96 JOURNAL OF COSMETIC SCIENCE ONE ATOM-AT-A-TIME CHEMISTRY OF THE HEAVIEST ELEMENTS Darleane C. Hoffman, Ph.D. Nuclear Science Division, MS- 70/I/3307, Lawrence Berkeley National Laboratory and Department of Chemistry, University of California, Berkeley, CA 94720 Uranium, discovered in 1789 in pitchblende from Saxony, Gemany by Martin Klaproth, was the heaviest known chemical element for more than 150 years until 1940 when E. M. McMillan and his student, P. H. Abelson, in experiments initially designed to investigate the newly discovered (1939) phenomenon of nuclear fission, chemically isolated and identified the new element neptunium at Berkeley in the products of neutron irradiation of uranium. Shortly thereafter in December 1940, G. T. Seaborg, E. M. McMillan, A. C. Wahl, and J. W. Kennedy identified an isot6pe of plutonium, and in February 1941 the first chemical separation of plutonium was performed by Seaborg's first graduate student, Art Wahl. Although these experiments were conducted as part of the investigators' academic research and without governmental financial support, the discoverers voluntarily withheld publication until 1946 because of wartime security concerns about the fissionability of plutonium. By 1961 the elements through lawrencium (atomic number, Z,=103) had been discovered, thus completing the actinide series. Since that time nine transaetinide (Z103) elements have been produced and identified so the elements through 112 are now known. The names and symbols for the transfermium elements approved by the International Union of Pure and Applied Chemistry (IUPAC) in August 1997 are: 101, Mendelevium, Md 102, Nobelium, No 103, Lawrencium, Lr 104, Rutherfordium, Rf:, 105, Dubnium, Db 106, Seaborgium, Sg 107, Bohrium, Bh 108, Hassium, I-Is 109, Meitnerium, Mt. To avoid confusion, I shall continue to use hahnium for element 105 in this presentation as hahnium has been used in all of our previous publications on the chemistry of element. 105. Hahnium was approved by the American Chemical Society in 1994 prior to the IUPAC approval of the compromise names given above. Hahnium was the name suggested by the Berkeley discoverers to honor Otto Hahn, one of the co- discoverers of nuclear fission. IUPAC is now considering claims to priority of discovery of elements 110, 111, and 112, and will then request suggestions from the discoverers for names for these elements. Based on the "actinide concept" proposed by Glenn Seaborg in 1945, the actinide series should end with element 103, Lr, with the filling of the 5f electron shell, a series analogous to the lanthanides in which the 4f electron shell is being filled. Then element i04, Rf, the first of the transactinides would be expected to be the first member of a new 6d transition series and should be placed in the periodic table under the group 4 elements, zirconium and hafnium. It would be predicted to have a most stable 4+ oxidation state in aqueous solution while Lr has a most stable 3+ oxidation state. Similarly, Ha and Sg would be the heaviest members of groups 5 and 6 and exhibit chemical properties similar to those elements. Early studies (1970s) continned that, indeed, Lr and Rf had chemical properties generally similar to those expected, but no studies of the chemical properties of Ha in aqueous sollstion were performed. Some early studies of the gas phase properties of both Rf and Ha were reported by a Russian group, but these were not definitive because their detection method did not positively identify the element they were

PREPRINTS OF THE 1998 ANNUAL SCIENTIFIC MEETING 97 studying. Subsequently, there was a hiatus in studies of chemical properties until 1985 when my group at Berkeley decided to investigate the'chemical properties of Lr and Rf in more detail, and then to perform studies of Ha in aqueous solution. The major impetus for our interest was the predictions based on relativistic calculations that the electronic structures, and hence the chemical properties, of these elements might show significant deviations from their lighter homologues. In fact, it was postulated that the most stable oxidation state for Ha might be 3+ rather than 5+ like the lighter group 5 elements, tantalum and niobium. Since relativistic effects are predicted to increase as Z 2 the heaviest elements should be the best place to assess the influence of these effects on ionic radii, complexing ability, oxidation states and redox properties. By this time, some longer-lived isotopes had also been discovered which made chemical studies more feasible than in the 1970s. My group then began its investigations of the chemical properties of the heaviest elements in 1985 to explore the architecture of the periodic table at its furthest reaches and to compare their properties with those of their lighter homologues. Because of the short half-lives of these elements, they must be studied at the accelerator where they are oroduced. Charged-oarticle beams must be used since they cannot be produced by neutron capture at reactors and the use of radioactive heavy actinide targets is often necessary. The production rates are so small that chemistry is performed on an *atom-at-a-time" basis and a knowledge of the nuclear decay properties of the element being investigated is required in order to detect these atoms and identify them. Our detailed studies showed anomalous trends in the chemical properties of Rf and Ha which could not be simply extrapolated from those of their lighter homologues in groups 4 and 5, of the periodic table, thus highlighting the importance of obtaining additional information for comparison with theoretical predictions of relativistic effects. These results sparked a renaissance of interest in chemical studies and we hosted many international collaborations at the 88-Inch Cyclotron at Berkeley. These have continued and our group has also participated in experiments at the UNILAC accelerator at Darmstadt, Germany. To date, we have studied the gas and aqueous phase chemical properties of Rf, Ha, and Sg, even though their longest known half-lives are only 78 seconds, 35 seconds, and 10 to 30 seconds, respectively. We use computer-controlled automated systems as well as "manual" separations for studying both aqueous and gas-phase properties and have found that Rf and Ha sometimes behave more like their pseudo-homologues, thorium (IV) and protactinium (V) than their group 4 and 5 homologues. Recent results have shown that the most stable oxidation state of Sg in aqueous solution is 6+, and like its homologues molybdenum and tungsten, Sg forms neutral or artionic oxy- or oxyhalide-compounds. However, somewhat unexpectedly, this is unlike the behavior of its possible pseudo-homologue U(VI).. We are now planning to try to produce a new isotope of Bh (element 107), predicted to have a half-life of-10 seconds, in order to study its chemistry. Based on new predictions of nuclear properties, it now appears that the half-lix•es of isotopes of elements 108 and 109, and even heavier, may be long enough for chemical studies but the production rates are dropping very rapidly. Now the trick will be to devise new and imaginative ways to increase the number of atoms produced in order to extend our studies of chemical properties to these elements..