and osmium (Os), which are known to form highly volatile tetroxides. Therefore, Hs is also expected to form a very volatile tetroxide (HsO 4 ) suitable for gas-phase isolation 1,14–16 , even though two earlier attempts to chemically identify Hs in the tetroxide form proved unsuccessful 17,18 . Fully relativistic density functional calculations 19 for the tetrox- ides of the group 8 elements indicated that the electronic structure of HsO 4 is very similar to that of OsO 4 , with covalent bonding being somewhat more pronounced in the former. The stability of the gaseous tetroxides was found 19 to increase in the order RuO 4 , OsO 4 , HsO 4 ; in agreement with extrapolations within group 8 of the periodic table 20 . The density functional calculations, in con- junction with a surface interaction model, suggest adsorption 1 8 7 6 5 3 4 4 9 10 2 11 Figure 1 Schematic drawing of the IVO-COLD set-up used to produce and isolate Hs isotopes in form of the volatile HsO 4 . The 26 Mg-beam (1) passes through the rotating vacuum window and 248 Cm-target (2) assembly. The target consisted of three banana- shaped segments (1.9 cm 2 area each) covered with 239 mg cm 22 , 730 mg cm 22 , and 692 mg cm 22 248 Cm, respectively. The 248 Cm (isotopic composition 246 Cm: 4.2%; 248 Cm: 95.8%) was deposited on 2.82 mg cm 22 beryllium (Be) backings by molecular plating. The target-window assembly rotated in the adjacent gas volume with 2,000 rev min 21 and was synchronized with the beam macrostructure of the accelerator in order to distribute each 6-ms beam pulse evenly over one target segment. In the fusion reaction 269,270 Hs nuclei are formed that recoil out of the target into a gas volume (3) and are flushed with a He/O 2 mixture (4) out of the chamber. The gas was passed through a cartridge containing P 2 O 5 as a drying agent before injecting it into IVO. In this way, the water vapour concentration was reduced to a measured value of ,1 p.p.m. throughout the experiment. The gas was injected into a quartz column (5) containing a quartz wool plug (6) heated to 600 8C by an oven (7). There, Hs is converted to HsO 4 which is volatile at room temperature and transported with the gas flow through a 10-m-long perfluoroalkoxy (PFA) Teflon capillary (8) to the COLD detector array registering the nuclear decay (a and spontaneous fission) of the Hs nuclides. The array consists of 24 detectors arranged in 12 pairs (9). A temperature gradient was established along the detector array by means of a thermostat (10) at the entrance and a liquid nitrogen cryostat (11) at the exit. The temperature was monitored by five thermocouples installed along the copper bar. Depending on the volatility of HsO 4 , the molecules adsorb at a characteristic temperature. letters to nature NATURE | VOL 418 | 22 AUGUST 2002 | www.nature.com/nature 859 © 2002 Nature Publishing Group