Unseptriium, Ust, is the temporary name for element 173. 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 approach used for Z = 176 and above can be used at lower Z.

A boundary in the (Z,A) plane is constructed in "The Final Element" (this wiki) defining a region of that plane outside of which no nuclei exist. It does not predict where nuclei can exist within that region; but the first-order, liquid-drop model used to create that boundary can be used to guess at where nuclides may exist. Criteria used to guide these guesses are described in "Nuclear Guesswork" (this wiki). The resulting A(Z) ranges developed should not be considered accurate, but they are consistent from element to element.


Ref. 1 predicts isotopes ranging from Ust 505 to Ust 440.

Ust 505 and Ust 502 appear to be artifacts. N = 318 has been predicted(2) to be a neutron closure, but N = 229 or 232 is far above that closure.

Ust 501 to Ust 493 is a gap, which might mean half-lives below 10^-09 sec or might mean the model is going ragged at its edges.

Ust 492 decays by alpha emission and is estimated from adjacent nuclides to have a half-life in the ns-ms range. A second gap from Ust 491 to Ust 488 separates the two bands. This gap, too, may reflect short alpha-decay half-lives or a breakdown of the model.

The main band lies between Ust 487 and Ust 470. Format used to display these is: isotope(s); half-life in seconds; dominant decay mode; comments.

Ust 487 - Ust 483; 10^-09 - 0.001; alpha; These are not unrealistic, particularly if N = 318 is also neutron-magic like N = 308, Half-lives are estimated from adjacent nuclides.

Ust 482 - Ust 474; 10^-06 - 0.001; alpha; Some half-lives, including those at the band edge must be estimated from adjacent nuclides.

Ust 473 - Ust 468; 10^-09 - 10^-06; alpha. Some half-lives, including those at the band edge must be estimated from adjacent nuclides.

Ust 467 - Ust 466; 10^-09 - 10^-06; fission.

This pattern is to be expected, given a neutron shell closure at N = 308.

Ust 440 is reported to decay mainly by proton emission and have a half life in the 10^-06 to 0.001 sec range. Its long half-life implies strongly that it is an artifact.


A nuclear drop containing 173 protons and more than 579 neutrons must decay by neutron emission with a half-life under 10^-14 sec. A drop with 173 protons and fewer than 228 neutrons must decay by spontaneous fission with a half-life under 10^-14 sec. Nuclear drops in the band from Ust 752 to Ust 401 are not required to decay either by neutron emission or by fission, so it is possible they will survive the 10^-14 sec needed for them to become nuclides.

Nuclear drops in the band Ust 752 to Ust 674 are likely to decay by neutron emission but are stable against fission. Nuclides in this band are unlikely. Drops in the band Ust 673 to Ust 629 are likely to decay by neutron emission and require a moderate amount of structural correction energy. Nuclides in this band are improbable.

Drops in the band Ust 583 to Ust 562, and also Ust 481 are unlikely to decay by neutron emission and are stable against fission. Nuclides in these bands are likely. Drops in the bands Ust 628 to Ust 584, Ust 561 to Ust 482, and Ust 480 to Ust 455 are unlikely to decay by neutron emission and require a moderate amount of structural correction energy. Drops in these bands are unlikely. Drops in the band Ust 454 to Ust 401 are unlikely to decay by neutron emission but require large structural correction. Nuclides in this band are improbable.


The two techniques described above were more or less consistent. Ref. 1 does predict far more nuclides than were estimated to be "likely". The technique for estimating where nuclides are likely to exist is conservative.


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

Isotopes in the band Ust 579 to Ust 562 can form by a series of beta decays from dripline in the band Uhu 579 to Uph 562. Formation of isotopes in this band is likely. Drops in the band Ust 583 to Ust 580 require at least one beta decay from a mother nucleus which is expected to fission. It is unlikely for isotopes in this band to form.

It is improbable that any isotopes reported in Ref. 1, notably Ust 481, will form.


Electron structure of Ust has received limited study, but it is likely to differ significantly from what's found at lower atomic numbers. It is likely that orbital theory breaks down between Z = 170 and Z = 175. (Z = 173 is the most probable value at which this theory breaks down.) While only the innermost electrons would be qualitatively different, other orbitals are likely to be affected sufficiently to change the ground state occupation. Ust is also large enough that nuclear shape may have an effect on electron structure, which might cause different isotopes of Ust to have different electronic structures. (That means it is no longer an element in the chemical sense.) Predictions of atomic or chemical properties of Ust are risky.

If these effects are small, and if the assumptions made in "Period 9 Elements" (this wiki) are valid, Ust will be the 9th period alkali metal.


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. Other references are found in the wiki articles cited.

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