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= Oganesson =
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Introduction
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Oganesson is a synthetic chemical element; it has symbol Og and atomic
number 118. It was first synthesized in 2002 at the Joint Institute
for Nuclear Research (JINR) in Dubna, near Moscow, Russia, by a joint
team of Russian and American scientists. In December 2015, it was
recognized as one of four new elements by the Joint Working Party of
the international scientific bodies IUPAC and IUPAP. It was formally
named on 28 November 2016. The name honors the nuclear physicist Yuri
Oganessian, who played a leading role in the discovery of the heaviest
elements in the periodic table.
Oganesson has the highest atomic number and highest atomic mass of all
known elements. On the periodic table of the elements it is a p-block
element, a member of group 18 and the last member of period 7. Its
only known isotope, oganesson-294, is highly radioactive, with a
half-life of 0.7 ms and, only five atoms have been successfully
produced. This has so far prevented any experimental studies of its
chemistry. Because of relativistic effects, theoretical studies
predict that it would be a solid at room temperature, and
significantly reactive, unlike the other members of group 18 (the
noble gases).
Early speculation
===================
The possibility of a seventh noble gas, after helium, neon, argon,
krypton, xenon, and radon, was considered almost as soon as the noble
gas group was discovered. Danish chemist Hans Peter Jørgen Julius
Thomsen predicted in April 1895, the year after the discovery of
argon, that there was a whole series of chemically inert gases similar
to argon that would bridge the halogen and alkali metal groups: he
expected that the seventh of this series would end a 32-element period
which contained thorium and uranium and have an atomic weight of 292,
close to the 294 now known for the first and only confirmed isotope of
oganesson. Danish physicist Niels Bohr noted in 1922 that this seventh
noble gas should have atomic number 118 and predicted its electronic
structure as 2, 8, 18, 32, 32, 18, 8, matching modern predictions.
Following this, German chemist Aristid von Grosse wrote an article in
1965 predicting the likely properties of element 118. It was 107 years
from Thomsen's prediction before oganesson was successfully
synthesized, although its chemical properties have not been
investigated to determine if it behaves as the heavier congener of
radon. In a 1975 article, American chemist Kenneth Pitzer suggested
that element 118 should be a gas or volatile liquid due to
relativistic effects.
Unconfirmed discovery claims
==============================
In late 1998, Polish physicist Robert Smolańczuk published
calculations on the fusion of atomic nuclei towards the synthesis of
superheavy atoms, including oganesson. His calculations suggested that
it might be possible to make element 118 by fusing lead with krypton
under carefully controlled conditions, and that the fusion probability
(cross section) of that reaction would be close to the lead-chromium
reaction that had produced element 106, seaborgium. This contradicted
predictions that the cross sections for reactions with lead or bismuth
targets would go down exponentially as the atomic number of the
resulting elements increased.
In 1999, researchers at Lawrence Berkeley National Laboratory made use
of these predictions and announced the discovery of elements 118 and
116, in a paper published in 'Physical Review Letters', and very soon
after the results were reported in 'Science'. The researchers reported
that they had performed the reaction
: + → + .
In 2001, they published a retraction after researchers at other
laboratories were unable to duplicate the results and the Berkeley lab
could not duplicate them either. In June 2002, the director of the lab
announced that the original claim of the discovery of these two
elements had been based on data fabricated by principal author Victor
Ninov. Newer experimental results and theoretical predictions have
confirmed the exponential decrease in cross sections with lead and
bismuth targets as the atomic number of the resulting nuclide
increases.
Discovery reports
===================
The first genuine decay of atoms of oganesson was observed in 2002 at
the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, by a
joint team of Russian and American scientists. Headed by Yuri
Oganessian, a Russian nuclear physicist of Armenian ethnicity, the
team included American scientists from the Lawrence Livermore National
Laboratory in California. The discovery was not announced immediately,
because the decay energy of 294Og matched that of 212mPo, a common
impurity produced in fusion reactions aimed at producing superheavy
elements, and thus announcement was delayed until after a 2005
confirmatory experiment aimed at producing more oganesson atoms. The
2005 experiment used a different beam energy (251 MeV instead of 245
MeV) and target thickness (0.34 mg/cm2 instead of 0.23 mg/cm2). On 9
October 2006, the researchers announced that they had indirectly
detected a total of three (possibly four) nuclei of oganesson-294 (one
or two in 2002 and two more in 2005) produced via collisions of
californium-249 atoms and calcium-48 ions.
: + → + 3 .
In 2011, IUPAC evaluated the 2006 results of the Dubna-Livermore
collaboration and concluded: "The three events reported for the 'Z' =
118 isotope have very good internal
redundancy but with no anchor to known nuclei do not satisfy the
criteria for discovery".
Because of the very small fusion reaction probability (the fusion
cross section is or ) the experiment took four months and involved a
beam dose of calcium ions that had to be shot at the californium
target to produce the first recorded event believed to be the
synthesis of oganesson. Nevertheless, researchers were highly
confident that the results were not a false positive, since the chance
that the detections were random events was estimated to be less than
one part in .
In the experiments, the alpha-decay of three atoms of oganesson was
observed. A fourth decay by direct spontaneous fission was also
proposed. A half-life of 0.89 ms was calculated: decays into by
alpha decay. Since there were only three nuclei, the half-life derived
from observed lifetimes has a large uncertainty: .
: → +
The identification of the nuclei was verified by separately creating
the putative daughter nucleus directly by means of a bombardment of
with ions,
: + → + 3 ,
and checking that the decay matched the decay chain of the nuclei.
The daughter nucleus is very unstable, decaying with a lifetime of 14
milliseconds into , which may experience either spontaneous fission or
alpha decay into , which will undergo spontaneous fission.
Confirmation
==============
In December 2015, the Joint Working Party of international scientific
bodies International Union of Pure and Applied Chemistry (IUPAC) and
International Union of Pure and Applied Physics (IUPAP) recognized the
element's discovery and assigned the priority of the discovery to the
Dubna-Livermore collaboration. This was on account of two 2009 and
2010 confirmations of the properties of the granddaughter of 294Og,
286Fl, at the Lawrence Berkeley National Laboratory, as well as the
observation of another consistent decay chain of 294Og by the Dubna
group in 2012. The goal of that experiment had been the synthesis of
294Ts via the reaction 249Bk(48Ca,3n), but the short half-life of
249Bk resulted in a significant quantity of the target having decayed
to 249Cf, resulting in the synthesis of oganesson instead of
tennessine.
From 1 October 2015 to 6 April 2016, the Dubna team performed a
similar experiment with 48Ca projectiles aimed at a mixed-isotope
californium target containing 249Cf, 250Cf, and 251Cf, with the aim of
producing the heavier oganesson isotopes 295Og and 296Og. Two beam
energies at 252 MeV and 258 MeV were used. Only one atom was seen at
the lower beam energy, whose decay chain fitted the previously known
one of 294Og (terminating with spontaneous fission of 286Fl), and none
were seen at the higher beam energy. The experiment was then halted,
as the glue from the sector frames covered the target and blocked
evaporation residues from escaping to the detectors. The production of
293Og and its daughter 289Lv, as well as the even heavier isotope
297Og, is also possible using this reaction. The isotopes 295Og and
296Og may also be produced in the fusion of 248Cm with 50Ti
projectiles.
A search beginning in summer 2016 at RIKEN for 295Og in the 3n
channel of this reaction was unsuccessful, though the study is planned
to resume; a detailed analysis and cross section limit were not
provided. These heavier and likely more stable isotopes may be useful
in probing the chemistry of oganesson.
Naming
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Using Mendeleev's nomenclature for unnamed and undiscovered elements,
oganesson is sometimes known as 'eka-radon' (until the 1960s as
'eka-emanation', emanation being the old name for radon). In 1979,
IUPAC assigned the systematic placeholder name 'ununoctium' to the
undiscovered element, with the corresponding symbol of 'Uuo', and
recommended that it be used until after confirmed discovery of the
element. 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 118", with the symbol of 'E118', '(118)', or simply '118'.
Before the retraction in 2001, the researchers from Berkeley had
intended to name the element 'ghiorsium' ('Gh'), after Albert Ghiorso
(a leading member of the research team).
The Russian discoverers reported their synthesis in 2006. According to
IUPAC recommendations, the discoverers of a new element have the right
to suggest a name.
In 2007, the head of the Russian institute stated the team were
considering two names for the new element: 'flyorium', in honor of
Georgy Flyorov, the founder of the research laboratory in Dubna; and
'moskovium', in recognition of the Moscow Oblast where Dubna is
located. He also stated that although the element was discovered as an
American collaboration, who provided the californium target, the
element should rightly be named in honor of Russia since the Flyorov
Laboratory of Nuclear Reactions at JINR was the only facility in the
world which could achieve this result. These names were later
suggested for element 114 (flerovium) and element 116 (moscovium).
Flerovium became the name of element 114; the final name proposed for
element 116 was instead 'livermorium', with 'moscovium' later being
proposed and accepted for element 115 instead.
Traditionally, the names of all noble gases end in "-on", with the
exception of helium, which was not known to be a noble gas when
discovered. The IUPAC guidelines valid at the moment of the discovery
approval however required 'all' new elements be named with the ending
"-ium", even if they turned out to be halogens (traditionally ending
in "-ine") or noble gases (traditionally ending in "-on"). While the
provisional name ununoctium followed this convention, a new IUPAC
recommendation published in 2016 recommended using the "-on" ending
for new group 18 elements, regardless of whether they turn out to have
the chemical properties of a noble gas.
The scientists involved in the discovery of element 118, as well as
those of 117 and 115, held a conference call on 23 March 2016 to
decide their names. Element 118 was the last to be decided upon; after
Oganessian was asked to leave the call, the remaining scientists
unanimously decided to have the element "oganesson" after him.
Oganessian was a pioneer in superheavy element research for sixty
years reaching back to the field's foundation: his team and his
proposed techniques had led directly to the synthesis of elements 107
through 118. Mark Stoyer, a nuclear chemist at the LLNL, later
recalled, "We had intended to propose that name from Livermore, and
things kind of got proposed at the same time from multiple places. I
don't know if we can claim that we actually proposed the name, but we
had intended it."
In internal discussions, IUPAC asked the JINR if they wanted the
element to be spelled "oganeson" to match the Russian spelling more
closely. Oganessian and the JINR refused this offer, citing the
Soviet-era practice of transliterating names into the Latin alphabet
under the rules of the French language ("Oganessian" is such a
transliteration) and arguing that "oganesson" would be easier to link
to the person.
In June 2016, IUPAC announced that the discoverers planned to give the
element the name 'oganesson' (symbol: 'Og'). The name became official
on 28 November 2016. In 2017, Oganessian commented on the naming:
The naming ceremony for moscovium, tennessine, and oganesson was held
on 2 March 2017 at the Russian Academy of Sciences in Moscow.
In a 2019 interview, when asked what it was like to see his name in
the periodic table next to Einstein, Mendeleev, the Curies, and
Rutherford, Oganessian responded:
Characteristics
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Other than nuclear properties, no properties of oganesson or its
compounds have been measured; this is due to its extremely limited and
expensive production and the fact that it decays very quickly. Thus
only predictions are available.
Nuclear stability and isotopes
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The stability of nuclei quickly decreases with the increase in atomic
number after curium, element 96, whose most stable isotope, 247Cm, has
a half-life four orders of magnitude longer than that of any
subsequent element. All nuclides with an atomic number above 101
undergo radioactive decay with half-lives shorter than 30 hours. No
elements with atomic numbers above 82 (after lead) have stable
isotopes. This is because of the ever-increasing Coulomb repulsion of
protons, so that the strong nuclear force cannot hold the nucleus
together against spontaneous fission for long. Calculations suggest
that in the absence of other stabilizing factors, elements with more
than 104 protons should not exist. However, researchers in the 1960s
suggested that the closed nuclear shells around 114 protons and 184
neutrons should counteract this instability, creating an island of
stability in which nuclides could have half-lives reaching thousands
or millions of years. While scientists have still not reached the
island, the mere existence of the superheavy elements (including
oganesson) confirms that this stabilizing effect is real, and in
general the known superheavy nuclides become exponentially
longer-lived as they approach the predicted location of the island.
Oganesson is radioactive, decaying via alpha decay and spontaneous
fission, with a half-life that appears to be less than a millisecond.
Nonetheless, this is still longer than some predicted values.
Calculations using a quantum-tunneling model predict the existence of
several heavier isotopes of oganesson with alpha-decay half-lives
close to 1 ms.
Theoretical calculations done on the synthetic pathways for, and the
half-life of, other isotopes have shown that some could be slightly
more stable than the synthesized isotope 294Og, most likely 293Og,
295Og, 296Og, 297Og, 298Og, 300Og and 302Og (the last reaching the 'N'
= 184 shell closure). Of these, 297Og might provide the best chances
for obtaining longer-lived nuclei, and thus might become the focus of
future work with this element. Some isotopes with many more neutrons,
such as some located around 313Og, could also provide longer-lived
nuclei. The isotopes from 291Og to 295Og might be produced as
daughters of element 120 isotopes that can be reached in the reactions
249-251Cf+50Ti, 245Cm+48Ca, and 248Cm+48Ca.
In a quantum-tunneling model, the alpha decay half-life of was
predicted to be with the experimental Q-value published in 2004.
Calculation with theoretical Q-values from the macroscopic-microscopic
model of Muntian-Hofman-Patyk-Sobiczewski gives somewhat lower but
comparable results.
Calculated atomic and physical properties
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Oganesson is a member of group 18, the zero-valence elements. The
members of this group are usually inert to most common chemical
reactions (for example, combustion) because the outer valence shell is
completely filled with eight electrons. This produces a stable,
minimum energy configuration in which the outer electrons are tightly
bound. It is thought that similarly, oganesson has a closed outer
valence shell in which its valence electrons are arranged in a 7s27p6
configuration.
Consequently, some expect oganesson to have similar physical and
chemical properties to other members of its group, most closely
resembling the noble gas above it in the periodic table, radon.
Following the periodic trend, oganesson would be expected to be
slightly more reactive than radon. However, theoretical calculations
have shown that it could be significantly more reactive. In addition
to being far more reactive than radon, oganesson may be even more
reactive than the elements flerovium and copernicium, which are
heavier homologs of the more chemically active elements lead and
mercury, respectively. The reason for the possible enhancement of the
chemical activity of oganesson relative to radon is an energetic
destabilization and a radial expansion of the last occupied
7p-subshell. More precisely, considerable spin-orbit interactions
between the 7p electrons and the inert 7s electrons effectively lead
to a second valence shell closing at flerovium, and a significant
decrease in stabilization of the closed shell of oganesson. It has
also been calculated that oganesson, unlike the other noble gases,
binds an electron with release of energy, or in other words, it
exhibits positive electron affinity, due to the relativistically
stabilized 8s energy level and the destabilized 7p3/2 level, whereas
copernicium and flerovium are predicted to have no electron affinity.
Nevertheless, quantum electrodynamic corrections have been shown to be
quite significant in reducing this affinity by decreasing the binding
in the anion Og− by 9%, thus confirming the importance of these
corrections in superheavy elements. 2022 calculations expect the
electron affinity of oganesson to be 0.080(6) eV.
Monte Carlo simulations of oganesson's molecular dynamics predict it
has a melting point of and a boiling point of due to relativistic
effects (if these effects are ignored, oganesson would melt at ≈).
Thus oganesson would probably be a solid rather than a gas under
standard conditions, though still with a rather low melting point.
Oganesson is expected to have an extremely broad polarizability,
almost double that of radon. Because of its tremendous polarizability,
oganesson is expected to have an anomalously low first ionization
energy of about 860 kJ/mol, similar to that of cadmium and less than
those of iridium, platinum, and gold. This is significantly smaller
than the values predicted for darmstadtium, roentgenium, and
copernicium, although it is greater than that predicted for flerovium.
Its second ionization energy should be around 1560 kJ/mol. Even the
shell structure in the nucleus and electron cloud of oganesson is
strongly impacted by relativistic effects: the valence and core
electron subshells in oganesson are expected to be "smeared out" in a
homogeneous Fermi gas of electrons, unlike those of the "less
relativistic" radon and xenon (although there is some incipient
delocalisation in radon), due to the very strong spin-orbit splitting
of the 7p orbital in oganesson. A similar effect for nucleons,
particularly neutrons, is incipient in the closed-neutron-shell
nucleus 302Og and is strongly in force at the hypothetical superheavy
closed-shell nucleus 472164, with 164 protons and 308 neutrons.
Studies have also predicted that due to increasing electrostatic
forces, oganesson may have a semibubble structure in proton density,
having few protons at the center of its nucleus. Moreover, spin-orbit
effects may cause bulk oganesson to be a semiconductor, with a band
gap of eV predicted. All the lighter noble gases are insulators
instead: for example, the band gap of bulk radon is expected to be
eV.
Predicted compounds
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The only confirmed isotope of oganesson, 294Og, has much too short a
half-life to be chemically investigated experimentally. Therefore, no
compounds of oganesson have been synthesized yet. Nevertheless,
calculations on theoretical compounds have been performed since 1964.
It is expected that if the ionization energy of the element is high
enough, it will be difficult to oxidize and therefore, the most common
oxidation state would be 0 (as for the noble gases); nevertheless,
this appears not to be the case.
Calculations on the diatomic molecule showed a bonding interaction
roughly equivalent to that calculated for , and a dissociation energy
of 6 kJ/mol, roughly 4 times of that of . Most strikingly, it was
calculated to have a bond length shorter than in by 0.16 Å, which
would be indicative of a significant bonding interaction. On the other
hand, the compound OgH+ exhibits a dissociation energy (in other words
proton affinity of oganesson) that is smaller than that of RnH+.
The bonding between oganesson and hydrogen in OgH is predicted to be
very weak and can be regarded as a pure van der Waals interaction
rather than a true chemical bond. On the other hand, with highly
electronegative elements, oganesson seems to form more stable
compounds than for example copernicium or flerovium. The stable
oxidation states +2 and +4 have been predicted to exist in the
fluorides and . The +6 state would be less stable due to the strong
binding of the 7p1/2 subshell. This is a result of the same spin-orbit
interactions that make oganesson unusually reactive. For example, it
was shown that the reaction of oganesson with to form the compound
would release an energy of 106 kcal/mol of which about 46 kcal/mol
come from these interactions. For comparison, the spin-orbit
interaction for the similar molecule is about 10 kcal/mol out of a
formation energy of 49 kcal/mol. The same interaction stabilizes the
tetrahedral Td configuration for , as distinct from the square planar
D4h one of xenon tetrafluoride, which is also expected to have; this
is because OgF4 is expected to have two inert electron pairs (7s and
7p1/2). As such, OgF6 is expected to be unbound, continuing an
expected trend in the destabilisation of the +6 oxidation state (RnF6
is likewise expected to be much less stable than XeF6). The Og-F bond
will most probably be ionic rather than covalent, rendering the
oganesson fluorides non-volatile. OgF2 is predicted to be partially
ionic due to oganesson's high electropositivity. Oganesson is
predicted to be sufficiently electropositive to form an Og-Cl bond
with chlorine.
A compound of oganesson and tennessine, OgTs4, has been predicted to
be potentially stable chemically.
See also
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* Island of stability
* Superheavy element
* Transuranium element
* Extended periodic table
External links
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*
[
https://www.sciencenews.org/article/5-ways-heaviest-element-periodic-table-really-bizarre
5 ways the heaviest element on the periodic table is really bizarre],
ScienceNews.org
*
[
https://web.archive.org/web/20061129112314/https://flerovlab.jinr.ru/flnr/elm118.html
Element 118: Experiments on discovery], archive of discoverers'
official web page
* [
https://www.nytimes.com/2006/10/17/science/17heavy.html Element
118, Heaviest Ever, Reported for 1,000th of a Second], 'The New York
Times'.
* [
https://education.jlab.org/itselemental/ele118.html It's Elemental:
Oganesson]
* [
https://www.periodicvideos.com/videos/118.htm Oganesson] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
https://iupac.org/publications/pac/75/10/1601/ On the Claims for
Discovery of Elements 110, 111, 112, 114, 116, and 118 (IUPAC
Technical Report)]
* [
https://www.webelements.com/oganesson/ WebElements: Oganesson]
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Oganesson
