- Unhexoctium, Uho, is the temporary name for element 168. A number of short-lived, alpha-decaying isotopes have been predicted. It appears likely that a different set of short-lived beta-decaying isotopes can form.
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 Uho 500 to Uho 457.
Uho 500 appears to be an artifact. N = 318 has been predicted(2) to be a neutron closure, but N = 229 or 232 is far above that closure.
Uho 499 to Uho 485 is a gap, which might mean half-lives below 10^-09 sec or might mean the model is going ragged at its edges.
The main band lies between Uho 484 and Uho 455. Format used to display these is: isotope(s); half-life in seconds; dominant decay mode; comments.
Uho 484 - Uho 478; 10^-09 - 0.001; alpha. (Due to masking by beta-stable nuclides, it is impossible to read predicted half-lives directly.) These are not unrealistic, particularly if N = 318 is also neutron-magic like N = 308.
Uho 477; 10^-06 - 0.001; alpha.
Uho 476 - Uho 475; 0.001 - 1; alpha. (Half-life must be estimated from adjacent nuclides.)
Uho 474; 10^-06 - 0.001; alpha.
Uho 473; 0.001 - 1; alpha.
Uho 472 - Uho 463; 10^-06 - 0.001; alpha.
Uho 462 - Uho 461; 10^-09 - 10^-06; fission.
Uho 460; <10^-09; unknown, but probably fission.
Uho 459; 10^-09 - 10^-06; fission.
Uho 458; <10^-09; unknown, but probably fission.
Uho 457; 10^-09 - 10^-06; fission.
This pattern is to be expected, given a neutron shell closure at N = 308.
Drops in the bands Uho 608 to Uho 543 and Uho 478 to Uho 470 are unlikely to decay by neutron emission and are stable against fission. Nuclides in these bands are likely. Drops in the bands Uho 542 to Uho 479 and Uho 469 to Uho 424 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 Uho 478 to Uho 470 as "likely". Ref. 1 predicts a much wider band of existing Uho isotopes.
Since a disintegrating neutron star can supply neutron-rich pieces of nuclear matter of the correct size (see "Neutron Star", this wiki), Uho isotopes can form where inhibition of neutron emission and fission allow beta decay from the neutron dripline.
Isotopes in the band Uho 608 to Uho 543 can form by a series of beta decays from the neutron dripline. Formation of isotopes in this band is likely. Other isotopes require beta decay from at least one mother nuclide which is expected to fission. Formation of those isotopes is unlikely.
It is improbable that any isotopes reported in Ref. 1, notably Uho 478 to Uho 470, will form.
Electron structure of Uho has been predicted by several sources (see "Extended Periodic Table" in Wikipedia). However, these predictions should be used with caution. Uho is large enough that nuclear shape may have an effect on electron structure, which might cause different isotopes of Uho to have different electronic structures. (That means it is no longer an element in the chemical sense.)
If this effect is small, Uho will be a p-block metal of the 8th period. Its electron configuration has been predicted(3) to be [Og] 5g18 6f14 7d10 8s2 8p21/2 9s2 9p21/2.
At one time, Uho was thought to complete Period 8, but this is no longer thought true.
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.