Unpentbium, Upb, is the temporary name for element 152. It is expected to be a transient element, one without long-lived isotopes or long-lived precoursers. Upb may form during neutron star mergers.
Between Z = 175 and Z near 130, one set of predictions for half-life and principal decay mode has been published(1). Ref. 1 is publicly available and can be found via a search by paper title. Anyone interested in this element should study pp 15 and 18, which allow a given element to be understood in the context of adjacent nuclides.
These data are limited to nuclides for which N <= 333. Half-lives are presented in bands covering 3 orders of magnitude (0.001 sec to 1 sec, for instance) and are accurate to within +/- 3 orders of magnitude, which seems rather crude until the enormous extrapolation from what is known is taken into account, Minimum half-life is set at 10^-09 sec, rather than 10^-14 sec; which introduces a little uncertainty, but not a great deal because fission half-lives tend to transition quickly from values well above 10^-09 sec to values well below 10^-14 sec; and, while alpha-decay half-lives change more slowly, alpha emission is rarely dominant except where fission is suppressed. Significantly, beta-decay half-lives do not decline far below 10^-03 sec, even for highly energetic decays, so there is little uncertainty about neutron-rich nuclides.
Ref. 1 does have one significant weakness. Nuclides which are beta-stable are identified by black squares, overwriting decay mode and half-life information. In many cases, these data can be estimated from adjacent nuclides.
No predictions exist for N > 333. The liquid-drop sketch developed in "The Final Element" (this wiki) for Z = 176 and above can be used to guess at where nuclides with Z < 175 and N > 333 may exist. Probability criteria for this purpose were set in "Nuclear Guesswork" (this wiki). Below Z = 171, it is necessary to look only at nuclear drops which are not expected to decay by neutron emission and require only normal amounts of structural correction energy in order to suppress spontaneous fission.
Ref. 1 predicts isotopes ranging from Upb 475 to Upb 421. Format used to display isotope properties is: isotope(s); half-life in seconds; dominant decay mode; comments.
Upb 475 - Upb 473; <10^-06; fission. Upb 474 has a half-live under 10^-09 sec.
Upb 472; 10^-09 - 10^-06; beta. Decay mode is an anomaly. Beta decay is not that rapid.
Upb 471 - Upb 470; 10^-06 - 0.001; fission. Fission half-lives this long are plausible only if there is a shell closure at N = 318(2). They are too far above N = 308 for it to stabilize nuclides against fission. If N = 308 is the only closure, these nuclides should either decay quickly by fission or decay decay in a millisecond time frame by beta emission.
Upb 469; 0.001 - 1; fission. A fission half-life this long is plausible only if there is a shell closure at N = 318. It is too far above N = 308 for that closure to stabilize it against fission. If N = 308 is the only closure, this nuclide should either decay quickly by fission or decay in a millisecond time frame by beta emission.
Upb 468 - Upb 445; 0.001 - 1; beta.
Upb 444; 0.001 - 1; fission. Ref 1 predicts a band of fission-decaying nuclides with N between 285 and 295. It appears to be possible for structure to destabilize a nuclide(2), so isotopes between Upb 444 and Upb 437 which decay in the manner predicted are not implausible.
Upb 443; 1 - 1000; fission. Ref 1 predicts a band of fission-decaying nuclides with N between 285 and 295. It appears to be possible for structure to destabilize a nuclide, so isotopes between Upb 444 and Upb 437 which decay in the manner predicted are not implausible. Beta decay is likely to be an important secondary decay mode.
Upb 442; 0.001 - 1; fission. Ref 1 predicts a band of fission-decaying nuclides with N between 285 and 295. It appears to be possible for structure to destabilize a nuclide, so isotopes between Upb 444 and Upb 437 which decay in the manner predicted are not implausible.
Upb 441; 1 - 1000; fission. Predicted mode and half-life are plausible, as stated for Upb 443 above.
Upb 440; 0.001 - 1; beta. Alpha emission and fission are likely to be important secondary decay modes for this nuclide.
Upb 439; 1 - 1000; alpha. Beta emission and fission are likely to be important secondary decay modes.
Upb 438 - Upb 437; 0.001 - 1; fission. Half-lives and decay modes are plausible as described above.
Upb 436 - Upb 431; 0.001 - 1; alpha. Even-N nuclides in this band are beta-stable, so their properties are estimated from adjacent nuclides. The estimate for Upb 432 is particularly uncertain.
Upb 430 - Upb 427; 0.001 - 1; alpha and fission. Even-N isotopes in this band decay by fission, odd-N isotopes decay by alpha emission. This is reasonable, given the very strong stabilization that an unpaired neutron has on fission. All isotopes in this band are beta-stable, so both mode and half-life are estimated from adjacent nuclides. Due to rapidly changing half-lives and decay modes for nuclides in this region, these estimates are particularly uncertain.
Upb 426 - Upb 423; 10^-06 - 0.001; fission. Except for Upb 423, all isotopes in this band are beta-stable. Decay modes and half-lives are estimated from the properties of adjacent nuclides.
Upb 422 - Upb 421; 10^-09 - 10^-06; fission. Properties of Upb 422 are estimated, since it is predicted to be beta-stable.
Below N = 308, this pattern is generally to be expected, given a neutron shell closure at N = 308. Above this point, predictions become more confusing. Presence of relatively long-lived, fission-decaying isotopes of Upb indicates a more sophisticated structure than implied by a simple liquid-drop picture.
Drops in the bands Upb 546 to Upb 527 and Upb 462 to Upb 450 are unlikely to decay by neutron emission and are stable against fission. Nuclides in these bands are likely. Drops in the bands Upb 526 to Upb 463 and Upb 449 to Upb 347, and the nuclide Upb 336 are unlikely to decay by neutron emission and require a moderate amount of structural correction energy. Drops in these bands are unlikely.
In the region where predictions and guesses overlap, the estimating technique lists only Upb 462 to Upb 450 as "likely". Ref. 1 predicts that Upb 475 to Upb 421 will exist, a much broader range.
Since a disintegrating neutron star can supply neutron-rich pieces of nuclear matter of the correct size (see "Neutron Star", this wiki), Upb isotopes can form where inhibition of fission allows beta decay from the neutron dripline.
Isotopes in the band Upb 546 to Upb 527 can form by a series of beta decays from the neutron dripline. Formation of isotopes in this band is likely. It is improbable that other isotopes in the band Upb 526 - Upb 486 can form.
Beta decay from the neutron dripline also forms nuclei in the region described in Ref. 1. Although only one decay mode is reported for each nuclide, branching decay can be expected unless the partial half-life against one mode of decay is much shorter than any of the others. In practice, that means beta decay series will extend either to nuclides which are stable against beta decay, nuclides lighter than beta-stable nuclides (which can decay by positron emission), or whose spontaneous fission partial half-lives are under 1 us.
Under this assumption, Upb 455 to Upb 431, Upb 429, Upb 427, and Upb 425 can form. It is improbable that other isotopes in the band Upb 485 to Upb 421 can form.
Electron structure of Upb has been predicted by several sources (see "Extended Periodic Table" in Wikipedia). However, these predictions should be used with caution. Upb is large enough that nuclear shape may have an effect on electron structure, which might cause different isotopes of Upb to have different electronic structures. (That means it is no longer an element in the chemical sense.)
If this effect is small, Upb will be an active (superactinide) metal of the 8th period. Its electron configuration has been predicted(3) to be [Og] 5g18 6f9 7d3 8s2 8p21/2.
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. “The Highest Limiting Z in the Extended Periodic Table”; Y.K. Gambhir, A. Bhagwat, and M. Gupta; Journal of Physics G: Nuclear and Particle Physics. 42 (12): 125105. DOI:10.1088/0954 3899/42/12/ 125105.
3. "Extended Periodic Table", Wikipedia.
4. Other references are found in the wiki articles cited.