Unbinilium, also known as eka-radium or element 120, is the hypothetical chemical element in the periodic table with 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 and Russian teams to synthesize unbinilium are planned to begin in the mid-2020s.
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 and 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 (flerovium through oganesson) 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 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 (JINR) 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.
+ → * → no atoms
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
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:
+ → * → no atoms
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: + → * → no atoms
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.