Bohrium

2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements

107 seaborgium ← bohrium → hassium Re ↑ Bh ↓ (Ups) Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	bohrium, Bh, 107
Element category	transition metals
Group, Period, Block	7, 7, d
Standard atomic weight	[270]  g·mol−1
Electron configuration	[Rn] 5f14 6d5 7s2
Electrons per shell	2, 8, 18, 32, 32, 13, 2
Physical properties
Phase	presumably a solid
Atomic properties
Crystal structure	unknown
Oxidation states	7
Miscellaneous
CAS registry number	54037-14-8
Selected isotopes
Main article: Isotopes of bohrium iso NA half-life DM DE ( MeV) DP 272Bh syn 9.8 s α 9.02 268Db 271Bh syn α 267Db 270Bh syn 61 s α 8.93 266Db 267Bh syn 17 s α 8.83 263Db 266Bh syn 0.9 s α 9.77,9.04 262Db 265Bh syn 0.9 s α 9.24 261Db 264Bh syn 0.97 s α 9.62,9.48 260Db 262mBh syn 9.6 ms α 10.37,10.24 258Db 262gBh syn 84 ms α 10.08,9.94,9.82,9.74,9.66 258Db 261Bh syn 11.8 ms α 10.40,10.10,10.03 257Db 260Bh syn 35 ms α 10.16 256Db
References

Bohrium (pronounced /ˈbɔəriəm/) 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.

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 honour 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 (pronounced /ˌjuːnɪlˈsɛptiəm/ 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.

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.

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	+