Unbiunium

Unbiunium, Ubu, is the temporary name for element 121. Isotopes are predicted between the dripline and Ubu 339; Ubu 337; between Ubu 319 and Ubu 317; and between Ubu 316 and Ubu 283. All of them have half-lives less than 1 day, and most are under 1 sec. None of its isotopes will persist longer than 1000 sec after the event which caused them to form

Wikipedia has an article "Unbibium". This article focuses on things Wikipedia's doesn't stress.

NUCLEAR PROPERTIES

INFORMATION SOURCES

There has been considerable study of the decay properties of Ubu. Most of these have been limited to N < 220, but I know of only one which which reports decay properties of Ubu to the neutron dripline⁽¹⁾. Maps on pp 15 & 18 address the entire (Z,N) region covered, but report only the dominant decay mode and report half-lives only to within a band three orders of magnitude wide (0.001 - 1 sec, for example). More detailed estimates of these properties can be extracted from maps on pp 11 & 12, but only for a limited range of Z and N. Half-life data are reported by colors, which makes numerical estimates laborious to produce. Ref. 1 will be used as the primary reference in this article. Predictions from other sources will be compared to it.

An independent map of half-lives and decay modes exists⁽²⁾. The map shows a stabilized region in a band apparently ending near Z = 132. That band ends at Z = 118. Neutron counts in the region lie between N = 191 and N = 202, but the most stabilized nuclides lie in a band 195 <= N <= 200. Ref. 1 does not show this region.

An interactive site exists which has tables of b-, b+, and alpha decay(1) half-lives for Z up to 135 and A up to 339⁽³⁾. Total half-lives and branch ratios can be computed from these to provide a third pair of maps. This model does not show particular stabilization around N = 200, aligning with Ref. 1 in this regard.

PREDICTED PROPERTIES

Even-N isotopes from the neutron dripline down to Ubu 392 decay predominantly by beta emission with half-lives in the 0.001 - 1 sec range. Half-lives aren't reported, but the properties of beta decay indicate that half-lives close to 0.001 sec are likely⁽³⁾. Odd-N drops in this band decay by neutron emission.

All isotopes in the band Ubu 391 to Ubu 366 are predicted to have half-lives in the 0.001 - 1 sec range. Decay modes are a mixture of fission and beta emission. Which mode dominates depends on N, with specific values of N associated with fission over a range of Z values.

Isotopes in the band Ubu 365 to Ubu 341 are predicted to decay by beta emission and have half-lives in the 0.001 - 1 sec range, except for Ubu 343, whose half-life is a few seconds.

Ubu 340 to Ubu 336 are predicted to decay by fission. Except for Ubu 340, which has a half-life in the range .001 - 1 sec, all have half-lives in the 10^-09 - 10^-06 sec range.

Ref 1 predicts a gap from Ubu 335 to Ubu 317, which is predicted to be occupied by nuclear drops or very short-lived nuclides.

Ref, 2 reports isotopes with half-lives above 10^-06 sec in the band Ubu 319 - Ubu 316. Of these, Ubu 319 has a half-life in the 1 sec - 1 day band; and all others have half-lives in the 0.001 - 1 sec band. Aside from these it predicts a wide band of isotopes predicted to have halflives under 10^-06 sec. These can't be compared with Ref. 1 due to the difference in half-life cutoff. All decay by fission. Ref. 2 differs from Rev 1 in predicting long half-lives for isotopes where Ref. 1 predicts short lives. Both agree that fission is the principal decay mode.

Ref 1 predicts Ubu 315 to Ubu 312 will decay by quickly and by fission.

It also predicts that Ubu 311 through Ubu 309 should decay by alpha emission with half-lives in the range 10^-06 - 0.001 sec. Ref. 2 predicts fission and shorter half-lives.

Ref. 1 predicts Ubu 308 and Ubu 307 should decay by alpha emission with half-lives in the range 10^-09 - 10^-06 sec. This dip in half-lives between heavier and lighter isotopes is typical for nuclides with slightly more than a closed shell of neutrons. Ref. 2 predicts fission in this range.

Ubu 306 to Ubu 290 are predicted to decay by alpha emission with half-lives in the 10^-06 - 0.001 sec range. Note that neutron count for Ubu 305 is 184.

Most isotopes in the band Ubu 289 to Ubu 283 are predicted to decay by alpha emission and have half-lives in the 10^-09 - 10^-06 sec range. Exceptions include Ubu 286, which decays in the 10^-06 - 0.001 sec range, and Ubu 285 & Ubu 283, which are predicted to decay by proton emission.

Wikipedia's article "Unbibium" also addresses theoretical properties of lighter Ubu isotopes.

OCCURRENCE

FORMATION

All even-N nuclear drops from the neutron dripline to Ubu 392 are predicted to be nuclides. All nuclear drops in the bands Ubu 391 to Ubu 336 are predicted to be nuclides. Some of the heavier isotopes in that band can form directly as a neutron star disintegrates. Most of them however require a chain of beta decays to form. All isotopes in this band can form; except for Ubu 338 and Ubu 336, whose beta chains are interrupted at lower Z. Ref 2 shows nuclides in the band from Ubu 319 to Ubu 317. Beta decay chains leading to them are all interrupted. None of them can form. Ref. 1 also predicts all drops in the band Ubu 316 to Ubu 285 are nuclides. None of them can form.

It is implausible that neutron capture can form any Ubu isotope.

PERSISTENCE

It is possible to simulate the evolution of nuclides after an element-forming event. Details of one such model are provided in "Nuclear Decay Chains at High A" in this wiki. Applying this model indicates that all Ubu isotopes will persist less than 1000 sec after a neutron star merger or similar event which led to their formation.

ATOMIC PROPERTIES

Ubu is expected to be an 8th period active metal. It is normally considered to be the first superactinide. Its electron configuration is expected to be [Og] 8s² 8p¹_1/2.

Wikipedia's article "Unbibium" addresses the element's atomic properties and chemistry in some detail.

REFERENCES

1. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011.

2. “Systematic Study of Decay Properties of Heaviest Elements.”; Y. M. Palenzuelaa, L. F. Ruiza, A. Karpov, and W. Greiner; Bulletin of the Russian Academy of Sciences, Physics.  Vol . 76, No.11, pp 1165 – 1177; 2012

3. "Nuclear Properties for Astrophysical Applications"; P. Moller & J. R. Nix; Los Alamos National Laboratory website; search by "LANL, T2", then "Nuclear Properties for Astrophysical Applications".

4. Other references are found in the wiki articles cited.

(09-16-20)