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Nuclear physics

Heaviest element could have fascinating atomic and nuclear spectra

12 Feb 2018
A stamp issued in 2017 by Armenia post honours the discovery of oganesson by Yuri Oganessian and colleagues
Superheavy discovery: oganesson is named after the Russian-Armenian physicist Yuri Oganessian

In 2002, a team of Russian and American scientists created the first ever atom of oganesson, which is the heaviest chemical element ever recorded to date. With an atomic number of 118, oganesson filled the final gap in the seventh period of the periodic table as a noble gas. However, the chemical and physical properties of the new element have proven incredibly difficult to explore. Making oganesson atoms is an arduous process, and even when synthesized, they had a half-life of just around 1 µs.

A team of physicists led by Paul Jerabek at Massey University in New Zealand has now predicted the atomic structure of oganesson with the help of computer simulations. Their calculations take account of relativistic and quantum theories to explore the structures of the oganesson nucleus and its orbiting electrons, revealing fascinating properties that are not observed in lighter elements. Their work could make it easier for experimentalists to measure the properties of other superheavy elements.

Both the atomic electrons, and the protons and neutrons inside the nucleus, are arranged in shells. These shells can be calculated using “fermion localization functions”, which map the density of electrons and nucleons. For lighter atoms, these calculations will reveal electron shells that are dense and distinct, separated with low-density bands. In much heavier atoms, however, the calculation becomes more complicated.

Exotic structures

In their study, Jerabek’s team used fermion localization functions to simulate the exotic structures they believed would be seen in oganesson. For the electron structure, the researchers assumed that the large electrostatic forces originating from the highly positively-charged nucleus would cause electron energy levels to overlap, forcing electron shells to become much smoother and less well-defined. Electrons passing close to the nucleus would also need to move faster to escape its electrostatic forces, introducing the need for relativistic effects.

To account for these effects, Jerabek and his colleagues created an electron localization function for their simulations – a modified form of a fermion localization function. Using the function, they compared the electronic structures of lighter noble gases – xenon and radon – with that of oganesson. They found that the distinction between electron shells become almost imperceptible. Instead, the electrons form a uniform-density “gas” around the nucleus. Practically, this would make oganesson highly polarizable, meaning there would be strong Van der Waals forces between atoms.

Opposing the strong force

Inside an oganesson nucleus, Jerabek’s team predict that electrostatic forces between protons, combined with those from orbiting electrons, would act against the strong nuclear force keeping the nucleus together. Again, the researchers concluded that these effects would smooth out the clearly-defined shell structure. Constructing a separate nucleon localization function to account for these unusual properties, Jerabek’s team again mapped the structure of an oganesson nucleus, comparing it with the nuclei of xenon and radon. Much like the electrons, the superheavy nucleus forms a uniform-density gas of protons and neutrons.

As well as representing a significant advance towards understanding the atomic structures of superheavy elements, the work could inspire new theoretical research into the strange physical and chemical properties which emerge for elements at the bottom of the periodic table. As technology improves, it could also help experimentalists to create instruments that measure the properties of superheavy atoms.

The simulations are described in Physical Review Letters

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