Unquadennium, Uqe, is the temporary name for element 149. It is expected to be a transient element, one without long-lived isotopes or long-lived precoursers. Uqe 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 Uqe 477 to Uqe 416. Format used to display isotope properties is: isotope(s); half-life in seconds; dominant decay mode; comments.
Uqe 477 - Uqe 468; <10^-06; fission.
Uqe 467 - Uqe 466; 0.001 - 1; fission. A fission half-life this long is plausible only if there is a shell closure at N = 318(2). It is too far above N = 308 for stabilization against fission. Beta decay seems more likely.
Uqe 465 - Uqe 442; 0.001 - 1; beta.
Uqe 441 - Uqe 439; 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(2), so short partial half-lives against fission in this region are plausible.
Uqe 438; 0.001 - 1; fission. Half-life is probably close to 1 second.
Uqe 437; 1 - 1000; alpha. Fission is probably a competing decay mode.
Uqe 436 - Uqe 435; 1 - 1000; fission. Alpha emission is likely to be a competing decay mode.
Uqe 434; 0.001 - 1; fission. Alpha emission is likely to be a competing decay mode.
Uqe 433 - Uqe 432; 1 - 1000; alpha.
Uqe 431; 1 - 1000; fission.
Uqe 430 - Uqe 426; 1 - 1000; alpha. Fission may not be a factor in these isotopes of Uqe.
Uqe 425; 0.001 - 1; fission.
Uqe 424 - Uqe 423; 0.001 - 1; alpha.
Uqe 422 - Uqe 420; 0.001 - 1; fission. Alpha emission is likely to be a competing decay mode.
Uqe 419; 0.001 - 1; alpha. Fission is likely to be a competing decay mode.
Uqe 418; 10^-06 - 0.001; fission.
Uqe 417; 10^-06 - 0.001; fission. This nuclide is predicted to be beta-stable, so its half-life and decay mode are estimated.
Uqe 416; 10^-09 - 10^-06; fission.
This pattern is generally to be expected, given a neutron shell closure at N = 308. Presence of a fission-decaying band in the N = 285 - 295 range indicates requires nuclear structure.
Drops in the bands Uqe 535 to Uqe 513 and Uqe 459 to Uqe 448 are unlikely to decay by neutron emission and are stable against fission. Nuclides in these bands are likely. Drops in the bands Uqe 512 to Uqe 460 and Uqe 447 to Uqe 327 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 Uqe 459 to Uqe 448 as "likely". Ref. 1 predicts that Uqe 477 to Uqe 416 will exist, a much broader range. In particular, fissioning nuclides between Uqe 477 and Uqe 466 aren't anticipated by the estimating technique used.
Since a disintegrating neutron star can supply neutron-rich pieces of nuclear matter of the correct size (see "Neutron Star", this wiki), Uqe isotopes can form where inhibition of fission allows beta decay from the neutron dripline.
Isotopes in the band Uqe 535 to Uqe 513 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 Uqe 512 - Uqe 483 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, Uqe 455 to Uqe 423, Uqe 421, Uqe 419, and Uqe 417 can form. It is improbable that other isotopes in the band Uqe 482 to Uqe 416 can form.
Electron structure of Uqe has been predicted by several sources (see "Extended Periodic Table" in Wikipedia). However, these predictions should be used with caution. Uqe is large enough that nuclear shape may have an effect on electron structure, which might cause different isotopes of Uqe to have different electronic structures. (That means it is no longer an element in the chemical sense.)
If this effect is small, Uqe will be an active (superactinide) metal of the 8th period. Its electron configuration has been predicted(3) to be [Og] 5g18 6f6 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.