Exploring the superheavy elements at the end of the periodic table

Relativity comes into play because the sheer mass of these large nuclei accelerates the electrons that inhabit thier spherical orbits. As a result, these electrons are stabilized and harder to remove. Meanwhile, electrons in nonspherical orbitals become destabilized because the contracted spherical orbitals act as stronger shields around the nucleus, decreasing the interaction between the nucleus and electrons in nonspherical orbitals. Another consequence is spin-orbit splitting, which means that the p orbitals of the SHEs do not all have the same energy but split into two energy levels, changing the order in which electrons fill those orbitals.

These relativistic effects are why some researchers think the organization of the periodic table might need some tweaking. The superheavies might not behave like their lighter homologs. And that’s one of the reasons that scientists are so keen to understand the properties of the elements they have created.

It could be that the properties of the SHEs do not match those of elements above them in the periodic table, says Michael Block, who studies the physical characteristics of the new elements at GSI. Some researchers around the world argue about how the periodic table should be arranged as the periodicity of the table breaks down. Should the organizing factor be proton number, electronic structure, or the elements’ properties?

Arranging the periodic table by elemental properties is tempting, but it requires chemists to characterize those properties. For SHEs, that means making isotopes that are stable enough to characterize. Scientists haven’t done any chemistry experiments on them yet because either the isotopes don’t last long enough, or stable isotopes with more neutrons in them are difficult to make. An isotope must last for more than a second, Düllmann says, for scientists to probe the properties of an element or of its compounds.

So how do scientists characterize these elements? One approach is simple gas-phase chromatography. And although the element is gone in a flash, planning the experiment and analyzing data can take a lot of time. “For our first experiment on 114, we observed two atoms,” he says. It took “2 days to extract the value, and then we spent 2 years to put an error bar on it.”

After a target element comes out of the accelerator, air or a specific gas captures and funnels it through a capillary toward an array of solid-state detectors made usually with gold or quartz surfaces. The gas carries the SHE nucleus across the surface until the element adsorbs. Researchers can measure where the element is adsorbed by observing its decay. Comparing the adsorption positions of SHE nuclei and the nuclei of lighter elements in the same periodic table group allows the researchers to understand how the SHE interacts with the surface.

For example, in one experiment, radon travels to the end of a set of detectors, but lead doesn’t travel far at all. Element 114, flerovium, travels just a bit farther along the detector than lead but not as far as radon, suggesting that it bonds to a gold surface as its homolog lead does but that the resulting bond is weaker (Inorg. Chem. 2014, DOI: 10.1021/ic4026766). Aksenov and Steinegger have some results from similar experiments with element 113, nihonium, but its properties have proved trickier to pin down. In their most recent experiments, the element never made it to the detectors. Instead, they say, it seems to have adsorbed to the capillary that linked the accelerator to the detectors (Eur. Phys. J. A2017, DOI: 10.1140/epja/i2017-12348-8).

Researchers also want to understand more basic questions about the SHE nuclei, such as their mass and shape and how those properties affect their electronic structure.

In November 2018, researchers at the Lawrence Berkeley National Laboratory announced the first direct measurements of the mass numbers for elements 113 and 115, nihonium and moscovium (Phys. Rev. Lett. 2018, DOI: 10.1103/PhysRevLett.121.222501). That’s important, says Jackie Gates, who was part of that team, because “although the elements of flerovium to oganesson, 114 through 118, have been added to the periodic table, we have not experimentally confirmed either the number of protons or the number of neutrons in the systems.”

Scientists often estimate the mass of short-lived SHEs by adding up the decay products of the new elements. The Berkeley team used a newly built piece of equipment designed and run by Gates. The machine is known as FIONA, an acronym that means “For the Identification of Nuclide A,” where “A” represents the element’s mass number. The researchers added FIONA to the Berkeley gas-field separator that isolates the newly created elements from the unwanted reaction products in the facility’s beam.

FIONA first traps the SHE in a small volume of about 1 mm3 and then accelerates the SHE into magnets that deflect the moving ion and send it toward a detection station. When the SHE decays on the detector, it gives away its position. Researchers can use that position to work out what the nucleus’s trajectory was. Because the trajectory is linked to the nucleus’s mass-to-charge ratio, knowing where the nucleus is when it decays tells you the mass.

“If I had just one atom of lead sitting somewhere,” Gates says, “I’m not certain how I would determine where that atom is.” But, because the superheavies decay, she says, all that the chemists who hunt them need is a detector.

Scientists are developing other techniques to try to understand the nuclear and electronic structure of SHEs. For example, using pulsed laser systems to reveal exactly how finely split the energies of the elements’ different orbitals are and in turn help researchers understand the superheavy nuclear structures in more detail. X-ray spectroscopy experiments can also be done on these short-lived elements to confirm the atomic number.

Uncovering the properties of these SHEs might not grab the same number of headlines as does discovering a new element, but the field is clearly invested in better characterizing them. The new SHE factory at the FLNR devoted to superheavies research is just one sign of that investment.

Back in Dubna, Steinegger and Aksenov are excited by not just what they might learn with the new device but also how other collaborators might come and use it. “We have so much work to do,” Aksenov adds. “We have to characterize all these elements!”

Düllmann agrees and argues that this basic research has great value, without the need to find applications. Applications will come, but they aren’t the focus of the field.

And what about pushing the periodic table’s limits and discovering elements 119 or 120? Scientists are developing new methods to try to make them, but researchers expect the elements will be too short lived for anyone to do much chemistry with them. Such elements might introduce g orbitals that will be interesting for chemists to explore, but, Düllmann says “We’ll be long dead by the time that’s possible.”