Ununoctium ( ), also known as
eka-radon or
element 118, is the temporary
IUPAC
name for the
transactinide element having the
atomic number 118 and temporary
element symbol Uuo.
On the
periodic table of
the elements, it is a
p-block element and
the last one of the
7th period. Ununoctium
is currently the only
synthetic
member of
Group 18. It has the
highest atomic number and highest
atomic
mass of all discovered elements.
The
radioactive ununoctium atom is
very unstable, and since 2002, only three atoms (possibly four) of
the isotope have been detected. While this allowed for very little
experimental characterization of its properties and possible
compounds, theoretical
calculations have allowed for many predictions, including some very
unexpected ones. For example, although ununoctium is a member of
Group 18, it is probably not a
noble gas,
as are all the other Group 18 elements. It was formerly thought to
be a gas but is now predicted to be a
solid
under
normal
conditions.
History
Unsuccessful attempts
In late 1998, Polish physicist Robert Smolańczuk published
calculations on the fusion of atomic nuclei towards the synthesis
of
superheavy atoms, including
element 118. His calculations suggested that it might be possible
to make element 118 by fusing
lead with
krypton under carefully controlled
conditions.
In 1999,
researchers at Lawrence Berkeley National
Laboratory
made use of these predictions and announced the
discovery of elements 116 and 118, in a
paper published in Physical
Review Letters, and very soon after the results were
reported in Science. The researchers
claimed to have performed the
reaction
- + → + .
The following year, they published a retraction after researchers
at other laboratories were unable to duplicate the results and the
Berkeley lab itself was unable to duplicate them as well. 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.
Discovery
First
decay of atoms of ununoctium was observed at the Joint Institute
for Nuclear Research
(JINR) in Dubna
, Russia, in
2002. On October 9, 2006, researchers from JINR and
Lawrence Livermore National
Laboratory
of California, USA, working at the JINR in Dubna
, announced
that they had indirectly detected a total of three (possibly four)
nuclei of ununoctium-294 (one or two in 2002 and two more in 2005)
produced via collisions of californium-249 atoms and calcium-48 ions:
- + → + 3 .
Because of the very small
fusion
reaction probability (the fusion
cross section is ~0.3–0.6
pb = (3–6)×10
−41 m
2)
the experiment took 4 months and involved a beam dose of
4×10
19 calcium ions that had to
be shot at the
californium target to
produce the first recorded event believed to be the synthesis of
ununoctium. Nevertheless, researchers are highly confident that the
results are not a
false positive,
since the chance that the detections were random events was
estimated to be less than one part in 100,000.
In the experiments, the alpha-decay of three atoms of ununoctium
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: 0.89 ms.
- → +
The identification of the nuclei was verified by separately
creating the putative
daughter nucleus
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 half-life of 14 milliseconds into
. The latter may experience either
spontaneous fission or alpha decay into
, which will undergo spontaneous fission.
In a quantum-tunneling model, the alpha decay half-life of
294118 was predicted to be 0.66 ms 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 low but comparable
results.
Following the success in obtaining ununoctium, the discoverers have
started similar experiments in the hope of creating
element 120 from and .Isotopes of the element
120 are predicted to have alpha decay half lives of the order of
micro-seconds.
Naming
Until the 1960s ununoctium was known as
eka-emanation
(emanation is the old name for radon). In 1979 the
IUPAC published recommendations according to which the
element was to be called
ununoctium, a
systematic element name, as a
placeholder until the discovery of
the element is confirmed and the IUPAC decides on a name.
Before the retraction in 2002, 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.
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 Moskovskaya
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 Flerov Laboratory of Nuclear
Reactions at JINR was the only facility in the world which could
achieve this result.
Characteristics
Nucleus stability and isotopes
There are no elements with an
atomic
number above 82 (after
lead) that have
stable isotopes. The stability of nuclei decreases with the
increase in atomic number, such that all isotopes with an atomic
number above
101 decay radioactive with a
half-life under a day. Nevertheless, because of
reasons not very well
understood yet, there is a slight increased nuclear stability
around elements 110–114, which leads to the appearance of what is
known in nuclear physics as the "
island of stability".
This concept, proposed
by UC
Berkeley
professor
Glenn Seaborg, explains why superheavy elements last longer than
predicted. Ununoctium is
radioactive and has
half-life that appears to be less than a
millisecond. Nonetheless, this is still longer
than some predicted values, thus giving further support to the idea
of this "island of stability".
Calculations using a quantum-tunneling model predict the existence
of several neutron-rich isotopes of element 118 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 , most likely , , , , , and . Of these, 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 ,
could also provide longer-lived nuclei.
Calculated atomic and physical properties
Ununoctium 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, ununoctium has a
closed outer valence shell in which its
valence electrons are arranged in a
7s
2, 7p
6 configuration.
Consequently, some expect ununoctium 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, ununoctium would be expected to be slightly more
reactive than radon. However, theoretical calculations have shown
that it could be quite reactive, so that it can probably not be
considered a noble gas. In addition to being far more reactive than
radon, ununoctium may be even more reactive than elements
114 and
112. The
reason for the apparent enhancement of the chemical activity of
element 118 relative to radon is an energetic destabilization and a
radial expansion of the last occupied 7p
subshell.the actual quote is:
"The reason for
the apparent enhancement of chemical activity of element 118
relative to radon is the energetic destabilization and radial
expansion of its occupied 7p3/2 spinor shell" More precisely, considerable
spin-orbit interactions
between the 7p electrons with the inert 7s
2 electrons,
effectively lead to a second valence shell closing at
element 114, and a significant decrease in
stabilization of the closed shell of element 118. It has also been
calculated that ununoctium, unlike other noble gases, binds an
electron with release of energy—or in other words, it exhibits
positive
electron
affinity.Nevertheless,
quantum electrodynamic corrections
have been shown to be quite significant in reducing this affinity
(by decreasing the binding in the
anion
Uuo
− by 9%) thus confirming the importance of these
corrections in
superheavy atoms.
See Pyykko
Ununoctium is expected to have by far the broadest
polarizability of all elements before it in
the periodic table, and almost twofold of radon. By extrapolating
from the other noble gases, it is expected that ununoctium has a
boiling point between 320 and 380 K. This is very different from
the previously estimated values of 263 K or 247 K. Even given
the large uncertainties of the calculations, it seems highly
unlikely that element 118 would be a gas under
standard conditions. And as the liquid
range of the other gases is very narrow, between 2 and 9 kelvins,
this element should be
solid at standard
conditions. If ununoctium forms a
gas under
standard conditions nevertheless, it would be one of the densest
substances gaseous at standard conditions (even if it is
monatomic like the other noble gases).
Because of its tremendous polarizability, ununoctium is expected to
have an anomalously low
ionization
energy (similar to that of
lead which is
70% of that of radon and significantly smaller than that of element
114) and a standard state
condensed
phase.
Predicted compounds
and have a square planar configuration.
No compounds of ununoctium have been synthesized yet, but
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 will be 0 (as for other
noble gases).
Calculations on the
dimeric 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 . But 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
UuoH
+ exhibits a dissociation energy (in other words
proton affinity of Uuo) that is
smaller than that of RnH
+.
The bonding between ununoctium and
hydrogen
in UuoH is very limp 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, ununoctium seems
to form more stable compounds than for example
element 112 or
element
114. The stable oxidation states +2 and +4 have been predicted
to exist in the
fluorinated compounds and .
This is a result of the same spin-orbit interactions that make
ununoctium unusually reactive. For example, it was shown that the
reaction of Uuo 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
and . The Uuo–F bond will most probably
be
ionic rather than
covalent, rendering the UuoF
n
compounds non-volatile. Unlike the other noble gases, ununoctium
was predicted to be sufficiently
electropositive to form a Uuo–Cl bond with
chlorine.
Since no more than four atoms of ununoctium have ever been
produced, it currently has no uses outside of basic scientific
research. It would constitute a
radiation hazard if enough were ever
assembled in one place.
See also
References
- It is debatable if the name of the group 'noble gases' will be
changed if ununoctium is shown to be non-volatile.
External links
- Element 118: experiments on discovery, archive
of discoverers' official web page
- Chemistry Blog: Independent analysis of 118
claim
- WebElements: Ununoctium
- It's Elemental: Ununoctium
- On the Claims for Discovery of Elements 110, 111, 112,
114, 116, and 118 (IUPAC Technical Report)
- " Element 118, Heaviest Ever, Reported for 1,000th of
a Second", NYTimes.com.