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Bohrium (pronounced /ˈbɔəriəm/ ( listen)) is a chemical element with the symbol Bh and atomic number 107 and is the heaviest member of group 7 (VIIB). It is a synthetic element whose most stable known isotope, 270Bh, has a half-life of 61 seconds. Chemical experiments have confirmed bohrium's predicted position as a heavier homologue to rhenium with the formation of a stable +7 oxidation state.[1] History The first convincing synthesis was in 1981 by a German research team led by Peter Armbruster and Gottfried Münzenberg at the Gesellschaft für Schwerionenforschung (Institute for Heavy Ion Research, GSI) in Darmstadt using the Dubna reaction. 20983Bi + 5424Cr → 262107Bh + n In 1989, the GSI team successfully repeated the reaction during their efforts to measure an excitation function. During these experiments, 261Bh was also identified in the 2n evaporation channel and it was confirmed that 262Bh exists as two states - a ground state and an isomeric state. The IUPAC/IUPAP Transfermium Working Group (TWG) report in 1992 officially recognised the GSI team as discoverers of element 107. Historically element 107 has been referred to as eka-rhenium. The German group suggested the name nielsbohrium with symbol Ns to honor the Danish physicist Niels Bohr. The Soviet scientists had suggested this name be given to element 105 (which was finally called dubnium) and the German team wished to recognise both Bohr and the fact that the Dubna team had been the first to propose the cold fusion reaction. There was an element naming controversy as to what the elements from 104 to 106 were to be called; the IUPAC adopted unnilseptium (symbol Uns) as a temporary, systematic element name for this element. In 1994 a committee of IUPAC rejected the name nielsbohrium since there was no precedence for using a scientist's complete name in the naming of an element and thus recommended that element 107 be named bohrium.[2] This was opposed by the discoverers who were adamant that they had the right to name the element. The matter was handed to the Danish branch of IUPAC who voted in favour of the name bohrium. There was some concern however that the name might be confused with boron and in particular the distinguishing of the names of their respective oxo-ions bohrate and borate. Despite this, the name bohrium for element 107 was recognized internationally in 1997.[3] The IUPAC subsequently decided that bohrium salts should be called bohriates. This section deals with the synthesis of nuclei of bohrium by so-called "cold" fusion reactions. These are processes which create compound nuclei at low excitation energy (~10-20 MeV, hence "cold"), leading to a higher probability of survival from fission. The excited nucleus then decays to the ground state via the emission of one or two neutrons only. The synthesis of element 107 was first attempted in 1976 by scientists at the Joint Institute for Nuclear Research at Dubna using this cold fusion reaction. Analysis was by detection of spontaneous fission (SF). They discovered two SF activities, one with a 1-2 ms half-life and one with a 5 s activity. Based on the results of other cold fusion reactions, they concluded that they were due to 261107 and 257105 respectively. However, later evidence gave a much lower SF branching for 261107 reducing confidence in this assignment. The assignment of the element 105 activity was later changed to 258105, presuming that the decay of element 107 was missed. The 2 ms SF activity was assigned to 258Rf resulting from the 33% EC branch.[4] The GSI team studied the reaction in 1981 in their discovery experiments. Five atoms of 262Bh were detected using the method of correlation of genetic parent-daughter decays.[5] In 1987, an internal report from Dubna indicated that the team had been able to detect the spontaneous fission of 261107 directly. The GSI team further studied the reaction in 1989 and discovered the new isotope 261Bh during the measurement of the 1n and 2n excitation functions but were unable to detect an SF branching for 261Bh.[6] They continued their study in 2003 using newly developed bismuth(III) fluoride (BiF3) targets, used to provide further data on the decay data for 262Bh and the daughter 258Db. The 1n excitation function was remeasured in 2005 by the team at LBNL after some doubt about the accuracy of previous data. They observed 18 atoms of 262Bh and 3 atoms of 261Bh and confirmed the two isomers of 262Bh. [7] The team at Dubna studied this reaction in 1976 in order to assist in their assignments of the SF activities from their experiments with a Cr-54 beam. They were unable to detect any such activity, indicating the formation of different isotopes decaying primarily by alpha decay. This reaction was studied for the first time in 2007 by the team at LBNL to search for the lightest bohrium isotope 260Bh. The team successfully detected 8 atoms of 260Bh decaying by correlated 10.16 MeV alpha particle emission to 256Db. The alpha decay energy indicates the continued stabilising effect of the N=152 closed shell.[8] The team at Dubna also studied this reaction in 1976 as part of their newly established cold fusion approach to new elements. As for the reaction using a Bi-209 target, they observed the same SF activities and assigned them to 261107 and 257105. Later evidence indicated that these should be reassigned to 258105 and 258104 (see above). In 1983, they repeated the experiment using a new technique: measurement of alpha decay from a descendant using chemical separation. The team were able to detect the alpha decay from a descendant of the 1n evaporation channel, providing some evidence for the formation of element 107 nuclei. This reaction was later studied in detail using modern techniques by the team at LBNL. In 2005 they measured 33 decays of 262Bh and 2 atoms of 261Bh, providing a 1n excitation function and some spectroscopic data of both 262Bh isomers. The 2n excitation function was further studied in a 2006 repeat of the reaction. [9] [10] The team found that this reaction had a higher 1n cross section than the corresponding reaction with a Bi-209 target, contrary to expectations. Further research is required to understand the reasons. This section deals with the synthesis of nuclei of bohrium by so-called "hot" fusion reactions. These are processes which create compound nuclei at high excitation energy (~40-50 MeV, hence "hot"), leading to a reduced probability of survival from fission and quasi-fission. The excited nucleus then decays to the ground state via the emission of 3-5 neutrons. This reaction was first studied in 2006 at the LBNL as part of their systematic study of fusion reactions using 238U targets. Results have not been published but preliminary results appear to indicate the observation of spontaneous fission, possibly from 264Bh.[11] Recently, the team at the Institute of Modern Physics (IMP), Lanzhou, have studied the nuclear reaction between americium-243 and magnesium-26 ions in order to synthesise the new isotope 265Bh [12] and gather more data on 266Bh. In two series of experiments, the team has measured partial excitation functions of the 3n,4n and 5n evaporation channels. This reaction was studied for the first time in 2008 by the team at RIKEN, Japan, in order to study the decay properties of 266Bh, which is a decay product in their claimed decay chains of ununtrium.[13] The decay of 266Bh by the emission of 9.04 MeV alpha particles was confirmed, although lines at 9.29 MeV (see below) and 9.77 MeV (see ununtrium) were not. The first attempts to synthesize element 107 by hot fusion pathways were performed in 1979 by the team at Dubna. The reaction was repeated in 1983. In both cases, they were unable to detect any spontaneous fission from nuclei of element 107. More recently, hot fusions pathways to bohrium have been re-investigated in order to allow for the synthesis of more long-lived, neutron rich isotopes to allow a first chemical study of bohrium. In 1999, the team at LBNL claimed the discovery of long-lived 267Bh (5 atoms) and 266Bh (1 atom).[14] In the following year, the same team attempted to confirm the synthesis and decay of 266Bh, but were unable to do so. Current research has been unable to confirm the 9.29 MeV decay claimed and the synthesis of 266Bh by this reaction is not accepted at this moment in time. The team at the Paul Scherrer Institute (PSI) in Bern, Switzerland later synthesized 6 atoms of 267Bh in the first definitive study of the chemistry of bohrium (see below). As an alternative means of producing long-lived bohrium isotopes suitable for a chemical study, the synthesis of 267Bh and 266Bh were attempted in 1995 by the team at GSI using the highly asymmetric reaction using an einsteinium-254 target. They were unable to detect any product atoms. Isotopes of bohrium have also been detected in the decay of heavier elements. Observations to date are shown in the table below: The only confirmed example of isomerism in bohrium is for the isotope 262Bh. Direct production populates two states, a ground state and an isomeric state. The ground state is confirmed as decaying by alpha emission with alpha lines at 10.08,9.82 and 9.76 MeV with a revised half life of 84 ms. The excited state decays by alpha emission with lines at 10.37 and 10.24 MeV with a revised half-life of 9.6 ms. The table below provides cross-sections and excitation energies for cold fusion reactions producing bohrium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel. The table below provides cross-sections and excitation energies for hot fusion reactions producing bohrium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel. Bohrium is element 107 in the Periodic Table. The two forms of the projected electronic structure are: Element 107 is projected to be the fourth member of the 6d series of transition metals and the heaviest member of group VII in the Periodic Table, below manganese, technetium and rhenium. All the members of the group readily portray their group oxidation state of +7 and the state becomes more stable as the group is descended. Thus bohrium is expected to form a stable +7 state. Technetium also shows a stable +4 state whilst rhenium exhibits stable +4 and +3 states. Bohrium may therefore show these lower states as well. The heavier members of the group are known to form volatile heptoxides M2O7, so bohrium should also form the volatile oxide Bh2O7. The oxide should dissolve in water to form perbohric acid, HBhO4. Rhenium and technetium form a range of oxyhalides from the halogenation of the oxide. The chlorination of the oxide forms the oxychlorides MO3Cl, so BhO3Cl should be formed in this reaction. Fluorination results in MO3F and MO2F3 for the heavier elements in addition to the rhenium compounds ReOF5 and ReF7. Therefore, oxyfluoride formation for bohrium may help to indicate eka-rhenium properties. In 2000, a team at the PSI conducted a chemistry reaction using atoms of 267Bh produced in the reaction between Bk-249 and Ne-22 ions. The resulting atoms were thermalised and reacted with a HCl/O2 mixture to form a volatile oxychloride. The reaction also produced isotopes of its lighter homologues, technetium (as 108Tc) and rhenium (as 169Re). The isothermal adsorption curves were measured and gave strong evidence for the formation of a volatile oxychloride with properties similar to that of rhenium oxychloride. This placed bohrium as a typical member of group 7.[1] 2 Bh + 3 O2 + 2 HCl → 2 BhO3Cl + H2 Summary of compounds References 1. ^ a b "Gas chemical investigation of bohrium (Bh, element 107)", Eichler et al., GSI Annual Report 2000. Retrieved on 2008-02-29 External links * WebElements.com - Bohrium
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