Ununoctium

Ununoctium (pronounced /uːnuːnˈɒktiəm/ 9] oon-oon-OK-tee-əm), also known as eka-radon or element 118, is the temporary IUPAC name[10] 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 294Uuo have been detected.[11] While this allowed for 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 is probably not a noble gas, as are all the other Group 18 elements.[1] It was formerly thought to be a gas but is now predicted to be a solid under normal conditions.[1]

History
See also: Discoveries of the chemical elements

Unsuccessful attempts

In late 1998, Polish physicist Robert Smolańczuk published calculations on the fusion of atomic nuclei towards the synthesis of superheavy atoms, including ununoctium.[12] His calculations suggested that it might be possible to make ununoctium by fusing lead with krypton under carefully controlled conditions.[12]

In 1999, researchers at Lawrence Berkeley National Laboratory made use of these predictions and announced the discovery of ununhexium and ununoctium, in a paper published in Physical Review Letters,[13] and very soon after the results were reported in Science.[14] The researchers claimed to have performed the reaction

⁸⁶₃₆Kr + ²⁰⁸₈₂Pb → ²⁹³₁₁₈Uuo + 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.[15] 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.[16]

Discovery

First decay of atoms of ununoctium was observed at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, in 2002.[17] On October 9, 2006, researchers from JINR and Lawrence Livermore National Laboratory of California, USA, working at the JINR in Dubna, announced[8] that they had indirectly detected a total of three (possibly four) nuclei of ununoctium-294 (one or two in 2002[18] and two more in 2005) produced via collisions of californium-249 atoms and calcium-48 ions:[19][20][21][22][23]

²⁴⁹98Cf + ⁴⁸20Ca → ²⁹⁴118Uuo + ³ n.

Schematic diagram of Ununoctium-294 alpha decay, with a half-life of 0.89 ms and a decay energy of 11.65 MeV. The resulting ununhexium-290 decays by alpha decay, with a half-life of 10.0 ms and a decay energy of 10.80 MeV, to ununquadium-286. Ununquadium-286 in turn has a half-life of 0.16 s and a decay energy of 10.16 MeV, and undergoes alpha decay to copernicium-282 with a 0.7 rate of spontaneous fission. Copernicium itself has a half-life of only 1.9 ms and has a 1.0 rate of spontaneous fission.
Radioactive decay pathway of isotope Uuo-294.[8] The decay energy and average half-life is given for the parent isotope and each daughter isotope. The fraction of atoms undergoing spontaneous fission (SF) is given in green.

Because of the very small fusion reaction probability (the fusion cross section is ~0.3–0.6 pb = (3–6)×10−41 m2) the experiment took 4 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.[7] 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.[24]

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: 294Uuo decays into 290Uuh by alpha decay. Since there were only three nuclei, the half-life derived from observed lifetimes has a large uncertainty: 0.89+1.07−0.31 ms.[8]

²⁹⁴118Uuo → ²⁹⁰116Uuh + ⁴He

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

²⁴⁵96Cm + ⁴⁸20Ca → ²⁹⁰116Uuh + ³n,

and checking that the 290Uuh decay matched the decay chain of the 294Uuo nuclei.[8] The daughter nucleus 290Uuh is very unstable, decaying with a half-life of 14 milliseconds into 286Uuq. The latter may experience either spontaneous fission or alpha decay into 282Cn, which will undergo spontaneous fission.[8]

In a quantum-tunneling model, the alpha decay half-life of 294Uuo was predicted to be 0.66+0.23−0.18 ms[25] with the experimental Q-value published in 2004.[26] Calculation with theoretical Q-values from the macroscopic-microscopic model of Muntian–Hofman–Patyk–Sobiczewski gives somewhat low but comparable results.[27]

Following the success in obtaining ununoctium, the discoverers have started similar experiments in the hope of creating unbinilium from 58Fe and 244Pu.[28] Isotopes of unbinilium are predicted to have alpha decay half lives of the order of micro-seconds.[29][30]

Target-Projectile combinations leading to Z=118 compound nuclei
Target Projectile CN Attempt result
208Pb 86Kr 294Uuo Failure to date
249Cf 48Ca 297Uuo Successful reaction

Naming

Until the 1960s ununoctium was known as eka-emanation (emanation is the old name for radon).[31] In 1979 the IUPAC published recommendations according to which the element was to be called ununoctium,[9] 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).[32]

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.[33] 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.[34][35]

Characteristics

Nucleus stability and isotopes
A 3D graph of stability of elements vs. number of protons Z and neutrons N, showing a "mountain chain" running diagonally through the graph from the low to high numbers, as well as an "island of stability" at high N and Z.
Ununoctium comes right at the end of the "island of stability" and thus its nuclei are slightly more stable than otherwise predicted.
Main article: Isotopes of ununoctium
See also: Island of stability

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 radioactively with a half-life under a day. Nevertheless, because of reasons not very well understood yet, there is a slight increased nuclear stability around atomic numbers 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.[36] Ununoctium is radioactive and has half-life that appears to be less than a millisecond. Nonetheless, this is still longer than some predicted values,[25][37] thus giving further support to the idea of this "island of stability".[38]

Calculations using a quantum-tunneling model predict the existence of several neutron-rich isotopes of ununuoctium with alpha-decay half-lives close to 1 ms.[29][30]

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.[25][39] Of these, 297Uuo might provide the best chances for obtaining longer-lived nuclei,[25][39] 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.[40]

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.[41] It is thought that similarly, ununoctium has a closed outer valence shell in which its valence electrons are arranged in a 7s2, 7p6 configuration.[1]

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.[42] 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.[5] In addition to being far more reactive than radon, ununoctium may be even more reactive than elements ununquadium and copernicium.[1] 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.[1][43] More precisely, considerable spin-orbit interactions between the 7p electrons with the inert 7s2 electrons, effectively lead to a second valence shell closing at ununquadium, and a significant decrease in stabilization of the closed shell of element 118.[1] 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.[44][45][46]

Ununoctium is expected to have by far the broadest polarizability of all elements before it in the periodic table, and almost twofold of radon.[1] By extrapolating from the other noble gases, it is expected that ununoctium has a boiling point between 320 and 380 K.[1] This is very different from the previously estimated values of 263 K[6] or 247 K.[47] Even given the large uncertainties of the calculations, it seems highly unlikely that ununoctium would be a gas under standard conditions.[1][48] 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[49] and significantly smaller than that of ununquadium[50]) and a standard state condensed phase.[1]

Predicted compounds
Skeletal model of a planar molecule with a central atom symmetrically bonded to four peripheral (fluorine) atoms.
XeF4 and RnF4 have a square planar configuration.
Skeletal model of a terahedral molecule with a central atom (Uuo) symmetrically bonded to four peripheral (fluorine) atoms.
UuoF4 is predicted to have a tetrahedral configuration.

No compounds of ununoctium have been synthesized yet, but calculations on theoretical compounds have been performed since 1964.[31] 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).[4]

Calculations on the dimeric molecule Uuo2 showed a bonding interaction roughly equivalent to that calculated for Hg2, and a dissociation energy of 6 kJ/mol, roughly 4 times of that of Rn2.[1] But most strikingly, it was calculated to have a bond length shorter than in Rn2 by 0.16 Å, which would be indicative of a significant bonding interaction.[1] 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+.[1]

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.[49] On the other hand, with highly electronegative elements, ununoctium seems to form more stable compounds than for example element 112 or element 114.[49] The stable oxidation states +2 and +4 have been predicted to exist in the fluorinated compounds UuoF2 and UuoF4.[51] 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 F2 to form the compound UuoF2, would release an energy of 106 kcal/mol of which about 46 kcal/mol come from these interactions.[49] For comparison, the spin-orbit interaction for the similar molecule RnF2 is about 10 kcal/mol out of a formation energy of 49 kcal/mol.[49] The same interaction stabilizes the tetrahedral Td configuration for UuoF4, as distinct from the square planar D4h one of XeF4 and RnF4.[51] The Uuo–F bond will most probably be ionic rather than covalent, rendering the UuoFn compounds non-volatile.[5][52] Unlike the other noble gases, ununoctium was predicted to be sufficiently electropositive to form a Uuo–Cl bond with chlorine.[5]

Since no more than four atoms of ununoctium have 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.[53]

See also

* Transactinide element
* Transuranic element
* Ununhexium

References

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46. ^ 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
47. ^ Takahashi, N. (2002). "Boiling points of the superheavy elements 117 and 118". Journal of Radioanalytical and Nuclear Chemistry 251 (2): 299–301. doi:10.1023/A:1014880730282.
48. ^ It is debatable if the name of the group 'noble gases' will be changed if ununoctium is shown to be non-volatile.
49. ^ a b c d e Han, Young-Kyu; Bae, Cheolbeom; Son, Sang-Kil; Lee, Yoon Sup (2000). "Spin–orbit effects on the transactinide p-block element monohydrides MH (M=element 113–118)". Journal of Chemical Physics 112 (6): 2684. doi:10.1063/1.480842.
50. ^ Nash, Clinton S.; Bursten, Bruce E. (1999). "Spin-Orbit Effects, VSEPR Theory, and the Electronic Structures of Heavy and Superheavy Group IVA Hydrides and Group VIIIA Tetrafluorides. A Partial Role Reversal for Elements 114 and 118". Journal of Physical Chemistry A 1999 (3): 402–410. doi:10.1021/jp982735k.
51. ^ a b Han, Young-Kyu; Lee, Yoon Sup (1999). "Structures of RgFn (Rg = Xe, Rn, and Element 118. n = 2, 4.) Calculated by Two-component Spin-Orbit Methods. A Spin-Orbit Induced Isomer of (118)F4". Journal of Physical Chemistry A 103 (8): 1104–1108. doi:10.1021/jp983665k.
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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.
* Eric Scerri, The Periodic Table, Its Story and Its Significance, Oxford University Press, New York, 2007.