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= Copernicium =
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Introduction
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Copernicium is a synthetic chemical element; it has symbol Cn and
atomic number 112. Its known isotopes are extremely radioactive, and
have only been created in a laboratory. The most stable known isotope,
copernicium-285, has a half-life of approximately 30 seconds.
Copernicium was first created in February 1996 by the GSI Helmholtz
Centre for Heavy Ion Research near Darmstadt, Germany. It was named
after the astronomer Nicolaus Copernicus on his 537th anniversary.
In the periodic table of the elements, copernicium is a d-block
transactinide element and a group 12 element. During reactions with
gold, it has been shown
to be an extremely volatile element, so much so that it is possibly a
gas or a volatile liquid at standard temperature and pressure.
Copernicium is calculated to have several properties that differ from
its lighter homologues in group 12, zinc, cadmium and mercury; due to
relativistic effects, it may give up its 6d electrons instead of its
7s ones, and it may have more similarities to the noble gases such as
radon rather than its group 12 homologues. Calculations indicate that
copernicium may show the oxidation state +4, while mercury shows it in
only one compound of disputed existence and zinc and cadmium do not
show it at all. It has also been predicted to be more difficult to
oxidize copernicium from its neutral state than the other group 12
elements. Predictions vary on whether solid copernicium would be a
metal, semiconductor, or insulator. Copernicium is one of the heaviest
elements whose chemical properties have been experimentally
investigated.
Discovery
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Copernicium was first created on 9 February 1996, at the Gesellschaft
für Schwerionenforschung (GSI) in Darmstadt, Germany, by Sigurd
Hofmann, Victor Ninov et al. This element was created by firing
accelerated zinc-70 nuclei at a target made of lead-208 nuclei in a
heavy ion accelerator. A single atom of copernicium was produced with
a mass number of 277. (A second was originally reported, but was found
to have been based on data fabricated by Ninov, and was thus
retracted.)
:Pb + Zn → Cn* → Cn + n
In May 2000, the GSI successfully repeated the experiment to
synthesize a further atom of copernicium-277.
This reaction was repeated at RIKEN using the Search for a Super-Heavy
Element Using a Gas-Filled Recoil Separator set-up in 2004 and 2013 to
synthesize three further atoms and confirm the decay data reported by
the GSI team.
This reaction had also previously been tried in 1971 at the Joint
Institute for Nuclear Research in Dubna, Russia to aim for 276Cn
(produced in the 2n channel), but without success.
The IUPAC/IUPAP Joint Working Party (JWP) assessed the claim of
copernicium's discovery by the GSI team in 2001 and 2003. In both
cases, they found that there was insufficient evidence to support
their claim. This was primarily related to the contradicting decay
data for the known nuclide rutherfordium-261. However, between 2001
and 2005, the GSI team studied the reaction 248Cm(26Mg,5n)269Hs, and
were able to confirm the decay data for hassium-269 and
rutherfordium-261. It was found that the existing data on
rutherfordium-261 was for an isomer,
now designated rutherfordium-261m.
In May 2009, the JWP reported on the claims of discovery of element
112 again and officially recognized the GSI team as the discoverers of
element 112. This decision was based on the confirmation of the decay
properties of daughter nuclei as well as the confirmatory experiments
at RIKEN.
Work had also been done at the Joint Institute for Nuclear Research in
Dubna, Russia from 1998 to synthesise the heavier isotope 283Cn in the
hot fusion reaction 238U(48Ca,3n)283Cn; most observed atoms of 283Cn
decayed by spontaneous fission, although an alpha decay branch to
279Ds was detected. While initial experiments aimed to assign the
produced nuclide with its observed long half-life of 3 minutes based
on its chemical behaviour, this was found to be not mercury-like as
would have been expected (copernicium being under mercury in the
periodic table), and indeed now it appears that the long-lived
activity might not have been from 283Cn at all, but its electron
capture daughter 283Rg instead, with a shorter 4-second half-life
associated with 283Cn. (Another possibility is assignment to a
metastable isomeric state, 283mCn.) While later cross-bombardments in
the 242Pu+48Ca and 245Cm+48Ca reactions succeeded in confirming the
properties of 283Cn and its parents 287Fl and 291Lv, and played a
major role in the acceptance of the discoveries of flerovium and
livermorium (elements 114 and 116) by the JWP in 2011, this work
originated subsequent to the GSI's work on 277Cn and priority was
assigned to the GSI.
Naming
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Using Mendeleev's nomenclature for unnamed and undiscovered elements,
copernicium should be known as 'eka-mercury'. In 1979, IUPAC published
recommendations according to which the element was to be called
'ununbium' (with the corresponding symbol of 'Uub'), 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 either called it
"element 112", with the symbol of 'E112', '(112)', or even simply
'112'.
After acknowledging the GSI team's discovery, the IUPAC asked them to
suggest a permanent name for element 112. On 14 July 2009, they
proposed 'copernicium' with the element symbol Cp, after Nicolaus
Copernicus "to honor an outstanding scientist, who changed our view of
the world".
During the standard six-month discussion period among the scientific
community about the naming,
it was pointed out that the symbol 'Cp' was previously associated with
the name 'cassiopeium' (cassiopium), now known as lutetium (Lu).
Moreover, Cp is frequently used today to mean the cyclopentadienyl
ligand (C5H5). Primarily because cassiopeium (Cp) was (until 1949)
accepted by IUPAC as an alternative allowed name for lutetium, the
IUPAC disallowed the use of Cp as a future symbol, prompting the GSI
team to put forward the symbol Cn as an alternative. On 19 February
2010, the 537th anniversary of Copernicus' birth, IUPAC officially
accepted the proposed name and symbol.
Isotopes
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{{Isotopes summary
|element=copernicium
|reaction ref=
|isotopes=
}}
Copernicium 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.
Eight different isotopes have been reported with mass numbers 277 and
280-286, and one unconfirmed metastable isomer in 285Cn has been
reported. Most of these decay predominantly through alpha decay, but
some undergo spontaneous fission, and copernicium-283 may have an
electron capture branch.
The isotope copernicium-283 was instrumental in the confirmation of
the discoveries of the elements flerovium and livermorium.
Half-lives
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All confirmed copernicium isotopes are extremely unstable and
radioactive; in general, heavier isotopes are more stable than the
lighter, and isotopes with an odd neutron number have relatively
longer half-lives due to additional hindrance against spontaneous
fission. The most stable known isotope, 285Cn, has a half-life of 30
seconds; 283Cn has a half-life of 4 seconds, and the unconfirmed
285mCn and 286Cn have half-lives of about 15 and 8.45 seconds
respectively. Other isotopes have half-lives shorter than one second.
281Cn and 284Cn both have half-lives on the order of 0.1 seconds, and
the remaining isotopes have half-lives shorter than one millisecond.
It is predicted that the heavy isotopes 291Cn and 293Cn may have
half-lives longer than a few decades, for they are predicted to lie
near the center of the theoretical island of stability, and may have
been produced in the r-process and be detectable in cosmic rays,
though they would be about 10−12 times as abundant as lead.
The lightest isotopes of copernicium have been synthesized by direct
fusion between two lighter nuclei and as decay products (except for
277Cn, which is not known to be a decay product), while the heavier
isotopes are only known to be produced by decay of heavier nuclei. The
heaviest isotope produced by direct fusion is 283Cn; the three heavier
isotopes, 284Cn, 285Cn, and 286Cn, have only been observed as decay
products of elements with larger atomic numbers.
In 1999, American scientists at the University of California,
Berkeley, announced that they had succeeded in synthesizing three
atoms of 293Og. These parent nuclei were reported to have successively
emitted three alpha particles to form copernicium-281 nuclei, which
were claimed to have undergone alpha decay, emitting alpha particles
with decay energy 10.68 MeV and half-life 0.90 ms, but their claim was
retracted in 2001 as it had been based on data fabricated by Ninov.
This isotope was truly produced in 2010 by the same team; the new data
contradicted the previous fabricated data.
The missing isotopes 278Cn and 279Cn are too heavy to be produced by
cold fusion and too light to be produced by hot fusion. They might be
filled from above by decay of heavier elements produced by hot fusion,
and indeed 280Cn and 281Cn were produced this way. The isotopes 286Cn
and 287Cn could be produced by charged-particle evaporation, in the
reaction 244Pu(48Ca,α'x'n) with 'x' equalling 1 or 2.
Predicted properties
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Very few properties of copernicium or its compounds have been
measured; this is due to its extremely limited and expensive
production and the fact that copernicium (and its parents) decays very
quickly. A few singular chemical properties have been measured, as
well as the boiling point, but properties of the copernicium metal
remain generally unknown and for the most part, only predictions are
available.
Chemical
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Copernicium is the tenth and last member of the 6d series and is the
heaviest group 12 element in the periodic table, below zinc, cadmium
and mercury. It is predicted to differ significantly from the lighter
group 12 elements. The valence s-subshells of the group 12 elements
and period 7 elements are expected to be relativistically contracted
most strongly at copernicium. This and the closed-shell configuration
of copernicium result in it probably being a very noble metal. A
standard reduction potential of +2.1 V is predicted for the Cn2+/Cn
couple. Copernicium's predicted first ionization energy of 1155 kJ/mol
almost matches that of the noble gas xenon at 1170.4 kJ/mol.
Copernicium's metallic bonds should also be very weak, possibly making
it extremely volatile like the noble gases, and potentially making it
gaseous at room temperature. However, it should be able to form
metal-metal bonds with copper, palladium, platinum, silver, and gold;
these bonds are predicted to be only about 15-20 kJ/mol weaker than
the analogous bonds with mercury. In opposition to the earlier
suggestion, ab initio calculations at the high level of accuracy
predicted that the chemistry of singly-valent copernicium resembles
that of mercury rather than that of the noble gases. The latter result
can be explained by the huge spin-orbit interaction which
significantly lowers the energy of the vacant 7p1/2 state of
copernicium.
Once copernicium is ionized, its chemistry may present several
differences from those of zinc, cadmium, and mercury. Due to the
stabilization of 7s electronic orbitals and destabilization of 6d ones
caused by relativistic effects, Cn2+ is likely to have a
[Rn]5f146d87s2 electronic configuration, using the 6d orbitals before
the 7s one, unlike its homologues. The fact that the 6d electrons
participate more readily in chemical bonding means that once
copernicium is ionized, it may behave more like a transition metal
than its lighter homologues, especially in the possible +4 oxidation
state. In aqueous solutions, copernicium may form the +2 and perhaps
+4 oxidation states. The diatomic ion , featuring mercury in the +1
oxidation state, is well-known, but the ion is predicted to be
unstable or even non-existent. Copernicium(II) fluoride, CnF2, should
be more unstable than the analogous mercury compound, mercury(II)
fluoride (HgF2), and may even decompose spontaneously into its
constituent elements. As the most electronegative reactive element,
fluorine may be the only element able to oxidise copernicium even
further to the +4 and even +6 oxidation states in CnF4 and CnF6; the
latter may require matrix-isolation conditions to be detected, as in
the disputed detection of HgF4. CnF4 should be more stable than CnF2.
In polar solvents, copernicium is predicted to preferentially form the
and anions rather than the analogous neutral fluorides (CnF4 and
CnF2, respectively), although the analogous bromide or iodide ions may
be more stable towards hydrolysis in aqueous solution. The anions and
should also be able to exist in aqueous solution. The formation of
thermodynamically stable copernicium(II) and (IV) fluorides would be
analogous to the chemistry of xenon. Analogous to mercury(II) cyanide
(Hg(CN)2), copernicium is expected to form a stable cyanide, Cn(CN)2.
Physical and atomic
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Copernicium should be a dense metal, with a density of 14.0 g/cm3 in
the liquid state at 300 K; this is similar to the known density of
mercury, which is 13.534 g/cm3. (Solid copernicium at the same
temperature should have a higher density of 14.7 g/cm3.) This results
from the effects of copernicium's higher atomic weight being cancelled
out by its larger interatomic distances compared to mercury. Some
calculations predicted copernicium to be a gas at room temperature due
to its closed-shell electron configuration, which would make it the
first gaseous metal in the periodic table. A 2019 calculation agrees
with these predictions on the role of relativistic effects, suggesting
that copernicium will be a volatile liquid bound by dispersion forces
under standard conditions. Its melting point is estimated at and its
boiling point at , the latter in agreement with the experimentally
estimated value of . The atomic radius of copernicium is expected to
be around 147 pm. Due to the relativistic stabilization of the 7s
orbital and destabilization of the 6d orbital, the Cn+ and Cn2+ ions
are predicted to give up 6d electrons instead of 7s electrons, which
is the opposite of the behavior of its lighter homologues.
In addition to the relativistic contraction and binding of the 7s
subshell, the 6d5/2 orbital is expected to be destabilized due to
spin-orbit coupling, making it behave similarly to the 7s orbital in
terms of size, shape, and energy. Predictions of the expected band
structure of copernicium are varied. Calculations in 2007 expected
that copernicium may be a semiconductor with a band gap of around 0.2
eV, crystallizing in the hexagonal close-packed crystal structure.
However, calculations in 2017 and 2018 suggested that copernicium
should be a noble metal at standard conditions with a body-centered
cubic crystal structure: it should hence have no band gap, like
mercury, although the density of states at the Fermi level is expected
to be lower for copernicium than for mercury. 2019 calculations then
suggested that in fact copernicium has a large band gap of 6.4 ± 0.2
eV, which should be similar to that of the noble gas radon (predicted
as 7.1 eV) and would make it an insulator; bulk copernicium is
predicted by these calculations to be bound mostly by dispersion
forces, like the noble gases. Like mercury, radon, and flerovium, but
not oganesson (eka-radon), copernicium is calculated to have no
electron affinity.
Experimental atomic gas phase chemistry
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Interest in copernicium's chemistry was sparked by predictions that it
would have the largest relativistic effects in the whole of period 7
and group 12, and indeed among all 118 known elements. Copernicium is
expected to have the ground state electron configuration [Rn] 5f14
6d10 7s2 and thus should belong to group 12 of the periodic table,
according to the Aufbau principle. As such, it should behave as the
heavier homologue of mercury and form strong binary compounds with
noble metals like gold. Experiments probing the reactivity of
copernicium have focused on the adsorption of atoms of element 112
onto a gold surface held at varying temperatures, in order to
calculate an adsorption enthalpy. Owing to relativistic stabilization
of the 7s electrons, copernicium shows radon-like properties.
Experiments were performed with the simultaneous formation of mercury
and radon radioisotopes, allowing a comparison of adsorption
characteristics.
The first chemical experiments on copernicium were conducted using the
238U(48Ca,3n)283Cn reaction. Detection was by spontaneous fission of
the claimed parent isotope with half-life of 5 minutes. Analysis of
the data indicated that copernicium was more volatile than mercury and
had noble gas properties. However, the confusion regarding the
synthesis of copernicium-283 has cast some doubt on these experimental
results. Given this uncertainty, between April-May 2006 at the JINR, a
FLNR-PSI team conducted experiments probing the synthesis of this
isotope as a daughter in the nuclear reaction 242Pu(48Ca,3n)287Fl.
(The 242Pu + 48Ca fusion reaction has a slightly larger cross-section
than the 238U + 48Ca reaction, so that the best way to produce
copernicium for chemical experimentation is as an overshoot product as
the daughter of flerovium.) In this experiment, two atoms of
copernicium-283 were unambiguously identified and the adsorption
properties were interpreted to show that copernicium is a more
volatile homologue of mercury, due to formation of a weak metal-metal
bond with gold. This agrees with general indications from some
relativistic calculations that copernicium is "more or less"
homologous to mercury. However, it was pointed out in 2019 that this
result may simply be due to strong dispersion interactions.
In April 2007, this experiment was repeated and a further three atoms
of copernicium-283 were positively identified. The adsorption property
was confirmed and indicated that copernicium has adsorption properties
in agreement with being the heaviest member of group 12.
These experiments also allowed the first experimental estimation of
copernicium's boiling point: 84 °C, so that it may be a gas at
standard conditions.
Because the lighter group 12 elements often occur as chalcogenide
ores, experiments were conducted in 2015 to deposit copernicium atoms
on a selenium surface to form copernicium selenide, CnSe. Reaction of
copernicium atoms with trigonal selenium to form a selenide was
observed, with -Δ'H'adsCn(t-Se) > 48 kJ/mol, with the kinetic
hindrance towards selenide formation being lower for copernicium than
for mercury. This was unexpected as the stability of the group 12
selenides tends to decrease down the group from ZnSe to HgSe.
See also
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* Island of stability
External links
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* [
https://www.periodicvideos.com/videos/112.htm Copernicium] at 'The
Periodic Table of Videos' (University of Nottingham)
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Copernicium
