Unbinilium

Unbinilium, also known as eka-radium or element 120, is a hypothetical chemical element; it has symbol Ubn and atomic number 120. Unbinilium and Ubn are the temporary systematic IUPAC name and symbol, which are used until the element is discovered, confirmed, and a permanent name is decided upon. In the periodic table of the elements, it is expected to be an s-block element, an alkaline earth metal, and the second element in the eighth period. It has attracted attention because of some predictions that it may be in the island of stability.
Unbinilium has not yet been synthesized, despite multiple attempts from German and Russian teams. Experimental evidence from these attempts shows that the period 8 elements would likely be far more difficult to synthesise than the previous known elements. New attempts by American, Russian, and Chinese teams to synthesize unbinilium are planned to begin in the mid-2020s; in particular, an attempt to synthesize the element was ongoing as of 2025, at Lawrence Berkeley National Laboratory in the United States.
Unbinilium's position as the seventh alkaline earth metal suggests that it would have similar properties to its lighter congeners; however, relativistic effects may cause some of its properties to differ from those expected from a straight application of periodic trends. For example, unbinilium is expected to be less reactive than barium and radium, be closer in behavior to strontium, and while it should show the characteristic +2 oxidation state of the alkaline earth metals, it is also predicted to show the +4 and +6 oxidation states, which are unknown in any other alkaline earth metal.

Introduction

History

Elements 114 to 118 were discovered in "hot fusion" reactions bombarding the actinides plutonium through californium with calcium-48, a quasi-stable neutron-rich isotope which could be used as a projectile to produce more neutron-rich isotopes of superheavy elements. This cannot easily be continued to elements 119 and 120, because it would require a target of the next actinides einsteinium and fermium. Tens of milligrams of these would be needed to create such targets, but only micrograms of einsteinium and picograms of fermium have so far been produced. More practical production of further superheavy elements would require bombarding actinides with projectiles heavier than ⁴⁸Ca, but this is expected to be more difficult. Attempts to synthesize elements 119 and 120 push the limits of current technology, due to the decreasing cross sections of the production reactions and their probably short half-lives, expected to be on the order of microseconds.

Synthesis attempts

Past

Following their success in obtaining oganesson by the reaction between ²⁴⁹Cf and ⁴⁸Ca in 2006, the team at the Joint Institute for Nuclear Research in Dubna started experiments in March–April 2007 to attempt to create unbinilium with a ⁵⁸Fe beam and a ²⁴⁴Pu target. The attempt was unsuccessful, and the Russian team planned to upgrade their facilities before attempting the reaction again.
In April 2007, the team at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany attempted to create unbinilium using a ²³⁸U target and a ⁶⁴Ni beam:
No atoms were detected. The GSI repeated the experiment with higher sensitivity in three separate runs in April–May 2007, January–March 2008, and September–October 2008, all with negative results, reaching a cross section limit of 90 fb.
In 2011, after upgrading their equipment to allow the use of more radioactive targets, scientists at the GSI attempted the rather asymmetrical fusion reaction:
It was expected that the change in reaction would quintuple the probability of synthesizing unbinilium, as the yield of such reactions is strongly dependent on their asymmetry. Although this reaction is less asymmetric than the ²⁴⁹Cf+⁵⁰Ti reaction, it also creates more neutron-rich unbinilium isotopes that should receive increased stability from their proximity to the shell closure at N = 184. Three signals were observed in May 2011; a possible assignment to ²⁹⁹Ubn and its daughters was considered, but could not be confirmed, and a different analysis suggested that what was observed was simply a random sequence of events.
In August–October 2011, a different team at the GSI using the TASCA facility tried a new, even more asymmetrical reaction:
Because of its asymmetry, the reaction between ²⁴⁹Cf and ⁵⁰Ti was predicted to be the most favorable practical reaction for synthesizing unbinilium, though it produces a less neutron-rich isotope of unbinilium than any other reaction studied. No unbinilium atoms were identified.
This reaction was investigated again in April to September 2012 at the GSI. This experiment used a ²⁴⁹Bk target and a ⁵⁰Ti beam to produce element 119, but since ²⁴⁹Bk decays to ²⁴⁹Cf with a half-life of about 327 days, both elements 119 and 120 could be searched for simultaneously:
Neither element 119 nor element 120 was observed.

Present

The team at the Lawrence Berkeley National Laboratory in Berkeley, California, United States had made plans to use the 88-inch cyclotron to make new elements using ⁵⁰Ti projectiles. First, the ²⁴⁴Pu+⁵⁰Ti reaction was tested, successfully creating two atoms of ²⁹⁰Lv in 2024. Since this was successful, an attempt to make element 120 in the ²⁴⁹Cf+⁵⁰Ti reaction was planned to begin in late 2025. The Lawrence Livermore National Laboratory, which previously collaborated with the JINR, will collaborate with the LBNL on this project. By June 2025, updates were underway at the LBNL in preparation for the search for element 120, and by September, the search had begun.

Planned

The JINR's plans to investigate the ²⁴⁹Cf+⁵⁰Ti reaction in their new facility were disrupted by the 2022 Russian invasion of Ukraine, after which collaboration between the JINR and other institutes completely ceased due to sanctions. Thus, ²⁴⁹Cf could no longer be used as a target, as it would have to be produced at the Oak Ridge National Laboratory in the United States. Instead, the ²⁴⁸Cm+⁵⁴Cr reaction will be used. In 2023, the director of the JINR, Grigory Trubnikov, stated that he hoped that the experiments to synthesise element 120 will begin in 2025. In preparation for this, the JINR reported success in the ²³⁸U+⁵⁴Cr reaction in late 2023, making a new isotope of livermorium, ²⁸⁸Lv. This was an unexpectedly good result; the aim had been to experimentally determine the cross-section of a reaction with ⁵⁴Cr projectiles and prepare for the synthesis of element 120. It is the first successful reaction producing a superheavy element using an actinide target and a projectile heavier than ⁴⁸Ca.
The team at the Heavy Ion Research Facility in Lanzhou, which is operated by the Institute of Modern Physics of the Chinese Academy of Sciences, also plans to synthesise elements 119 and 120. The reactions used will involve actinide targets and first-row transition metal projectiles.

Naming

would call unbinilium eka-radium. The 1979 IUPAC recommendations temporarily call it unbinilium until it is discovered, the discovery is confirmed and a permanent name chosen. Although the IUPAC systematic names are widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, scientists who work theoretically or experimentally on superheavy elements typically call it "element 120", with the symbol E120, or 120.

Predicted properties

Nuclear stability and isotopes

The stability of nuclei decreases greatly with the increase in atomic number after curium, element 96, whose half-life is four orders of magnitude longer than that of any currently known higher-numbered element. All isotopes with an atomic number above 101 undergo radioactive decay with half-lives of less than 30 hours. No elements with atomic numbers above 82 have stable isotopes. Nevertheless, because of reasons not yet well understood, there is a slight increase of nuclear stability around atomic numbers 110–114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by University of California professor Glenn Seaborg, explains why superheavy elements last longer than predicted.
Isotopes of unbinilium are predicted to have alpha decay half-lives of the order of microseconds. In a quantum tunneling model with mass estimates from a macroscopic-microscopic model, the alpha-decay half-lives of several unbinilium isotopes have been predicted to be around 1–20 microseconds. Some heavier isotopes may be more stable; Fricke and Waber predicted ³²⁰Ubn to be the most stable unbinilium isotope in 1971. Since unbinilium is expected to decay via a cascade of alpha decays leading to spontaneous fission around copernicium, the total half-lives of unbinilium isotopes are also predicted to be measured in microseconds. This has consequences for the synthesis of unbinilium, as isotopes with half-lives below one microsecond would decay before reaching the detector. Nevertheless, new theoretical models show that the expected gap in energy between the proton orbitals 2f_7/2 and 2f_5/2 is smaller than expected, so that element 114 no longer appears to be a stable spherical closed nuclear shell, and this energy gap may increase the stability of elements 119 and 120. The next doubly magic nucleus is now expected to be around the spherical ³⁰⁶Ubb, but the expected low half-life and low production cross section of this nuclide makes its synthesis challenging.
Given that element 120 fills the 2f_5/2 proton orbital, much attention has been given to the compound nucleus ³⁰²Ubn* and its properties. Several experiments have been performed between 2000 and 2008 at the Flerov Laboratory of Nuclear Reactions in Dubna studying the fission characteristics of the compound nucleus ³⁰²Ubn*. Two nuclear reactions have been used, namely ²⁴⁴Pu+⁵⁸Fe and ²³⁸U+⁶⁴Ni. The results have revealed how nuclei such as this fission predominantly by expelling closed shell nuclei such as ¹³²Sn. It was also found that the yield for the fusion-fission pathway was similar between ⁴⁸Ca and ⁵⁸Fe projectiles, suggesting a possible future use of ⁵⁸Fe projectiles in superheavy element formation.
In 2008, the team at GANIL, France, described the results from a new technique which attempts to measure the fission half-life of a compound nucleus at high excitation energy, since the yields are significantly higher than from neutron evaporation channels. It is also a useful method for probing the effects of shell closures on the survivability of compound nuclei in the super-heavy region, which can indicate the exact position of the next proton shell. The team studied the nuclear fusion reaction between uranium ions and a target of natural nickel:
The results indicated that nuclei of unbinilium were produced at high excitation energy which underwent fission with measurable half-lives just over 10⁻¹⁸ s. Although very short, the ability to measure such a process indicates a strong shell effect at Z = 120. At lower excitation energy, the effect of the shell will be enhanced and ground-state nuclei can be expected to have relatively long half-lives. This result could partially explain the relatively long half-life of ²⁹⁴Og measured in experiments at Dubna. Similar experiments have indicated a similar phenomenon at element 124 but not for flerovium, suggesting that the next proton shell does in fact lie beyond element 120. In September 2007, the team at RIKEN began a program utilizing ²⁴⁸Cm targets and have indicated future experiments to probe the possibility of 120 being the next proton magic number using the aforementioned nuclear reactions to form ³⁰²Ubn*, as well as ²⁴⁸Cm+⁵⁴Cr. They also planned to further chart the region by investigating the nearby compound nuclei ²⁹⁶Og*, ²⁹⁸Og*, ³⁰⁶Ubb*, and ³⁰⁸Ubb*.
The most likely isotopes of unbinilium to be synthesised in the near future are ²⁹⁵Ubn through ²⁹⁹Ubn, because they can be produced in the 3n and 4n channels of the ^249–251Cf+⁵⁰Ti, ²⁴⁵Cm+⁵⁴Cr, and ²⁴⁸Cm+⁵⁴Cr reactions.