Bohrium

Bohrium is a chemical element with symbol Bh and atomic number 107. It is named after Danish physicist Niels Bohr. It is a synthetic element (an element that can be created in a laboratory but is not found in nature) and radioactive; the most stable known isotope, ²⁷⁰Bh, has a half-life of approximately 61 seconds.

In the periodic table of the elements, it is a d-block transactinide element. It is a member of the 7th period and belongs to the group 7 elements. Chemistry experiments have confirmed that bohrium behaves as the heavier homologue to rhenium in group 7. The chemical properties of bohrium are characterized only partly, but they compare well with the chemistry of the other group 7 elements.

History

Official discovery

Bohrium was first convincingly synthesized in 1976 by a Russian research team led by Yuri Oganessian. The team bombarded a target of bismuth-209 with accelerated nuclei of chromium-54 to produce 5 atoms of the isotope bohrium-262:

209
83Bi + 54
24Cr â†' 262
107Bh + n

The IUPAC/IUPAP Transfermium Working Group (TWG) recognised the GSI collaboration as official discoverers in their 1992 report.

Proposed names

The German group suggested the name nielsbohrium with symbol Ns to honor the Danish physicist Niels Bohr. The Soviet scientists at the Joint Institute for Nuclear Research in Dubna, Russia 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 to solve the controversial problem of the naming of element 105. The Dubna team agreed with the German group's naming proposal for element 107.

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 recommended that element 107 be named bohrium, not nielsbohrium, since there was no precedence for using a scientist's complete name in the naming of an element. This was opposed by the discoverers as there was some concern that the name might be confused with boron and in particular the distinguishing of the names of their respective oxyanions, bohrate and borate. The matter was handed to the Danish branch of IUPAC which, despite this, voted in favour of the name bohrium, and thus the name bohrium for element 107 was recognized internationally in 1997. The IUPAC subsequently decided that bohrium salts should be called bohriates instead of bohrates.

Isotopes

Bohrium has no stable or naturally-occurring isotopes. Several radioactive isotopes have been synthesized in the laboratory, either by fusing two atoms or by observing the decay of heavier elements. Eleven different isotopes of bohrium have been reported with atomic masses 260â€"262, 264â€"267, 270â€"272, 274, one of which, bohrium-262, has a known metastable state. All of these decay only through alpha decay, although some unknown bohrium isotopes are predicted to undergo spontaneous fission.

Stability and half-lives

The lighter isotopes usually have shorter half-lives; half-lives of under 100 ms for ²⁶⁰Bh, ²⁶¹Bh, ²⁶²Bh, and ^262mBh were observed. ²⁶⁴Bh, ²⁶⁵Bh, ²⁶⁶Bh, and ²⁷¹Bh are more stable at around 1 s, and ²⁶⁷Bh and ²⁷²Bh have half-lives of about 10 s. The heaviest isotopes are the most stable, with ²⁷⁰Bh and ²⁷⁴Bh having measured half-lives of about 61 s and 54 s respectively. The unknown isotopes ²⁷³Bh and ²⁷⁵Bh are predicted to have even longer half-lives of around 90 minutes and 40 minutes respectively. Before its discovery, ²⁷⁴Bh was also predicted to have a long half-life of 90 minutes, but it was found to have a shorter half-life of only about 54 seconds.

The proton-rich isotopes with masses 260, 261, and 262 were directly produced by cold fusion, those with mass 262 and 264 were reported in the decay chains of meitnerium and roentgenium, while the neutron-rich isotopes with masses 265, 266, 267 were created in irradiations of actinide targets. The four most neutron-rich ones with masses 270, 271, 272, and 274 appear in the decay chains of ²⁸²113, ²⁸⁷115, ²⁸⁸115, and ²⁹⁴117 respectively. These eleven isotopes have half-lives ranging from 8 miliseconds to 1 minute.

Chemical properties

Extrapolated

Bohrium is 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 (M = metal), 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.

Bohrium is expected to be a solid under normal conditions and assume a hexagonal close-packed crystal structure (^c/_a = 1.62), similar to its lighter congener rhenium.

Experimental

In 1995, the first report on attempted isolation of the element was unsuccessful.

In 2000, it was confirmed that although relativistic effects are important, the 107th element does behave like a typical group 7 element.

In 2000, a team at the PSI conducted a chemistry reaction using atoms of ²⁶⁷Bh produced in the reaction between ²⁴⁹Bk and ²²Ne 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.

2 Bh + 3 O
2 + 2 HCl â†' 2 BhO
3Cl + H
2

See also

References

External links

• Los Alamos National Laboratory â€" Bohrium
• Properties of BhO₃Cl
• Bohrium at The Periodic Table of Videos (University of Nottingham)
• WebElements.com â€" Bohrium

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