element 105

Bohrium

Bohrium is a chemical element in the periodic table that has the symbol Bh and atomic number 107. It is a synthetic element whose most stable isotope, Bh-270, has a half-life of 61 seconds. Chemical experiments confirmed bohrium's predicted position as a member of group 7 of the periodic table, as a heavier homologue to rhenium.

Official discovery

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) in Darmstadt using the Dubna reaction.

, ^{209}_{83}mathrm{Bi} + , ^{54}_{24}mathrm{Cr} , to , ^{262}_{107}mathrm{Bh} + , ^{1}_{0}mathrm{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 isomers.

The IUPAC/IUPAP Transfermium Working Group (TWG) report in 1992 officially recognised the GSI team as discoverers of element 107.

Proposed names

Historically element 107 has been referred to as eka-rhenium.

The Germans suggested the name nielsbohrium with symbol Ns to honor the Danish physicist Niels Bohr. The Soviets had suggested this name be given to element 105 (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 101 to 109 were to be called; thus IUPAC adopted unnilseptium (or /ˌʌnɪlˈsɛptiəm/, 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. 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. The IUPAC subsequently decided that bohrium salts should be called bohriates.

Electronic structure

Bohrium is element 107 in the Periodic Table. The two forms of the projected electronic structure are:

Bohr model: 2, 8, 18, 32, 32, 13, 2

Quantum mechanical model: 1s22s22p63s23p64s23d10 4p65s24d105p66s24f145d10 6p67s25f146d5

Extrapolated chemical properties of eka-rhenium/dvi-technetium

Oxidation states

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 +VII and the state becomes more stable as the group is descended. Thus bohrium is expected to form a stable +VII state. Technetium also shows a stable +IV state whilst rhenium portrays stable +IV and +III states. Bohrium may therefore show these lower states as well.

Chemistry

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.

Industrial and commercial use

Like seaborgium and hassium, its neighbours, bohrium has no industrial or commercial use due to its extremely short half-life. Few atoms have ever been made, but if enough were found in one area, bohrium would constitute a radiation hazard.

Experimental chemistry

Gas phase chemistry

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.

mathrm{Bh} + tfrac{3}{2}cdotmathrm{O}_{2} + mathrm{HCl}to mathrm{BhO}_{3}mathrm{Cl} + mathrm{H}

Summary of compounds

Formula Names(s)
BhO3Cl bohrium oxychloride ; bohrium(VII) chloride trioxide

History of synthesis of isotopes by cold fusion

209Bi(54Cr,xn)263-xBh (x=1,2)

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 convidence 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. 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. 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. 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.

209Bi(53Cr,xn)262-xBh

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.

209Bi(52Cr,xn)261-xBh (x=1)

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.

208Pb(55Mn,xn)263-xBh (x=1)

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. . 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.

Synthesis of isotopes by hot fusion

243Am(26Mg,xn)269-xBh (x=3,4,5)

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 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.

249Bk(22Ne,xn)271-xBh (x=4,5)

The first attempts to synthesise 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 and 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 announced the discovery of long-lived 267Bh (5 atoms) and 266Bh (1 atom). In the following year, the same team attempted to confirm the synthesis and decay of 266Bh. However, they were unable to do so and the identification of 266Bh in the first experiment is questionable. The team at the Paul Scherrer Institute (PSI) in Bern, Switzerland later synthesised 6 atoms of 267Bh in the first definitive study of the chemistry of bohrium (see below).

254Es(16O,xn)270-xBh

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.

Synthesis of isotopes as decay products

Isotopes of bohrium have also been detected in the decay of heavier elements. Observations to date are shown in the table below:

Evaporation Residue Observed Bh isotope
288115 272Bh
287115 271Bh (missed)
282113 270Bh
278113 266Bh
272Rg 264Bh
266Mt 262Bh

Chronology of isotope discovery

Isotope Year discovered discovery reaction
260Bh 2007 209Bi(52Cr,n)
261Bh 1989 209Bi(54Cr,2n)
262Bhg,m 1981 209Bh(54Cr,n)
263Bh unknown
264Bh 1994 209Bi(64Ni,n)
265Bh 2004 243Am(26Mg,4n)
266Bh 2004 209Bi(70Zn,n)
267Bh 2000 249Bk(22Ne,5n)
268Bh unknown
269Bh unknown
270Bh 2006 237Np(48Ca,3n)
271Bh unknown
272Bh 2003 243Am(48Ca,3n)

Isomerism in bohrium nuclides

262Bh

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.

Chemical yields of isotopes

Cold Fusion

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.

Projectile Target CN 1n 2n 3n
55Mn 208Pb 263Bh 590 pb , 14.1 MeV ~35 pb
54Cr 209Bi 263Bh 510 pb , 15.8 MeV ~50 pb
52Cr 209Bi 261Bh 59 pb , 15.0 MeV

Hot Fusion

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.

Projectile Target CN 3n 4n 5n
26Mg 243Am 271Bh + + +
22Ne 249Bk 271Bh ~96 pb +

References

External links

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