Ununoctium (118)

The essentials

Experiments conducted at Dubna in Russia at the Flerov Laboratory of Nuclear Reactions (by workers from the Joint Institute for Nuclear Research in Russia and the Lawrence Livermore National Laboratory in the USA) indicate that element 118 (ununoctium, Uuo) was produced. Not too much though, one atom in the spring of 2002 and two more in 2005.

Name: Ununoctium Symbol: Uuo Atomic number: 118 Atomic weight: [ 294 ] Standard state: presumably a gas at 298 K CAS Registry ID: 54144-19-3

Group in periodic table: 18 Group name: Noble gas Period in periodic table: 7 Block in periodic table: p-block Color: unknown, but probably a colorless gas Classification: Non-metallic

Historical information

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.

Radioactive decay pathway of the isotope ununoctium-294. 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.

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 ~0.3–0.6 pb or (3–6)×10⁻⁴¹ 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
−0.31 ms.

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

Physical properties

• Boiling point: ~350 K (extrapolated)
• Density of solid: 4.9 to 5.1 (extrapolated) g cm^-3

Orbital properties

• Ground state electron configuration:  [Rn].5f¹⁴.6d¹⁰.7s².7p⁶ (a guess based upon that of radon)
• Shell structure:  2.8.18.32.32.18.8
• Term symbol:   ¹S₀ (a guess based upon guessed electronic structure)

Isolation

Experiments conducted at Dubna in Russia at the Flerov Laboratory of Nuclear Reactions (by workers from the Joint Institute for Nuclear Research in Russia and the Lawrence Livermore National Laboratory in the USA) indicate that element 118 (ununoctium, Uuo) was produced. Not too much though, one atom in the spring of 2002 and two more in 2005.

The 2002 experiment involved firing a beam of ⁴⁸₂₀Ca at ²⁴⁹₉₈Cf. The experiment took 4 months and involved a beam of 2.5 x 10¹⁹ calcium ions to produce the single event believed to be the synthesis of element 118 (ununoctium) as the ²⁹⁴₁₁₈Uuo isotope. Three neutrons are released during this process.

²⁴⁹₉₈Cf + ⁴⁸₂₀Ca → ²⁹⁴₁₁₈Uuo + 3¹n

This ununoctium isotope then loses three alpha particles in rapid succesion:

²⁹⁴₁₁₈Uuo → ²⁹⁰₁₁₆Lv + ⁴₂He (1.29 milliseconds)

²⁹⁰₁₁₆Lv → ²⁸⁶₁₁₄Fl + ⁴₂He (14.4 milliseconds)

²⁸⁶₁₁₄Fl → ²⁸²₁₁₂Uub + ⁴₂He (230 milliseconds)

The ²⁸²₁₁₂Cn species then undergoes spontaneous fission (denoted SF) to other species. An important part of this work was additional work synthesising isotopes of element 116 through irradiation of ²⁴⁵Cm (as opposed to ²⁴⁹Cm referred to above).

²⁴⁵₉₈Cf + ⁴⁸₂₀Ca → ²⁹⁰₁₁₆Lv + 3¹n

Earlier, a team of Berkeley Lab scientists announced in 1999 the observation of what appeared to be element 118 but retracted the claim after several confirmation experiments failed to reproduce the results. This means that the following apparently is wrong. In this work it was claimed that elements 118 and 116 were formed by accelerating a beam of krypton-86 (⁸⁶₃₆Kr) ions to an energy of 449 million electron volts and directing the beam onto targets of lead-208 (²⁰⁸₈₂Pb). After 11 days work, just three atoms of the new element were identified. The production rates for element 118 are approximately one in every 10¹² interactions.

²⁰⁸₈₂Pb + ⁸⁶₃₆Kr → ²⁹³₁₁₈Uuo + ¹n