This article is about an undiscovered element. Once it is discovered, this article will be edited with more information.

Unsepttrium, Ust, is the temporary name for element 173.


At least one set of theoretical values for half-lives and decay modes of Ust have been constructed for neutron count up to N = 333(1).  It predicts isotopes ranging from Ust 440 to Ust 505 in three bands: Ust 440, Ust 466 to Ust 492, and Ust 502 to Ust 505.  Examination of pp 15 & 18 of Ref. 1 indicates that Ust 466 through Ust 492 are part of the expected pattern of alpha-decaying nuclei centered on the N = 308 shell closure.  Isotopes between Ust 473 and Ust 482 are predicted to have sub-millisecond half-lives and to decay by alpha emission.  The other isotopes in this band have sub-microsecond half-lives and most decay by alpha emission, which is consistent with neutron shell closure at Ust 481.  Ust 440 is very neutron-poor, and not located in the vicinity of any suggested shell closure; it appears to be an artifact of the sort common near the edges of models.  Ust 502 to Ust 505 might indicate a shell closure near N = 330, but they are also located in a region whose patterns of half-lives and decay modes indicates that the model may have reached the limits of its capability.

What Ref. 1 can’t do is describe heavy isotopes of Ust.  It is possible to use a first-order, liquid-drop approach to guess at what lies there.  At least two computations of the neutron dripline’s location up to Z = 175 exist(2),(3), and since they give similar results, the maximum possible size of a Ust nucleus can be set slightly above the values computed, allowing only a small margin for error.  This gives Ust 619 as the heaviest possible Ust isotope.  Similarly, a realistic lower bound can be set by requiring that the amount of energy needed to stabilize a nucleus be no more than twice what is needed to stabilize Usp 471.  Within this range, the liquid-drop model can be used to indicate the amount of structural correction energy needed to allow a drop of nuclear matter to survive for the 10^-14 sec needed for electromagnetic interactions (such as binding an electron) to become important.  Structural correction required for Ust 619 is nearly 1.5 MeV, which means all Ust drops will fission quickly without structural stabilization.

In general, it is not possible to describe structural correction energy.  What can be predicted are neutron and proton shell closures, for which correction energy is expected to be particularly large.  Neutron shell closures have been predicted at N = 406(3),(4), 370(3), 318(5), and 308(1).  The isotope Ust 579 requires 1.5 MeV of structural correction, which means isotopes in the Ust 569 to Ust 584 band are likely.  (See “Formation” for additional significance of these nuclei.)  Ust 543 requires 2 MeV of structural correction, which means isotopes in the band Ust 533 to Ust 548 are also likely.  All isotopes in both bands should beta-decay with half-lives under a second.  On the other hand, Ust 491 requires 6 MeV of correction energy, which means it is likely to stabilize some nuclei in its vicinity.  Ref. 1 does not show a pattern of nuclides which indicate a shell closure at N = 318.

Ust 481 requires 7.5 MeV of correction energy, which is realistic for a strong neutron shell closure, such as the one predicted at N = 308, so the liquid-drop picture isn’t unrealistic.


Electron structure of Ust has been predicted to be alkali-metal-like (see Wikipedia, "Extended Periodic Table"). Ust is commonly identified as the point where conventional electron structure breaks down.  Its most tightly bound electrons cannot occupy 1s orbitals;  instead, they apparently occupy orbitals whose pattern oscillates around the nucleus. While other electrons can occupy conventional orbitals, they will be exposed to the full nuclear charge a portion of the time, which will probably change those orbitals, at least quantitatively. 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 stuctures.   Predictions of atomic or chemical properties of Ust are risky.


Ions of this element may form when material from roughly 1 km depth is ejected from a disintegrating neutron star during a merger. There is a possibility that beta decay from dripline nuclides stabilized by the N = 406 closure, enhanced by the Z = 164 proton shell closure, will allow some isotopes in the vicinity of Ust 569 to Ust 577 to form in quantity during such a merger.  It improbable that nuclides between Ust 533 and Ust 548, or lighter, can form in this way.  Fusion or multinucleon transfer reactions in the polar jets emanating from a neutron star or black hole might produce lighter isotopes, including those in the Ust 466 to Ust 492 band. Quantities produced by this method are very small.


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. "Neutron and Proton Drip Lines Using the Modified Bethe-Weizsacker Mass Formula; D.N. Basu et al; Int.J.Mod.Phys.; arXiv:nucl-th/0306061; url:

3.  “Single Particle Levels of Spherical Nuclei in the Superheavy and Extremely Superheavy Mass Region”; H. Koura and S. Chiba; Journal of the Physical Society of Japan; DOI 10.7566/JPSJ.82.014201; Jan. 2013.

4.  "Magic Numbers of Ultraheavy Nuclei"; V. Yu Denisov; Physics of Atomic Nuclei, v. 68, no. 7, pp 1133-1137; 2005.

5.  “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.


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