Ununoctium

Ununoctium is the temporary IUPAC name for the transactinide element with the atomic number 118 and temporary element symbol Uuo. It is also known as eka-radon or element 118, and 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 the elements discovered so far.

The radioactive ununoctium atom is very unstable, due to its high mass, and since 2005, only three or possibly four atoms of the isotope ²⁹⁴Uuo have been detected. Although this allowed very little experimental characterization of its properties and possible compounds, theoretical calculations have resulted in many predictions, including some unexpected ones. For example, although ununoctium is a member of group 18, it may possibly not be a noble gas, unlike all the other group 18 elements. It was formerly thought to be a gas under normal conditions but is now predicted to be a solid due to relativistic effects.

§History

§Unsuccessful synthesis attempts

In late 1998, Polish physicist Robert Smolańczuk published calculations on the fusion of atomic nuclei towards the synthesis of superheavy atoms, including ununoctium. His calculations suggested that it might be possible to make ununoctium 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 livermorium and ununoctium, in a paper published in Physical Review Letters, and very soon after the results were reported in Science. The researchers reported to have performed the reaction

86
36Kr + 208
82Pb â†' 293
118Uuo + n.

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 reports

The first decay of atoms of ununoctium was observed at the Joint Institute for Nuclear Research (JINR) by Yuri Oganessian and his group in Dubna, Russia, in 2002. On October 9, 2006, researchers from JINR and Lawrence Livermore National Laboratory of California, US, 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.

249
98Cf + 48
20Ca â†' 294
118Uuo + 3 n.

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 ~0.3â€"0.6 pb or (3â€"6)×10^âˆ'41 m²) the experiment took four months and involved a beam dose of 4×10¹⁹ 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 100000.

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: 294
Uuo decays into 290
Lv by alpha decay. Since there were only three nuclei, the half-life derived from observed lifetimes has a large uncertainty: 0.89+1.07
âˆ'0.31 ms.

294
118Uuo â†' 290
116Lv + 4
2He

The identification of the 294
Uuo nuclei was verified by separately creating the putative daughter nucleus 290
Lv directly by means of a bombardment of 245
Cm with 48
Ca ions,

245
96Cm + 48
20Ca â†' 290
116Lv + 3 n,

and checking that the 290
Lv decay matched the decay chain of the 294
Uuo nuclei. The daughter nucleus 290
Lv is very unstable, decaying with a lifetime of 14 milliseconds into 286
Fl, which may experience either spontaneous fission or alpha decay into 282
Cn, which will undergo spontaneous fission.

In a quantum-tunneling model, the alpha decay half-life of 294
Uuo was predicted to be 0.66+0.23
âˆ'0.18 ms with the experimental Q-value published in 2004. Calculation with theoretical Q-values from the macroscopic-microscopic model of Muntianâ€"Hofmanâ€"Patykâ€"Sobiczewski gives somewhat low but comparable results.

§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 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 Flerov Laboratory of Nuclear Reactions at JINR was the only facility in the world which could achieve this result. These names were later proposed for element 114 (flerovium) and element 116 (moscovium). However, the final name proposed for element 116 was instead livermorium.

No name has yet been officially suggested for the element as no claims for discovery have yet been accepted by the IUPAC. According to current guidelines from IUPAC, the ultimate name for all new elements should end in "-ium", which means the name for ununoctium will almost certainly end in "-ium", not "-on", even if ununoctium turns out to be a noble gas, which traditionally have names ending in "-on" (with the exception of helium, which was not known to be a noble gas when it was discovered).

§Characteristics

§Nuclear stability and isotopes

The stability of nuclei decreases greatly with the increase in atomic number after plutonium, the heaviest primordial element, so that all isotopes with an atomic number above 101 decay radioactively with a half-life under a day, with an exception of dubnium-268. No elements with atomic numbers above 82 (after lead) have stable isotopes. Nevertheless, because of reasons not very well understood yet, there is a slightly increased nuclear stability around atomic numbers 110â€"114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by University of California professor Glenn Seaborg, explains why superheavy elements last longer than predicted. Ununoctium is radioactive and has a 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 ununoctium 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 ²⁹⁴Uuo, most likely ²⁹³Uuo, ²⁹⁵Uuo, ²⁹⁶Uuo, ²⁹⁷Uuo, ²⁹⁸Uuo, ³⁰⁰Uuo and ³⁰²Uuo. Of these, ²⁹⁷Uuo 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 ³¹³Uuo 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 7s²7p⁶ 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 probably cannot be considered a noble gas. In addition to being far more reactive than radon, ununoctium may be even more reactive than elements flerovium and copernicium. The reason for the apparent enhancement of the chemical activity of ununoctium relative to radon is an energetic destabilization and a radial expansion of the last occupied 7p-subshell. More precisely, considerable spinâ€"orbit interactions between the 7p electrons with the inert 7s² electrons, effectively lead to a second valence shell closing at flerovium, 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 energyâ€"or in other words, it exhibits positive electron affinity.

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 ununoctium 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 gaseous substances 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 flerovium) and a standard state condensed phase.

§Predicted compounds

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); nevertheless, this appears not to be the case.

Calculations on the diatomic molecule Uuo
2 showed a bonding interaction roughly equivalent to that calculated for Hg
2, and a dissociation energy of 6 kJ/mol, roughly 4 times of that of Rn
2. But most strikingly, it was calculated to have a bond length shorter than in Rn
2 by 0.16 Ã…, 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 predicted to be 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 copernicium or flerovium. The stable oxidation states +2 and +4 have been predicted to exist in the fluorides UuoF
2 and UuoF
4. The +6 state would be less stable due to the strong binding of the 7p_1/2 subshell. This is a result of the same spin-orbit interactions that make ununoctium unusually reactive. For example, it was shown that the reaction of ununoctium with F
2 to form the compound UuoF
2 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 RnF
2 is about 10 kcal/mol out of a formation energy of 49 kcal/mol. The same interaction stabilizes the tetrahedral T_d configuration for UuoF
4, as distinct from the square planar D_4h one of XeF
4, which RnF
4 is also expected to have. The Uuoâ€"F bond will most probably be ionic rather than covalent, rendering the UuoF_n compounds non-volatile. UuoF₂ is predicted to be partially ionic due to ununoctium's high electropositivity. Unlike the other noble gases (except possibly xenon), ununoctium was predicted to be sufficiently electropositive to form a Uuoâ€"Cl bond with chlorine.

§See also

• Transactinide element
• Transuranic element

§Notes

§References

§Further reading

• Eric Scerri, The Periodic Table, Its Story and Its Significance, Oxford University Press, New York, 2007.

§External links

• Element 118: experiments on discovery, archive of discoverers' official web page
• Chemistry Blog: Independent analysis of 118 claim
• It's Elemental: Ununoctium
• Ununoctium at The Periodic Table of Videos (University of Nottingham)
• 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.
• WebElements: Ununoctium

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