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Unpentnilium, Upn, is the temporary name for element 150. It is expected to be a transient element, one without long-lived isotopes or long-lived precoursers. Upn may form during neutron star mergers.

NUCLEAR PROPERTIES

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.

PREDICTED PROPERTIES

Ref. 1 predicts isotopes ranging from Upn 477 to Upn 417. Format used to display isotope properties is: isotope(s); half-life in seconds; dominant decay mode; comments.

Upn 477 - Upn 474; <10^-06; fission.

Upn 473; 0.001 - 1; beta. Beta-decaying nuclides with millisecond-scale half-lives are probable in this region, but not a single, isolated nuclide.

Upn 472 - Upn 470; <10^-06; fission.

Upn 469 - Upn 468; 10^-06 - 0.001; 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.

Upn 467; 0.001 - 1; fission. This decay mode seems improbable. It is just below N = 318 and too far above N = 308 for fission to dominate. Beta decay is more probable.

Upn 466 - Upu 443; 0.001 - 1; beta.

Upn 442; 10^-06 - 0.001; 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 the short half-life of this even-N nuclide is plausible.

Upn 441; 1 - 1000; fission. Relatively long partial half-lives against fission are probable for odd-N nuclides. In light of the shorter beta-decay half-lives predicted for lighter Upn isotopes, the long half-life predicted for this isotope is somewhat unexpected.

Upn 440;10^-06 - 0.001; fission.

Upn 439; 1 - 1000; fission. Beta decay with a half-life under one second would seem more likely.

Upn 438; 0.001 - 1; beta.

Upn 437; 1 - 1000; fission. Half-life implies a longer than expected partial half life against beta decay. Beta decay is likely to be an important mode, whether dominant or not.

Upn 436; 0.001 - 1; fission.

Upn 435; 0.001 - 1; beta.

Upn 434; 0.001 - 1; fission. This nuclide is predicted to be beta-stable, so its half-life and decay mode are estimated.

Upn 433; 0.001 - 1; beta.

Upn 432; 0.001 - 1; fission. 0.001 - 1; fission. This nuclide is predicted to be beta-stable, so its half-life and decay mode are estimated. Alpha emission is likely to be an important, and maybe dominant, decay mode.

Upn 431; 1 - 1000; alpha. Fission is likely to be an important secondary decay mode.

Upn 430; 1 - 1000; fission. This nuclide is predicted to be beta-stable, so its half-life and decay mode are estimated. Even-Z / even-N nuclides tend to have short fission half-lives so fission decay seems the more likely to dominate. Alpha emission is likely to be an important decay mode.

Upn 429; 1 - 1000; alpha. Fission is likely to be an important secondary decay mode.

Upn 428; 1 - 1000; fission. This nuclide is predicted to be beta-stable, so its half-life and decay mode are estimated.

Upn 427 - Upn 424; 0.001 - 1; fission. Even-N isotopes are beta-stable, so their properties are estimated. Fission seems more likely, although an important alpha decay branch is likely in this band.

Upn 423; 0.001 - 1; alpha.

Upn 422 - Upn 419; 10^-06 - 0.001; fission. All properties are estimated except for those of Upn 421.

Upn 418 - Upn 417; <10^-06; fission.

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 Upn indicates a more sophisticated structure than implied by a simple liquid-drop picture.

GUESSED PROPERTIES

Drops in the bands Upn 538 to Upn 519 and Upn 460 to Upn 449 are unlikely to decay by neutron emission and are stable against fission. Nuclides in these bands are likely. Drops in the bands Upn 518 to Upn 461 and Upn 448 to Upn 330 are unlikely to decay by neutron emission and require a moderate amount of structural correction energy. Drops in these bands are unlikely.

COMPARISON

In the region where predictions and guesses overlap, the estimating technique lists only Upn 460 to Upn 449 as "likely". Ref. 1 predicts that Upn 477 to Upn 417 will exist, a much broader range. In particular, beta decay, rather than fission, is predicted for Upn isotopes as heavy as Upn 466.

FORMATION

Since a disintegrating neutron star can supply neutron-rich pieces of nuclear matter of the correct size (see "Neutron Star", this wiki), Upn isotopes can form where inhibition of fission allows beta decay from the neutron dripline.

Isotopes in the band Upn 538 to Upn 519 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 Upn 518 - Upn 484 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, Upn 455 to Upn 423, Upn 421, and Upn 419 can form. It is improbable that other isotopes in the band Upn 483 to Upn 417 can form.

ATOMIC PROPERTIES

Electron structure of Upn has been predicted by several sources (see "Extended Periodic Table" in Wikipedia). However, these predictions should be used with caution. Upn is large enough that nuclear shape may have an effect on electron structure, which might cause different isotopes of Upn to have different electronic structures. (That means it is no longer an element in the chemical sense.)

If this effect is small, Upn will be an active (superactinide) metal of the 8th period. Its electron configuration has been predicted(3) to be [Og] 5g18 6f7 7d3 8s2 8p21/2.

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.  “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.

(06-22-20)

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