Ununhexium

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

116 ununpentium ← ununhexium → ununseptium Po ↑ Uuh ↓ (Uhh) Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	ununhexium, Uuh, 116
Element category	presumably poor metal
Group, Period, Block	16, 7, p
Standard atomic weight	[293] g·mol−1
Electron configuration	perhaps [Rn] 5f14 6d10 7s2 7p4 (guess based on polonium)
Electrons per shell	2, 8, 18, 32, 32, 18, 6
CAS registry number	54100-71-9
Selected isotopes
Main article: Isotopes of ununhexium iso NA half-life DM DE ( MeV) DP 293Uuh syn 61 ms α 10.54 289Uuq 292Uuh syn 18 ms α 10.66 288Uuq 291Uuh syn 18 ms α 10.74 287Uuq 290Uuh syn 7.1 ms α 10.84 286Uuq
References

Ununhexium (pronounced /ˌjuːnənˈhɛksiəm/ or /ˌʌnənˈhɛksiəm/) is the temporary name of a synthetic superheavy element in the periodic table that has the temporary symbol Uuh and has the atomic number 116.

Four isotopes are currently known with masses 290-293. The most stable is Uuh-293 with a half-life of 63 ms.

Discovery profile

On July 19, 2000, scientists at Dubna (FLNR) detected a single decay from an atom of ununhexium following the irradiation of a Cm-248 target with Ca-48 ions. The results were published in December, 2000. This 10.54 MeV alpha-emitting activity was originally assigned to ²⁹²Uuh due to the correlation of the daughter to previously assigned ²⁸⁸Uuq. However, that assignment was later altered to ²⁸⁹Uuq, and hence this activity was correspondingly changed to ²⁹³Uuh. Two further atoms were reported by the institute during their second experiment between April-May 2001.

In the same experiment they also detected a decay chain which corresponded to the first observed decay of ununquadium and assigned to ²⁸⁹Uuq. This activity has not been observed again in a repeat of the same reaction. However, its detection in this series of experiments indicates the possibility of the decay of a meta-stable isomer of ununhexium, namely ^293m116, or a rare decay branch of the already discovered ground state isomer, in which the first alpha particle was missed. Further research is required to positively assign this activity.

The team repeated the experiment in April-May 2005 and detected 8 atoms of ununhexium. The measured decay data confirmed the assignment of the discovery isotope as ²⁹³116. In this run, the team also observed ²⁹²116 in the 4n channel for the first time.

Theoretical calculation in a quantum tunneling model supports the experimental data.

The IUPAC/IUPAP Joint Working Party (JWP) are currently assessing the claim of discovery for this element by the Dubna team.

Naming

Current names

The element with Z=116 is historically known as eka-polonium. Ununhexium (Uuh) is a temporary IUPAC systematic element name. Research scientists usually refer to the element simply as element 116 (E116).

Proposed names by Claimants

Claims to the discovery of element 116 have been put forward by Dmitriev of the Dubna team. The JWP will decide to whom the right to suggest a name will be given. The IUPAC have the final say on the official adoption of a name. No name for element 116 has yet been suggested by the Dubna team.

Disallowed names

According to IUPAC rules, names used for previous elements that have ultimately not been adopted are not allowed to be proposed for future use. The table below summarises those names which are probably not allowed to be proposed by the claimant laboratories under the rules.

Plausible names

Many speculative names appear in popular literature. The table below lists these names in the case where they obey IUPAC rules and are plausible with regard to the claimant laboratories. Rumored suggestions linked to the claimant laboratories are also included.

Electronic structure

Ununhexium has 6 full shells, 7 s+5 p+4 d+2 f=18 full subshells, and 116 orbitals:

Bohr model: 2, 8, 18, 32, 32, 18, 6

Quantum mechanical model: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰ 4p⁶5s²4d¹⁰5p⁶6s²4f¹⁴5d¹⁰ 6p⁶7s²5f¹⁴6d¹⁰7p⁴

Extrapolated chemical properties of eka-polonium

Oxidation states

Element 116 is projected to be the fourth member of the 7p series of non-metals and the heaviest member of group 16 (VIA) in the Periodic Table, below polonium. The group oxidation state of +VI is known for all the members apart from oxygen which lacks available d- orbitals for expansion and is limited to a maximum +II state, exhibited in the fluoride OF₂. The +IV is known for sulfur, selenium, tellurium, and polonium, undergoing a shift in stability from reducing for S(IV) and Se(IV) to oxidising in Po(IV). Tellurium(IV) is the most stable for this element. This suggests a decreasing stability for the higher oxidation states as the group is descended and element 116 should portray an oxidising +IV state and a more stable +II state. The lighter members are also known to form a −II state as oxide, sulfide, selenide, and telluride. Polonide formation is nonconfirmed or only transient. The extrapolated electronegativity of ununhexium should eliminate this low oxidation state.

Chemistry

The possible chemistry of element 116 can be extrapolated from that of polonium. It should therefore undergo oxidation to a dioxide, UuhO₂, although a trioxide, UuhO₃ is plausible, but unlikely. The stability of a +II state should manifest itself in the formation of a simple monoxide, UuhO. Fluorination will likely result in a tetrafluoride, UuhF₄ and/or a difluoride, UuhF₂. Chlorination and bromination may well stop at the corresponding dihalides, UuhCl₂ and UuhBr₂. Oxidation by iodine should certainly stop at UuhI₂ and may even be inert to this element.

History of synthesis of isotopes by cold fusion

²⁰⁸Pb(⁸²Se,xn)^290−x116

In 1995, the team at GSI attempted the synthesis of ²⁹⁰116 as a radiative capture (x=0) product. No atoms were detected providing a cross section limit of 4.8 pb.

History of synthesis of isotopes by hot fusion

²³⁸U(⁵⁴Cr,xn)^292−x116

There are sketchy indications that this reaction was attempted by the team at GSI in 2006. There are no published results on the outcome, presumably indicating that no atoms were detected. This is expected from a study of the systematics of cross sections for U-238 targets.

²⁴⁸Cm(⁴⁸Ca,xn)^296−x116 (x=3,4)

The first attempt to synthesise element 116 was performed in 1977 by Ken Hulet and his team at the Lawrence Livermore National Laboratory (LLNL). They were unable to detect any atoms of ununhexium. Yuri Oganessian and his team at the Flerov Laboratory of Nuclear Reactions (FLNR) subsequently attempted the reaction in 1978 and were met by failure. In 1985, a joint experiment between Berkeley and Peter Armbruster's team at GSI, the result was again negative with a calculated cross-section limit of 10–100 pb.

In 2000, Russian scientists at Dubna finally succeeded in detecting a single atom of element 116, assigned to the isotope ²⁹²116. In 2001, they repeated the reaction and formed a further 2 atoms in a confirmation of their discovery experiment. A third atom was tentatively assigned to ²⁹³116 on the basis of a missed parental alpha decay. In April 2004, the team ran the experiment again at higher energy and were able to detect a new decay chain, assigned to ²⁹²116. On this basis, the original data were reassigned to ²⁹³116. The tentative chain is therefore possibly associated with a rare decay branch of this isotope. In this reaction, 3 further atoms of ²⁹³116 were detected.

²⁴⁵Cm(⁴⁸Ca,xn)^293−x116 (x=2,3)

In order to assist in the assignment of isotope mass numbers for ununhexium, in March-May 2003 the Dubna team bombarded a Cm-245 target with Ca-48 ions. They were able to observe two new isotopes, assigned to ²⁹¹116 and ²⁹⁰116. This experiment was successfully repeated in Feb-March 2005 where 10 atoms were created with identical decay data to those reported in the 2003 experiment.

Synthesis of ununhexium as a decay product

Ununhexium has also been observed in the decay of ununoctium. In October 2006 it was announced that 3 atoms of ununoctium had been detected by the bombardment of californium-249 with calcium-48 ions, which then rapidly decayed into ununhexium.

The observation of ²⁹⁰116 allowed the assignment of the product to ²⁹⁴118 and proved the synthesis of a nucleus with Z=118 (see ununoctium).

Chronology of isotope discovery

Yields of isotopes

Hot fusion

The table below provides cross-sections and excitation energies for hot fusion reactions producing ununhexium isotopes directly. Data in bold represent maxima derived from excitation function measurements. + represents an observed exit channel.

Retracted isotopes

²⁸⁹116

In 1999, researchers at Lawrence Berkeley National Laboratory announced the synthesis of ²⁹³118 (see ununoctium), in a paper published in Physical Review Letters. The claimed isotope ²⁸⁹116 decayed by 11.63MeV alpha emission with a halflife of 0.64 ms. 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 the principal author Victor Ninov. As such, this ununhexium isotope is currently unknown or deconfirmed.

Future experiments

The team at Dubna are planning to revisit the synthesis in 2008. They will bombard a plutonium-244 target with titanium-50 ions. This experiment will allow them to assess the feasibility of using projectiles with Z>20 required in the synthesis of SHE's with Z>118. There are also plans to repeat the Cm-248 reaction at different projectile energies in order to probe the 2n channel, leading to the new isotope ²⁹⁴116. In addition they hope to complete the excitation function of the 4n channel product, ²⁹²116, which will allow them to assess the stabilising effect of the N=184 shell on the yield of evaporation residues.