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= Roentgenium =
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Introduction
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Roentgenium () is a synthetic chemical element; it has symbol Rg and
atomic number 111. It is extremely radioactive and can only be created
in a laboratory. The most stable known isotope, roentgenium-282, has a
half-life of 130 seconds, although the unconfirmed roentgenium-286 may
have a longer half-life of about 10.7 minutes. Roentgenium was first
created in December 1994 by the GSI Helmholtz Centre for Heavy Ion
Research near Darmstadt, Germany. It is named after the physicist
Wilhelm Röntgen (also spelled Roentgen), who discovered X-rays. Only a
few roentgenium atoms have ever been synthesized, and they have no
practical application.
In the periodic table, it is a d-block transactinide element. It is a
member of the 7th period and is placed in the group 11 elements,
although no chemical experiments have been carried out to confirm that
it behaves as the heavier homologue to gold in group 11 as the ninth
member of the 6d series of transition metals. Roentgenium is
calculated to have similar properties to its lighter homologues,
copper, silver, and gold, although it may show some differences from
them.
Official discovery
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Roentgenium was first synthesized by an international team led by
Sigurd Hofmann at the Gesellschaft für Schwerionenforschung (GSI) in
Darmstadt, Germany, on December 8, 1994. The team bombarded a target
of bismuth-209 with accelerated nuclei of nickel-64 and detected three
nuclei of the isotope 272111:
: + → 272111 +
This reaction had previously been conducted at the Joint Institute for
Nuclear Research in Dubna (then in the Soviet Union) in 1986, but no
atoms of 272111 had then been observed. In 2001, the IUPAC/IUPAP Joint
Working Party (JWP) concluded that there was insufficient evidence for
the discovery at that time. The GSI team repeated their experiment in
2002 and detected three more atoms. In their 2003 report, the JWP
decided that the GSI team should be acknowledged for the discovery of
this element.
Naming
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Using Mendeleev's nomenclature for unnamed and undiscovered elements,
roentgenium should be known as 'eka-gold'. In 1979, IUPAC published
recommendations according to which the element was to be called
'unununium' (with the corresponding symbol of 'Uuu'), a systematic
element name as a placeholder, until the element was discovered (and
the discovery then confirmed) and a permanent name was decided on.
Although widely used in the chemical community on all levels, from
chemistry classrooms to advanced textbooks, the recommendations were
mostly ignored among scientists in the field, who called it 'element
111', with the symbol of 'E111', '(111)' or even simply '111'.
The name 'roentgenium' (Rg) was suggested by the GSI team in 2004, to
honor the German physicist Wilhelm Conrad Röntgen, the discoverer of
X-rays. This name was accepted by IUPAC on November 1, 2004.
Isotopes
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{{Isotopes summary
|element=roentgenium
|reaction ref=
|isotopes=
}}
Roentgenium has no stable or naturally occurring isotopes. Several
radioactive isotopes have been synthesized in the laboratory, either
by fusion of the nuclei of lighter elements or as intermediate decay
products of heavier elements. Nine different isotopes of roentgenium
have been reported with atomic masses 272, 274, 278-283, and 286 (283
and 286 unconfirmed), two of which, roentgenium-272 and
roentgenium-274, have known but unconfirmed metastable states. All of
these decay through alpha decay or spontaneous fission, though 280Rg
may also have an electron capture branch.
Stability and half-lives
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All roentgenium isotopes are extremely unstable and radioactive; in
general, the heavier isotopes are more stable than the lighter. The
most stable known roentgenium isotope, 282Rg, is also the heaviest
known roentgenium isotope; it has a half-life of 100 seconds. The
unconfirmed 286Rg is even heavier and appears to have an even longer
half-life of about 10.7 minutes, which would make it one of the
longest-lived superheavy nuclides known; likewise, the unconfirmed
283Rg appears to have a long half-life of about 5.1 minutes. The
isotopes 280Rg and 281Rg have also been reported to have half-lives
over a second. The remaining isotopes have half-lives in the
millisecond range.
The missing isotopes between 274Rg and 278Rg are too light to be
produced by hot fusion and too heavy to be produced by cold fusion. A
possible synthesis method is to populate them from above, as daughters
of nihonium or moscovium isotopes that can be produced by hot fusion.
The isotopes 283Rg and 284Rg could be synthesised using
charged-particle evaporation, using the 238U+48Ca reaction where a
proton is evaporated alongside some neutrons.
Predicted properties
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Other than nuclear properties, no properties of roentgenium or its
compounds have been measured; this is due to its extremely limited and
expensive production and the fact that roentgenium (and its parents)
decays very quickly. Properties of roentgenium metal remain unknown
and only predictions are available.
Chemical
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Roentgenium is the ninth member of the 6d series of transition metals.
Calculations on its ionization potentials and atomic and ionic radii
are similar to that of its lighter homologue gold, thus implying that
roentgenium's basic properties will resemble those of the other group
11 elements, copper, silver, and gold; however, it is also predicted
to show several differences from its lighter homologues.
Roentgenium is predicted to be a noble metal. The standard electrode
potential of 1.9 V for the Rg3+/Rg couple is greater than that of 1.5
V for the Au3+/Au couple. Roentgenium's predicted first ionisation
energy of 1020 kJ/mol almost matches that of the noble gas radon at
1037 kJ/mol. Its predicted second ionization energy, 2070 kJ/mol, is
almost the same as that of silver. Based on the most stable oxidation
states of the lighter group 11 elements, roentgenium is predicted to
show stable +5 and +3 oxidation states, with a less stable +1 state.
The +3 state is predicted to be the most stable. Roentgenium(III) is
expected to be of comparable reactivity to gold(III), but should be
more stable and form a larger variety of compounds. Gold also forms a
somewhat stable −1 state due to relativistic effects, and it has been
suggested roentgenium may do so as well: nevertheless, the electron
affinity of roentgenium is expected to be around 1.6 eVpar,
significantly lower than gold's value of 2.3 eVpar, so roentgenides
may not be stable or even possible.
The 6d orbitals are destabilized by relativistic effects and
spin-orbit interactions near the end of the fourth transition metal
series, thus making the high oxidation state roentgenium(V) more
stable than its lighter homologue gold(V) (known only in gold
pentafluoride, Au2F10) as the 6d electrons participate in bonding to a
greater extent. The spin-orbit interactions stabilize molecular
roentgenium compounds with more bonding 6d electrons; for example, is
expected to be more stable than , which is expected to be more stable
than . The stability of is homologous to that of ; the silver
analogue is unknown and is expected to be only marginally stable to
decomposition to and F2. Moreover, Rg2F10 is expected to be stable to
decomposition, exactly analogous to the Au2F10, whereas Ag2F10 should
be unstable to decomposition to Ag2F6 and F2. Gold heptafluoride,
AuF7, is known as a gold(V) difluorine complex AuF5·F2, which is lower
in energy than a true gold(VII) heptafluoride would be; RgF7 is
instead calculated to be more stable as a true roentgenium(VII)
heptafluoride, although it would be somewhat unstable, its
decomposition to Rg2F10 and F2 releasing a small amount of energy at
room temperature. Roentgenium(I) is expected to be difficult to
obtain. Gold readily forms the cyanide complex , which is used in its
extraction from ore through the process of gold cyanidation;
roentgenium is expected to follow suit and form .
The probable chemistry of roentgenium has received more interest than
that of the two previous elements, meitnerium and darmstadtium, as the
valence s-subshells of the group 11 elements are expected to be
relativistically contracted most strongly at roentgenium. Calculations
on the molecular compound RgH show that relativistic effects double
the strength of the roentgenium-hydrogen bond, even though spin-orbit
interactions also weaken it by 0.7 eVpar. The compounds AuX and RgX,
where X = F, Cl, Br, O, Au, or Rg, were also studied. Rg+ is predicted
to be the softest metal ion, even softer than Au+, although there is
disagreement on whether it would behave as an acid or a base. In
aqueous solution, Rg+ would form the aqua ion [Rg(H2O)2]+, with an
Rg-O bond distance of 207.1 pm. It is also expected to form Rg(I)
complexes with ammonia, phosphine, and hydrogen sulfide.
Physical and atomic
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Roentgenium is expected to be a solid under normal conditions and to
crystallize in the body-centered cubic structure, unlike its lighter
congeners which crystallize in the face-centered cubic structure, due
to its being expected to have different electron charge densities from
them. It should be a very heavy metal with a density of around 22-24
g/cm3; in comparison, the densest known element that has had its
density measured, osmium, has a density of 22.61 g/cm3. The atomic
radius of roentgenium is expected to be around 114 pm.
Experimental chemistry
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Unambiguous determination of the chemical characteristics of
roentgenium has yet to have been established due to the low yields of
reactions that produce roentgenium isotopes. For chemical studies to
be carried out on a transactinide, at least four atoms must be
produced, the half-life of the isotope used must be at least 1 second,
and the rate of production must be at least one atom per week. Even
though the half-life of 282Rg, the most stable confirmed roentgenium
isotope, is 100 seconds, long enough to perform chemical studies,
another obstacle is the need to increase the rate of production of
roentgenium isotopes and allow experiments to carry on for weeks or
months so that statistically significant results can be obtained.
Separation and detection must be carried out continuously to separate
out the roentgenium isotopes and allow automated systems to experiment
on the gas-phase and solution chemistry of roentgenium, as the yields
for heavier elements are predicted to be smaller than those for
lighter elements. However, the experimental chemistry of roentgenium
has not received as much attention as that of the heavier elements
from copernicium to livermorium, despite early interest in theoretical
predictions due to relativistic effects on the 'n's subshell in group
11 reaching a maximum at roentgenium. The isotopes 280Rg and 281Rg are
promising for chemical experimentation and may be produced as the
granddaughters of the moscovium isotopes 288Mc and 289Mc respectively;
their parents are the nihonium isotopes 284Nh and 285Nh, which have
already received preliminary chemical investigations.
See also
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* Island of stability
External links
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* [
http://www.periodicvideos.com/videos/111.htm Roentgenium] at 'The
Periodic Table of Videos' (University of Nottingham)
License
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All content on Gopherpedia comes from Wikipedia, and is licensed under CC-BY-SA
License URL:
http://creativecommons.org/licenses/by-sa/3.0/
Original Article:
http://en.wikipedia.org/wiki/Roentgenium
