Ununoctium

2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements

118 ununseptium ← Ununoctium → ununennium Rn ↑ Uuo ↓ (uho) Periodic table - Extended periodic table
General
Name, symbol, number	Ununoctium, Uuo, 118
Chemical series	noble gases
Group, period, block	18,  7, p
Appearance	Unknown, probably colorless
Standard atomic weight	(294) g·mol−1
Electron configuration	[Rn] 7s2 5f14 6d10 7p6
Electrons per shell	2, 8, 18, 32, 32, 18, 8
Physical properties
Phase	liquid (or solid – predicted)
Density (near r.t.)	(predicted) 13.65 g·cm−3
Boiling point	(extrapolated) 320–380 K (50–110 ° C, 120–220 ° F)
Critical point	(extrapolated) 439 K, 6.8 MPa
Heat of fusion	(extrapolated) 23.5 kJ·mol−1
Heat of vaporization	(extrapolated) 19.4 kJ·mol−1
Oxidation states	0, +2, +4
Ionization energies	1st: (calc.) 820–1130 kJ·mol−1
	2nd: (extrapolated) 1450 kJ·mol−1
Atomic radius	(predicted) 152 pm
Covalent radius	(extrapolated) 230 pm
Miscellaneous
CAS registry number	54144-19-3
Selected isotopes
Main article: Isotopes of Ununoctium iso NA half-life DM DE ( MeV) DP 294Uuo syn ~0.89 ms α 11.65 ± 0.06 290Uuh
References

Ununoctium (pronounced /ˌjuːnəˈnɒktiəm/ or /ˌʌnəˈnɒktiəm/), 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 and has the highest atomic number and highest atomic mass assigned to a discovered element.

The radioactive ununoctium atom is very unstable, and since 2002, only three atoms of the isotope ²⁹⁴Uuo have been detected. While this allowed for very little experimental characterization of its properties and its compounds, theoretical calculations have allowed for many predictions, including some very unexpected ones. For example, although ununoctium is a member of the noble gas group, it could have a higher chemical reactivity than some elements outside this group. Furthermore, it is predicted that it might not even be a gas under normal conditions.

History

Unsuccessful attempts

In late 1998, the Polish physicist Robert Smolanczuk published some calculations on the fusion of atomic nuclei towards the synthesis of superheavy atoms, including the 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

8636Kr + 20882Pb → 293118Uuo + n

The following year, they published a retraction after other researchers were unable to duplicate the results. 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

On October 9, 2006, researchers from Joint Institute for Nuclear Research (JINR) and Lawrence Livermore National Laboratory of California, USA, working at the JINR in Dubna, Russia, announced in Physical Review C that they had indirectly detected a total of three nuclei of ununoctium-294 (one in 2002 and two more in 2005) produced via collisions of californium-249 atoms and calcium-48 ions:

24998Cf + 4820Ca → 294118Uuo + 3 n

Radioactive decay pathway of isotope Uuo-294. The decay energy and average halflife is given for the parent isotope and each daughter isotopes. 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 0.5 pb = 5×10⁻⁴¹ 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. 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 decay of three atoms of ununoctium was observed. A halflife of 0.89 ms was calculated: ²⁹⁴Uuo decays into ²⁹⁰Uuh by alpha decay. Since there were only three nuclei, the halflife derived from observed lifetimes has a large uncertainty: 0.89^+1.07_−0.31 ms.

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 + 3 n

and checking that the ²⁹⁰Uuh decay matched the decay chain of the ²⁹⁴Uuo nuclei. The daughter nucleus ²⁹⁰Uuh is very unstable, decaying with a halflife of 14 milliseconds into ²⁸⁶Uuq, which may undergo spontaneous fission or alpha decay into ²⁸²Uub, which will undergo spontaneous fission.

In a quantum tunneling model, the alpha decay half-life of ²⁹⁴118 was predicted to be 0.66(+0.23,-0.18)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 ⁵⁸Fe and ²⁴⁴Pu. Isotopes of the element 120 are predicted to have alpha decay half lives of the order of micro-seconds.

Naming

Element 118 is still called eka-radon, but before the 1960s it was also known as eka-emanation (for the old name for radon). Only in 1979 the IUPAC published recommendations according to which the element started to be called ununoctium. The name ununoctium is a systematic element name, used as a placeholder until it is confirmed by other research groups 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). Several years later, when the Russian discoverers reported their synthesis in 2006, rumors appeared that they were planning on calling it after the place of the discovery, dubnadium (Dn) (very similar to the name of the 105^th element, dubnium (Db)). Nevertheless, during an interview with a Russian newspaper, the head of the Russian institute stated the team were considering two names for the new element, Flyorium in honour of Georgy Flyorov, the founder of the research institute; and moskovium (also spelled moscovium or even moscowium), in recognition of the Moscow Oblast where Dubna lies. 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 honour of Russia since the Flerov Laboratory of Nuclear Reactions at JINR is the only facility in the world which can achieve this result.

Characteristics

Nucleus stability and isotopes

Element 118 comes right at the end of the "island of stability" and thus its nuclei are slightly more stable than predicted.

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 halflife under a day. Nevertheless, due to 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"..

Quantum tunneling model predicts existence of several neutron-rich isotopes of the element 118 with alpha decay half lives close to ms.

Theoretical calculations done on the synthetic pathways for, and the halflife 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.

Properties

Ununoctium is a member of the zero-valence elements that are called noble or inert gases. Consequently, it is expected that ununoctium will 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. The members of this group are inert to most common chemical reactions (such as combustion, for example) 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 7s², 7p⁶ configuration.

The expected electron shell diagram for ununoctium. Note the eight electrons in the outer shell.

Following the periodic trend, it would be expected for ununoctium to be slightly more reactive than radon; but theoretical calculations have shown that it could be quite reactive for its "noble" labeling. 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. More precisely, considerable spin-orbit interactions between the 7p electrons with the inert 7s² 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.

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 this to 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. Because of its tremendous polarizability, ununoctium is expected to have an anomalously low ionization potential (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. Nevertheless, even if ununoctium is a monatomic gas under normal conditions, it would be one of the gaseous substances with the highest molecular masses—in fact, only UF₆ with a molecular mass of 352 would surpass it.

Compounds and uses

XeF₄ and RnF₄ have a square planar configuration

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 as early as 1964. It is expected that if the ionization energy of the element will be high enough, it will be very hard to oxidize and therefore, the most common oxidation state to be 0 (as for other noble gases).

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. But most strikingly, it was calculated to have a bond length shorter than in Rn₂ by .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 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, 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 UuoF₂ and UuoF₄. 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. 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. The same interaction stabilizes the tetrahedral T_d configuration for UuoF₄, as opposed to the square planar D_4h one of XeF₄ and RnF₄. 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 only three 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.