Why are synthesized chemical elements unstable

For the first time connection between a superheavy element and carbon established

The effect of the theory of relativity on chemistry can now be examined more closely

19.09.2014

At the Japanese RIKEN Nishina Center, an international team of researchers led by groups from Mainz and Darmstadt has successfully synthesized a chemical compound between a superheavy element and carbon for the first time. For this purpose, seaborgium (element 106) was artificially produced and combined with carbon monoxide. Eighteen seaborgium atoms reacted with carbon monoxide to form seaborgium hexacarbonyl, a compound in which six carbon monoxide molecules each bond to a seaborgium atom. The scientists examined the gas phase properties and the adsorption behavior on a silicon dioxide surface and compared the results with those of the hexacarbonyl complexes of molybdenum and tungsten, which are in the same group of the periodic table and therefore suggest similar chemical properties. The work has opened up new perspectives in order to investigate the chemical properties of the elements at the end of the periodic table in much more detail than before and thus to investigate the influence of the theory of relativity, which is most pronounced in the heavy elements. The new results were published in the journal Science released.

Chemical experiments with superheavy elements - with atomic numbers beyond 104 - represent a major challenge. First, the element to be investigated must be artificially produced in a particle accelerator. The production rates are at most a few atoms per day, and even less for the heaviest elements. In addition, the atoms are unstable: In the current work, the lifespan was only about ten seconds. Despite the great effort, science is very interested in the study of the superheavy elements because it enables a test of the influence of Einstein's theory of relativity on chemistry. The many positively charged protons in the atomic nucleus of the "superheavies" accelerate the electrons in the atomic shell to high speeds - up to around 80 percent of the speed of light. According to the theory of relativity, this makes the electrons heavier than if they were at rest, which affects their location in the atomic shell and consequently the chemical properties. This is examined in comparison with homologous elements that have a similar structure in their atomic shell and are in the same group of the periodic table. Such studies provide access to the fundamental pillars of the periodic table of the elements, the basic arrangement of the elements for chemists around the world.

Against this background, a search for new systems in which relativistic effects are clearly expressed has been looking for a number of years. In preparation for the current experiments, the super-heavy element chemistry working groups at the Institute for Nuclear Chemistry at Johannes Gutenberg University Mainz (JGU), the Helmholtz Institute Mainz (HIM) and the GSI Helmholtz Center for Heavy Ion Research (GSI) in Darmstadt developed in collaboration with Swiss colleagues from the Paul Scherrer Institute, Villigen, and the University of Bern developed a novel experimental method. First test experiments at the research reactor TRIGA Mainz were particularly successful with short-lived molybdenum atoms. The method was further developed at the University of Bern and in accelerator experiments at the GSI. Dr. Alexander Yakushev from the GSI group explains: "A major challenge in such experiments is the accelerator's intense ion beam, which also destroys moderately stable chemical compounds. To prevent this, tungsten atoms, the heavier brothers of molybdenum, were first placed in the gas-filled TASCA Separator separated from the beam at GSI. The chemistry experiments were then carried out behind TASCA, under ideal conditions for investigating the new compounds. " The focus was on the formation of hexacarbonyl complexes. Theoretical work that began in the 1990s predicted that seaborgium should form such complexes with relatively high stability. The seaborgium is bound to the six carbon monoxide molecules by metal-carbon bonds, which are also typical for organometallic compounds. Many such compounds have the desired bonding situation that heavy-element chemists had long dreamed of.

The heavy elements group at the RIKEN Nishina Center (RNC) in Japan optimized the production of the seaborgium in the nuclear fusion of a neon ion beam (element 10) with a curium target (element 96) and the separation of the seaborgium in their gas-filled separator GARIS. Dr. Hiromitsu Haba, head of the team at RIKEN, explains: "In the conventional approach to the production of superheavy elements, the unequivocal detection of individual atoms of the superheavy elements such as the seaborgium is often made impossible by many undesirable reaction products To measure seaborgium and thus its rate of production and its decay properties. GARIS thus opened up the possibility of tackling new types of chemical studies with seaborgium. "

In 2013, the two teams, together with colleagues from Switzerland, Japan, the USA and China, conducted experiments at the RNC to determine whether they could synthesize compounds such as seaborgium hexacarbonyl. After two weeks of experimenting around the clock, during which the German chemical equipment was coupled to the Japanese separator, the team had detected eighteen seaborgium atoms that could be transported as volatile carbonyl complexes in the gas stream. The gas phase properties as well as the adsorption behavior of the complex on a silicon dioxide surface were investigated and were similar to those of the hexacarbonyls of Seaborgium's lighter homologues molybdenum and tungsten. These are very characteristic compounds of the elements in the sixth group of the periodic table. The measured properties are in agreement with theoretical calculations in which the effects of relativity are taken into account.

Dr. Hideto En'yo, Director of the RNC, explains: "The breakthrough that was achieved in this experiment would have been impossible without the close cooperation of the fourteen research centers around the world." HIM director Prof. Dr. Frank Maas says: "The experiment represents a milestone in the chemical investigation of superheavy elements. The researchers showed that many new compounds of these elements are within reach of the novel experimental techniques. The perspectives that are open to the investigation of the chemical bond, not only in the super heavy elements, open up, are fascinating. "

After this first successful step towards more detailed studies of the superheavy elements, the team is already making plans for further studies of other new compounds, including even heavier elements than seaborgium.