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Unbiquadium, also known as element 124 or eka-uranium, is a hypothetical chemical element; it has placeholder symbol Ubq and atomic number 124. Unbiquadium and Ubq are the temporary IUPAC name and symbol, respectively, until the element is discovered, confirmed, and a permanent name is decided upon. In the periodic table, unbiquadium is expected to be a g-block superactinide and the sixth element in the 8th period. Unbiquadium has attracted attention, as it may lie within the island of stability, leading to longer half-lives, especially for 308Ubq which is predicted to have a magic number of (184).
Despite several searches, unbiquadium has not been synthesized, nor have any naturally occurring been found to exist. It is believed that the synthesis of unbiquadium will be far more challenging than that of lighter undiscovered elements, and nuclear instability may pose further difficulties in identifying unbiquadium, unless the island of stability has a stronger stabilizing effect than predicted in this region.
As a member of the superactinide series, unbiquadium is expected to bear some resemblance to its possible lighter congener uranium. The valence electrons of unbiquadium are expected to participate in chemical reactions fairly easily, though relativistic effects may significantly influence some of its properties; for example, the electron configuration has been calculated to differ considerably from the one predicted by the Aufbau principle.
The team reported that they had been able to identify compound nucleus fissioning with half-lives > 10−18 s. This result suggests a strong stabilizing effect at Z = 124 and points to the next proton shell at Z > 120, not at Z = 114 as previously thought. A compound nucleus is a loose combination of that have not arranged themselves into nuclear shells yet. It has no internal structure and is held together only by the collision forces between the target and projectile nuclei. It is estimated that it requires around 10−14 s for the nucleons to arrange themselves into nuclear shells, at which point the compound nucleus becomes a nuclide, and this number is used by IUPAC as the minimum half-life a claimed isotope must have to potentially be recognised as being discovered. Thus, the GANIL experiments do not count as a discovery of element 124.
The fission of the compound nucleus 312124 was also studied in 2006 at the tandem ALPI heavy-ion accelerator at the Laboratori Nazionali di Legnaro (Legnaro National Laboratories) in Italy:
Similarly to previous experiments conducted at the JINR (Joint Institute for Nuclear Research), fission product clustered around doubly magic nuclei such as 132Sn ( Z = 50, N = 82), revealing a tendency for superheavy nuclei to expel such doubly magic nuclei in fission.see Flerov lab annual reports 2000–2004 inclusive http://www1.jinr.ru/Reports/Reports_eng_arh.html The average number of neutrons per fission from the 312124 compound nucleus (relative to lighter systems) was also found to increase, confirming that the trend of heavier nuclei emitting more neutrons during fission continues into the superheavy mass region.
The possible extent of primordial superheavy elements on Earth today is uncertain. Even if they are confirmed to have caused the radiation damage long ago, they might now have decayed to mere traces, or even be completely gone. It is also uncertain if such superheavy nuclei may be produced naturally at all, as spontaneous fission is expected to terminate the r-process responsible for heavy element formation between mass number 270 and 290, well before elements such as unbiquadium may be formed.
The production of new superheavy elements will require projectiles heavier than 48Ca, which was successfully used in the discovery of elements 114–118, though this necessitates more symmetric reactions which are less favorable. Hence, it is likely that the reactions between 58Fe and a 249californium or 251Cf target are most promising. Studies on the fission of various superheavy compound nucleus have found that the dynamics of 48Ca- and 58Fe-induced reactions are similar, suggesting that 58Fe projectiles may be viable in producing superheavy nuclei up to Z = 124 or possibly 125. It is also possible that a reaction with 251Cf will produce the compound nucleus 309Ubq* with 185 neutrons, immediately above the N = 184 shell closure. For this reason, the compound nucleus is predicted to have relatively high survival probability and low neutron separation energy, leading to the 1n–3n channels and isotopes 306–308Ubq with a relatively high cross section. These dynamics are highly speculative, as the cross section may be far lower should trends in the production of elements 112–118 continue or the be lower than expected, regardless of shell effects, leading to decreased stability against spontaneous fission (which is of growing importance). Nonetheless, the prospect of reaching the N = 184 shell on the proton-rich side of the chart of nuclides by increasing proton number has long been considered; already in 1970, Soviet nuclear physicist Georgy Flyorov suggested bombarding a plutonium target with zinc projectiles to produce isotopes of element 124 at the N = 184 shell.
In this region of the periodic table, N = 184 and N = 228 have been proposed as closed neutron shells, and various atomic numbers have been proposed as closed proton shells, including Z = 124. The island of stability is characterized by longer half-lives of nuclei located near these magic numbers, though the extent of stabilizing effects is uncertain due to predictions of weakening of the proton shell closures and possible loss of doubly magic. More recent research predicts the island of stability to instead be centered at beta-stable copernicium isotopes 291Cn and 293Cn, which would place unbiquadium well above the island and result in short half-lives regardless of shell effects. A 2016 study on the decay properties of unbiquadium isotopes 284–339Ubq predicts that 284–304Ubq lie outside the proton drip line and thus may be proton emission, 305–323Ubq may undergo alpha decay, with some chains terminating as far as flerovium, and heavier isotopes will decay by spontaneous fission. These results, as well as those from a quantum-tunneling model, predict no half-lives over a millisecond for isotopes lighter than 319Ubq, as well as especially short half-lives for 309–314Ubq in the sub-microsecond range due to destabilizing effects immediately above the shell at N = 184. This renders the identification of many unbiquadium isotopes nearly impossible with current technology, as detectors cannot distinguish rapid successive signals from alpha decays in a time period shorter than microseconds.
Increasingly short spontaneous fission half-lives of superheavy nuclei and the possible domination of fission over alpha decay will also probably determine the stability of unbiquadium isotopes. While some fission half-lives constituting a "sea of instability" may be on the order of 10−18 s as a consequence of very low , especially in even–even nuclei due to pairing effects, stabilizing effects at N = 184 and N = 228 may allow the existence of relatively long-lived isotopes. For N = 184, fission half-lives may increase, though alpha half-lives are still expected to be on the order of microseconds or less, despite the shell closure at 308Ubq. It is also possible that the island of stability may shift to the N = 198 region, where total half-lives may be on the order of seconds, in contrast to neighboring isotopes that would undergo fission in less than a microsecond. In the neutron-rich region around N = 228, alpha half-lives are also predicted to increase with increasing neutron number, meaning that the stability of such nuclei would primarily depend on the location of the beta-stability line and resistance to fission. One early calculation by P. Moller, a physicist at Los Alamos National Laboratory, estimates the total half-life of 352Ubq (with N = 228) to be around 67 seconds, and possibly the longest in the N = 228 region.
One predicted oxidation state of unbiquadium is +6, which would exist in the UbqX6 (X = a halogen), analogous to the known +6 oxidation state in uranium. Like the other early superactinides, the binding energies of unbiquadium's valence electrons are predicted to be small enough that all six should easily participate in chemical reactions. The predicted electron configuration of the Ubq5+ ion is Og 6f1.
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