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= Bohrium =
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Introduction
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Bohrium is a synthetic chemical element; it has symbol Bh and atomic
number 107. It is named after Danish physicist Niels Bohr. As a
synthetic element, it can be created in particle accelerators but is
not found in nature. All known isotopes of bohrium are highly
radioactive; the most stable known isotope is 270Bh with a half-life
of approximately 2.4 minutes, though the unconfirmed 278Bh may have a
longer half-life of about 11.5 minutes.
In the periodic table, it is a d-block transactinide element. It is a
member of the 7th period and belongs to the group 7 elements as the
fifth member of the 6d series of transition metals. 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.
Discovery
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Two groups claimed discovery of the element. Evidence of bohrium was
first reported in 1976 by a Soviet research team led by Yuri
Oganessian, in which targets of bismuth-209 and lead-208 were
bombarded with accelerated nuclei of chromium-54 and manganese-55,
respectively. Two activities, one with a half-life of one to two
milliseconds, and the other with an approximately five-second
half-life, were seen. Since the ratio of the intensities of these two
activities was constant throughout the experiment, it was proposed
that the first was from the isotope bohrium-261 and that the second
was from its daughter dubnium-257. Later, the dubnium isotope was
corrected to dubnium-258, which indeed has a five-second half-life
(dubnium-257 has a one-second half-life); however, the half-life
observed for its parent is much shorter than the half-lives later
observed in the definitive discovery of bohrium at Darmstadt in 1981.
The IUPAC/IUPAP Transfermium Working Group (TWG) concluded that while
dubnium-258 was probably seen in this experiment, the evidence for the
production of its parent bohrium-262 was not convincing enough.
In 1981, a German research team led by Peter Armbruster and Gottfried
Münzenberg at the GSI Helmholtz Centre for Heavy Ion Research (GSI
Helmholtzzentrum für Schwerionenforschung) in Darmstadt bombarded a
target of bismuth-209 with accelerated nuclei of chromium-54 to
produce 5 atoms of the isotope bohrium-262:
: + → +
This discovery was further substantiated by their detailed
measurements of the alpha decay chain of the produced bohrium atoms to
previously known isotopes of fermium and californium. The IUPAC/IUPAP
Transfermium Working Group (TWG) recognised the GSI collaboration as
official discoverers in their 1992 report.
Proposed names
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In September 1992, 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, and simultaneously help 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 precedent 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 names of the
respective oxyanions of boron and bohrium remain unchanged despite
their homophony.
Isotopes
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{{Isotopes summary
|element=bohrium
|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.
Twelve different isotopes of bohrium have been reported with atomic
masses 260-262, 264-267, 270-272, 274, and 278, one of which,
bohrium-262, has a known metastable state. All of these but the
unconfirmed 278Bh decay only through alpha decay, although some
unknown bohrium isotopes are predicted to undergo spontaneous fission.
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
2.4 min and 40 s respectively, and the even heavier unconfirmed
isotope 278Bh appearing to have an even longer half-life of about 11.5
minutes.
The most 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 five most neutron-rich ones with
masses 270, 271, 272, 274, and 278 (unconfirmed) appear in the decay
chains of 282Nh, 287Mc, 288Mc, 294Ts, and 290Fl respectively. The
half-lives of bohrium isotopes range from about ten milliseconds for
262mBh to about one minute for 270Bh and 274Bh, extending to about
11.5 minutes for the unconfirmed 278Bh, which may have one of the
longest half-lives among reported superheavy nuclides.
Predicted properties
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Very few properties of bohrium or its compounds have been measured;
this is due to its extremely limited and expensive production and the
fact that bohrium (and its parents) decays very quickly. A few
singular chemistry-related properties have been measured, but
properties of bohrium metal remain unknown and only predictions are
available.
Chemical
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Bohrium is the fifth member of the 6d series of transition metals and
the heaviest member of group 7 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 higher +7 oxidation state is more likely to exist in
oxyanions, such as perbohrate, , analogous to the lighter
permanganate, pertechnetate, and perrhenate. Nevertheless,
bohrium(VII) is likely to be unstable in aqueous solution, and would
probably be easily reduced to the more stable bohrium(IV).
The lighter group 7 elements are known to form volatile heptoxides
M2O7 (M = Mn, Tc, Re), 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. Since the oxychlorides are asymmetrical, and they should
have increasingly large dipole moments going down the group, they
should become less volatile in the order TcO3Cl > ReO3Cl >
BhO3Cl: this was experimentally confirmed in 2000 by measuring the
enthalpies of adsorption of these three compounds. The values are for
TcO3Cl and ReO3Cl are −51 kJ/mol and −61 kJ/mol respectively; the
experimental value for BhO3Cl is −77.8 kJ/mol, very close to the
theoretically expected value of −78.5 kJ/mol.
Physical and atomic
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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. Early predictions by Fricke estimated
its density at 37.1 g/cm3, but newer calculations predict a somewhat
lower value of 26-27 g/cm3.
The atomic radius of bohrium is expected to be around 128 pm. Due to
the relativistic stabilization of the 7s orbital and destabilization
of the 6d orbital, the Bh+ ion is predicted to have an electron
configuration of [Rn] 5f14 6d4 7s2, giving up a 6d electron instead of
a 7s electron, which is the opposite of the behavior of its lighter
homologues manganese and technetium. Rhenium, on the other hand,
follows its heavier congener bohrium in giving up a 5d electron before
a 6s electron, as relativistic effects have become significant by the
sixth period, where they cause among other things the yellow color of
gold and the low melting point of mercury. The Bh2+ ion is expected to
have an electron configuration of [Rn] 5f14 6d3 7s2; in contrast, the
Re2+ ion is expected to have a [Xe] 4f14 5d5 configuration, this time
analogous to manganese and technetium. The ionic radius of
hexacoordinate heptavalent bohrium is expected to be 58 pm
(heptavalent manganese, technetium, and rhenium having values of 46,
57, and 53 pm respectively). Pentavalent bohrium should have a larger
ionic radius of 83 pm.
Experimental chemistry
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In 1995, the first report on attempted isolation of the element was
unsuccessful, prompting new theoretical studies to investigate how
best to investigate bohrium (using its lighter homologs technetium and
rhenium for comparison) and removing unwanted contaminating elements
such as the trivalent actinides, the group 5 elements, and polonium.
In 2000, it was confirmed that although relativistic effects are
important, bohrium behaves like a typical group 7 element. A team at
the Paul Scherrer Institute (PSI) conducted a chemistry reaction using
six 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. The adsorption enthalpies of the
oxychlorides of technetium, rhenium, and bohrium were measured in this
experiment, agreeing very well with the theoretical predictions and
implying a sequence of decreasing oxychloride volatility down group 7
of TcO3Cl > ReO3Cl > BhO3Cl.
:2 Bh + 3 + 2 HCl → 2 +
The longer-lived heavy isotopes of bohrium, produced as the daughters
of heavier elements, offer advantages for future radiochemical
experiments. Although the heavy isotope 274Bh requires a rare and
highly radioactive berkelium target for its production, the isotopes
272Bh, 271Bh, and 270Bh can be readily produced as daughters of more
easily produced moscovium and nihonium isotopes.
External links
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*
* [
http://www.periodicvideos.com/videos/107.htm Bohrium] at 'The
Periodic Table of Videos' (University of Nottingham)
License
=========
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http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Bohrium
