At 150 years old, the periodic table of chemical elements is still growing. In 2016, four new elements with atomic numbers 113, 115, 117 and 118 were added to it: nihonium, moscovium, tennessine, and oganesson. It took a decade and worldwide effort to confirm these elements, and now scientists wonder: how far can the periodic table go? Some answers can be found in a paper published in the journal Nature Physics by Michigan State University’s Professor Witek Nazarewicz.
All elements with more than 104 protons are labeled as ‘superheavy,’ and are part of a vast, totally unknown land that chemists as well as nuclear and atomic physicists are trying to uncover.
It is predicted that atoms with up to 172 protons can physically form a nucleus that is bound together by the nuclear force. That force is what prevents its disintegration, but only for a few fractions of a second.
These lab-made nuclei are very unstable, and spontaneously decay soon after they are formed.
For the ones heavier than oganesson, this might be so quick that it prevents them from having enough time to attract and capture an electron to form an atom. They will spend their entire lifetime as congregations of protons and neutrons.
If that is the case, this would challenge the way scientists today define and understand ‘atoms.’
They can no longer be described as a central nucleus with electrons orbiting it much like planets orbit the Sun. And as to whether these nuclei can form at all, it is still a mystery.
Researchers are slowly but surely crawling into that region, synthesizing element by element, not knowing what they will look like, or where the end is going to be.
The search for element 119 continues at several institutions.
“Nuclear theory lacks the ability to reliably predict the optimal conditions needed to synthesize them, so you have to make guesses and run fusion experiments until you find something. In this way, you could run for years,” Professor Nazarewicz said.
If element 119 is confirmed, it will add an eighth period to the periodic table.
“The discovery might not be too far off. Soon. Could be now, or in two to three years. We don’t know. Experiments are ongoing,” the scientist said.
Another exciting question remains: can superheavy nuclei be produced in space?
It is thought that these can be made in neutron star mergers, a stellar collision so powerful that it literally shakes the very fabric of the Universe.
In stellar environments like this where neutrons are abundant, a nucleus can fuse with more and more neutrons to form a heavier isotope. It would have the same proton number, and therefore is the same element, but heavier.
The challenge here is that heavy nuclei are so unstable that they break down long before adding more neutrons and forming these superheavy nuclei. This hinders their production in stars.
The hope is that through advanced simulations, researchers will be able to see these elusive nuclei through the observed patterns of the synthesized elements.
As experimental capabilities progress, they will pursue these heavier elements to add to the remodeled table.
In the meantime, they can only wonder what fascinating applications these exotic systems will have.
“We don’t know what they look like, and that’s the challenge. But what we have learned so far could possibly mean the end of the periodic table as we know it,” Professor Nazarewicz said.
Witold Nazarewicz et al. 2018. The limits of nuclear mass and charge. Nature Physics 14: 537-541; doi: 10.1038/s41567-018-0163-3