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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[1]
Standard atomic weight	(predicted) (314)[2] g·mol−1
Electron configuration	perhaps [Rn] 7s2 5f14 6d10 7p6[3]
Electrons per shell	2, 8, 18, 32, 32, 18, 8 [3]
Phase	expected to be a gas[1]
Density (near r.t.)	(predicted) (13.65)[2] g·cm−3
Melting point	(243)[2] (predicted) K ((-30.3) °C,  °F)
Boiling point	(250)[2] (predicted) K ((-22.6) °C,  °F)
Oxidation states	0 [4]
Electronegativity	no data (Pauling scale)
Ionization energies	1st: (predicted) (963)[2] kJ/mol
Atomic radius (calc.)	152[2] pm
Thermal conductivity	(300 K) (0.001807)[2] W·m−1·K−1
CAS registry number	54144-19-3[1]
Selected isotopes
Main article: Isotopes of ununoctium iso NA half-life DM DE (MeV) DP 294Uuo [5] syn ~0.89 ms α 11.65 ± 0.06 290Uuh
References

Ununoctium (pronounced /ˌjuːnəˈnɒktiəm/ ^[6][7]) is the temporary IUPAC name for the transactinide element having atomic number of 118. It can also be referred to as eka-radon and it is currently the only synthetic member of the noble gases group, and the highest atomic number assigned to a reputedly discovered element. Its temporary IUPAC element symbol is Uuo. Only three atoms of this element have been detected.^[8] The name ununoctium is a systematic element name, used as a placeholder until it will be confirmed by other research groups and the IUPAC decides on a name. Moskowium (Mk) has been proposed by their Russian discoverers as the permanent name for the element.^[9][10]

In late 1998, the Polish theoreticist Robert Smolanczuk published some calculations on the fusion of atomic nuclei towards the synthesis of superheavy atoms, including the element 118.^[11] His calculations suggested that it might be possible to make element 118 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 elements 116 and 118, 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:

The following year, they published a retraction after other researchers were unable to duplicate the results.^[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] The American group had intended to name it ghiorsium after Albert Ghiorso (a member of the research team) before having to retract their claim.^[17]

On October 16, 2006, researchers working at the Joint Institute for Nuclear Research (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^[18] and two more in 2005) produced via collisions of californium-249 atoms and calcium-48 ions:^{[19][20][21][22][23]}

Radioactive decay pathway of isotope Uuo-294.^[5] The decay energy and average half-life is given for the parent isotope and each daughter isotopes. For each step, 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 of 4×10¹⁹ calcium ions that had to be shot at the californium atom to produce the single recorded event believed to be the synthesis of the element 118.^[1] Nevertheless, researchers are highly confident that the result is not a false positive, since the chance that the detection was a random event was estimated to be less than one part in 100,000.^[24]

The research team consisted of workers from JINR and the Lawrence Livermore National Laboratory in California, USA, but the decay products of three atoms of ununoctium (not the atoms themselves), were observed in Dubna. A half-life of 0.89 ms was observed: ²⁹⁴Uuo decays into ²⁹⁰Uuh by alpha decay. Since there were only three nuclei, the half-life was derived from observed lifetimes and has a large uncertainty: 0.89_-0.31^+1.07 ms.^[5]

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,

Element 118 comes right at the end of the island of stability.

and checking that the ²⁹⁰Uuh decay matched the decay chain of the ²⁹⁴Uuo nuclei.^[5] The daughter nucleus ²⁹⁰Uuh is very unstable, decaying with a half-life of 14 milliseconds into ²⁸⁶Uuq, which may undergo spontaneous fission or alpha decay into ²⁸²Uub, which will undergo spontaneous fission.^[5]

Although the half-life seems to be less than a milisecond, it is still longer than predicted, thus giving further support to the idea of “island of stability.”^[25] This concept, proposed by late UC Berkeley professor Glenn Seaborg, explains why superheavy elements such as element 118 last longer than predicted.^[26]

Ununoctium, like all noble gases except helium, has a "full" (eight electron) valence (outermost) electron shell (also called closed shell), thus making it a highly unreactive element

Ununoctium has all its electrons in a closed shell which makes it a member of the noble gas group. Therefore it is most likely that it will have similar properties as other members of the group, resembling in its chemical properties the noble gas above it in the periodic table, radon.^[27] There are no known compounds of ununoctium, and because it is likely to have a high ionization energy, the most common oxidation state will likely be 0.^[4] It is believed to be a gas under normal conditions,^[2] thus making it among the gaseous substances with highest molecular masses (in fact only UF₆ with a molecular mass of 352 would surpass it).

Since only three atoms of ununoctium have ever been produced, it currently has no uses outside of basic scientific research. Also, because it does not occur at all in the biosphere it never presents a risk, but it would constitute a radiation hazard if enough was ever assembled in one place.^[28]

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Controversy-Plagued Element 118, the Heaviest Atom Yet, Finally Discovered

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Ununoctium

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