Unquadoctium, Uqo, is the temporary name for element 148. It is expected to be a transient element, one without long-lived isotopes or long-lived precoursers. Uqo 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 Uqo 475 to Uqo 415. Format used to display isotope properties is: isotope(s); half-life in seconds; dominant decay mode; comments.
Uqo 475 - Uqo 466; <10^-06; fission.
Uqo 465; 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.
Uqo 464 - Uqo 442; 0.001 - 1; beta.
Uqo 441 - Uqo 437; 10^-06 - 1000; fission. Odd-N isotopes are predicted to have half-lives in the range 1 - 1000 sec; even-N isotopes are predicted to have half-lives in the range 10^-06 - 0.001 sec. 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. However there is a gap of at least three orders of magnitude between odd-N and even-N isotopes, which implies that one or the other is erroneous.
Uqo 436 - Uqo 434; 0.001 - 1; mixed. Uqo 435 is reported to decay by fission, the others by beta emission. It is likely that all isotopes in this band decay by a mixture of both modes.
Uqo 433; 1 - 1000; fission. A half-life is close to 1 second continues the pattern for Uqo 436 - Uqo 434, so is likely.
Uqo 432 - Uqo 427; 0.001 - 1; mixed. Fission is predicted to dominate decay for Uqo 430, with beta emission dominating the others. All nuclides in this band are likely to decay by a mixture of both modes.
Uqo 426; 0.001 - 1; fission. Adjacent Uqo isotopes decay by different principal modes. This isotope may decay by a mixture of alpha emission as well as beta emission and fission.
Uqo 425 - Uqo; 0.001 - 1000; mixed. Alpha emission dominates for odd-N nuclides. All decay modes are likely, as are half-lives in the vicinity of 1 sec.
Uqo 422 - Uqo 420; 0.001 - 1; fission. Even-N isotopes in this band are beta-stable, so their properties are estimated from adjacent nuclides. Mixed alpha emission and fission is likely.
Uqo 419 - Uqo 417; 10^-06 - 0.001; fission.
Uqo 416; <10^-09 ; fission. Even-n / even-Z isotopes tend to have short fission half-lives.
Uqo 415; 10^-09 - 10^-06; fission. This nuclide is reported to be beta-stable, so has a half-life >10^-09 sec. Fission is the most likely decay mode.
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 Uqo 531 to Uqo 506 and Uqo 458 to Uqo 447 are unlikely to decay by neutron emission and are stable against fission. Nuclides in these bands are likely. Drops in the bands Uqo 505 to Uqo 459 and Uqo 446 to Uqo 324 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 Uqo 458 to Uqo 447 as "likely". Ref. 1 predicts that a continuous band from Uqo 465 to Uqo 415 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), Uqo isotopes can form where inhibition of fission allows beta decay from the neutron dripline.
Isotopes in the band Uqo 531 to Uqo 506 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 Uqo 505 - Uqo 482 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, Uqo 455 to Uqo 417 can form. It is improbable that other isotopes in the band Uqo 481 to Uqo 415 can form.
Electron structure of Uqo has been predicted by several sources (see "Extended Periodic Table" in Wikipedia). However, these predictions should be used with caution. Uqo is large enough that nuclear shape may have an effect on electron structure, which might cause different isotopes of Uqo to have different electronic structures. (That means it is no longer an element in the chemical sense.)
If this effect is small, Uqo will be an active (superactinide) metal of the 8th period. Its electron configuration has been predicted(3) to be [Og] 5g18 6f6 7d2 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.