Reviews M. Schädel DOI: 10.1002/anie.200461072 Superheavy Elements Chemistry of Superheavy Elements Matthias Schädel* Keywords: Dedicated to Professor Günter Herrmann atom-at-a-time chemistry · periodic on the occasion of his 80th birthday table · relativistic effects · superheavy elements · transactinides Angewandte Chemie 368 www.angewandte.org 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2006, 45, 368 – 401 Angewandte Superheavy Elements Chemie The number of chemical elements has increased considerably in the From the Contents last few decades.Most excitingly, these heaviest, man-made elements at the far-end of the Periodic Table are located in the area of the long- 1. Introduction and Historical Remarks 369 awaited superheavy elements.While physical techniques currently play a leading role in these discoveries, the chemistry of superheavy 2. Nuclear Aspects 372 elements is now beginning to be developed.Advanced and very sensitive techniques allow the chemical properties of these elusive 3. Atom-at-a-Time Chemistry 374 elements to be probed.Often, less than ten short-lived atoms, chemi- 4. Objectives for Superheavy cally separated one-atom-at-a-time, provide crucial information on Element Chemistry 375 basic chemical properties.These results place the architecture of the far-end of the Periodic Table on the test bench and probe the 5. Experimental Techniques 376 increasingly strong relativistic effects that influence the chemical 6. Chemical Properties 380 properties there.This review is focused mainly on the experimental work on superheavy element chemistry.It contains a short contribu- 7. Summary and Perspectives 394 tion on relativistic theory, and some important historical and nuclear aspects. with the quantized treatment of indi- vidual nucleons—protons and neu- 1. Introduction and Historical Remarks trons—in nuclear shell models. Similar to electrons in atoms and molecules, and based on the same quantum mechanical How many chemical elements do we know? How many law, protons and neutrons form closed shells with “magic elements are sufficiently chemically characterized to justify numbers”, for example, 2, 8, 20, 28, 50, and 82. As with atoms their position in the Periodic Table? Simple questions at every having closed electron shells, nuclei with closed shells exhibit chemist should be able to answer. But do you—do we—really an extra and sometimes very pronounced stability (see know? ref. [12] and references therein for a concise discussion of The race for new elements beyond uranium started in the the liquid-drop model and the shell contributions). mid-1930s involving groups in Rome, Berlin, and Paris. In the mid-1960s, this nuclear-shell theory received a large Among the mistakes which led these scientists astray, were boost from computer calculations based upon these new presumptions about the structure of the Periodic Table at its theoretical understandings of the atomic nucleus. Until 1965 it far end—the transuranium elements were assumed to belong was conceivable that superheavy elements may exist around to Group 7 and the following Groups. The unexpected Z = 126 (see Myers and Swiateckis calculations of nuclear discovery of nuclear fission[1] marked the first obstacle, and, masses and deformations, ref. [13]). However, from then on, at the same time, brought new insight and opportunities[2,3] . new results focused on the Z = 114 nucleus with a neutron Soon after, the first transuranium elements, neptunium and number of N = 184 as the center of an “island of stability”. plutonium were synthesized. The road to the discovery of Contributions came from Sobiczewski and co-workers[14] and, heavier elements, successfully applied in the synthesis and during a conference at Lysekil[15] in 1966, from Meldner[16] and separation of americium and curium, was opened when others.[15]). First estimates[17–22] yielded relatively long half- Seaborg introduced the actinide concept.[4] This drastically lives—as long as a billion years! These times encouraged the revised the Periodic Table (see ref. [5,6] for an account of this search for superheavy elements (SHE) and their investigation development, and ref. [7] for a detailed summary of the with chemical techniques. Among experimentalists, the hunt chemistry of the actinides, thorium through lawrencium— started with searches for superheavy elements both in nature elements with atomic numbers Z = 90–103—which follow and at accelerators (see refs. [12,23–28] for reviews of this actinium in the “actinide series”, and ref. [8] for a complete early phase work). coverage of the chemistry of transactinide elements). At about the same time, the first Dirac–Fock and Dirac– The idea of the existence of chemical elements much Fock–Slater calculations were performed for atoms to deter- heavier than uranium emerged very early, at first as illu- mine the electronic structure of superheavy elements.[29–34] sionary dreams in science-fiction literature. It was not until These results are summarized in ref. [35] They show that the mid-1950s—when much was learned about the atomic extrapolating chemical properties along groups of elements in nucleus from investigations of its decay especially its fission properties—that a scientifically sound discussion of the possible existence of nuclei dubbed “superheavy” began [*] Dr. M. Schädel KPII–Kernchemie with contributions by John Wheeler[9] and Gertrude Scharff- [10] Gesellschaft für Schwerionenforschung mbH Goldhaber. After the early success of treating the atomic Planckstrasse 1, 64291 Darmstadt (Germany) nucleus as a charged liquid drop (liquid-drop model) in Fax : (+49)6159-71-2903 describing the nuclear fission process[11] a new quality appears E-mail: [email protected] Angew. Chem. Int. Ed. 2006, 45, 368 – 401 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 369 Reviews M. Schädel the Periodic Table could be a valid approach for estimating JWP[46] has requested a confirmation experiment. The find- the chemical properties of superheavy elements. Simultane- ings by the SHIP group were strongly supported by results ously, the importance of a relativistic treatment of the from the first chemical separation and investigation of electronic orbitals was recognized. Several authors discussed element 108 (this experiment will be discussed in detail in relativistic effects which might result in unexpected chemical the chemistry Section of this Review).[54] A direct confirma- properties; see ref. [36–39] One of the articles was entitled tion of the production and the decay of the isotope 277112 was “Are elements 112, 114, and 118 relatively inert gases?”.[40] In obtained by Morita and co-workers[55] at The Institute of the last decade a breakthrough towards the theoretical Physical and Chemical Research (RIKEN) in Wako (Japan) predictions of chemical properties was achieved with the with the same technique as used for elements 110 and 111.[49] development of relativistic quantum molecular theories With high confidence, we can anticipate that the discovery of applied for heavy and superheavy elements; reviews are element 112 will be accepted soon and that the assigned given in.[41–44] priority for the discovery will go to the SHIP group. Reviews Let us come back to the question, how many elements do of this groups work, including the discoveries of element 107 we know today? To answer this we have to be aware that the (bohrium, Bh), element 108 (hassium, Hs), and element 109 “discovery” of an element 1) “is not always a single, simply (meitnerium, Mt) can be found in ref. [56–62] identifiable event or even culmination of a series of A world-record low cross-section—and therefore researches … but may rather be the product of several extremely difficult to repeat and to confirm—was reached series of investigations … ”[45] and 2) that the judgment of by Morita and co-workers in their recently reported finding of what is sufficient evidence to convince the scientific com- one atom of element 113.[63] All the above mentioned nuclides munity that the formation of a new element has, indeed, been are the ones in the upper-left part of Figure 1, which shows the established, may vary from group to group.[45] Because of uppermost part of the chart of nuclides. From a chemists conflicting discovery claims and associated disputes over the point of view, an important characteristic feature of the naming of the elements, a working group was jointly nuclides produced in nuclear reactions with Pb and Bi targets established in 1986 by the International Union of Pure and yields only short-lived products with millisecond half-lives. Applied Physics (IUPAP) and the International Union of This life-time prohibits chemical studies with virtually all of Pure and Applied Chemistry (IUPAC). At first, this Trans- the presently available techniques. However, new technolog- fermium Working Group (TWG) established a set of criteria ical developments will also allow, to some extent, to exploit that must be satisfied before the discovery of a new element is nuclides produced from some types of nuclear reactions for recognized. Secondly, beginning with element 101, it evalu- chemical investigations. ated all discovery claims until the year 1991.[45] This work was But there are even more chemical elements—and longer continued by the IUPAC/IUPAP Joint Working Party (JWP). lived isotopes of known elements—on the horizon and these Based on their recent report, the last “discovered” chemical are especially exciting for chemists. Oganessian et al. have element[46] has