by Kit Chapman
For now, the element hunters seem to have a path forward. Everybody expects that elements 119 and 120 will emerge in the next five years. Beyond that, nobody knows. And that raises one last question. How far does the periodic table go before it breaks … or are we there already?
Notes
1 It would be convenient if the elements had been discovered in numerical order, but they weren’t. In summary, the order was: 114, 116, 115, 113, 118 and 117. Don’t be surprised if element 120 is discovered before element 119.
2 Despite the name, the fridges usually contain packed lunches; the team don’t drink while operating the particle accelerator these days – although there used to be a monitor next to the fridge to check on the beam …
3 Quantum tunnelling is a key part of understanding the atom, but delves into the strange world of quantum physics and is too complex to go into here.
CHAPTER TWENTY ONE
Beyond Superheavy
In 2017 a paper emerged from Massey University in New Zealand and Michigan State University in the US. It marked the end of chemistry as we know it.
Element 118, oganesson, it claimed, does not have electron shells.
Oganesson sits in the group of the periodic table occupied by the noble gases: the likes of helium, neon and argon. These, chemistry students are taught, have filled electron shells, meaning they don’t usually interact with things. But using state-of-the-art simulations, the New Zealand and US team compared the classic, expected appearance of the noble gases, and then a model that took into account relativistic effects. As the nuclei became heavier, the effects became more pronounced. By the time the model reached oganesson, the supposed electron shells are more like electron soup. It’s called a Fermi gas, after the man who first imagined it could exist. Even today, Enrico Fermi’s brilliant mind is being proved right.
If true, oganesson means the periodic table stops being relevant in terms of predicting properties. Its neat rows and patterns break. The electrons would be easy to polarise, meaning that oganesson would be reactive. It would form compounds with other elements and molecules easily. It wouldn’t even be a gas at room temperature – it would probably be a solid. Sadly, only five or so atoms of oganesson have ever been created, and it is so unstable it lasts for less than a millisecond. That’s too small an amount, and too little time, to test the theory.
One of the authors of the oganesson paper was Witold Nazarewicz. He’s one of the world’s leading physicists, who has gone from a PhD in his native Poland to chief scientist of the University of Michigan’s Facility for Rare Isotope Beams. When it starts up in 2021, the hope is that it will be able to produce neutron-rich radioactive beams that will allow researchers to find a way to get closer to the island of stability.
Nazarewicz says the strangeness around oganesson is only the beginning. Eventually, the periodic table is probably going to collapse. ‘We can only calculate nuclear properties using the best current models,’ Nazarewicz explains. These are the same models that first said the periodic table would end at element 100; that the nucleus was like a drop of water; that it had shells and magic numbers. The models change according to the best evidence available, with each iteration becoming more accurate and precise – even if it takes us far outside our comfort zone. ‘We can calculate,’ Nazarewicz adds. ‘And theorists should be allowed to calculate things that are very exotic.’
It’s these calculations that turn everything on its head. Using a combination of nuclear surface effects, quantum mechanics and Coulomb repulsion, Nazarewicz and his colleagues are mapping out the shape a nucleus will twist itself into while trying to hold onto its protons. There are a host of ideas about how the heaviest atoms may contort – nuclei stretching out, folding in on themselves, even warping into a doughnut shape with a hole in the middle. By the time you get to the elements around 140 or so, the latest models suggest that things become very strange. ‘The heavier you are, the more unstable [things become],’ Nazarewicz says. ‘Coulomb repulsion means you might have to deal with exotic topologies. Proton density might create a hole, or even a void. We don’t know. There will be nuclides – proton–neutron cluster systems – but they might not have electrons.’
It’s an astonishing claim. The very definition of a new element is based on the time a nucleus takes to attract electrons. Instead of having new elements, you’d suddenly have a section of the periodic table that would be … blank?
‘There would be nothing!’ Nazarewicz confirms. ‘There would be missing windows, there would be gaps [in the table]. Elements are basically for chemists. There’s no chemistry without electrons. You’ll have a nucleus but no atom. The end of chemistry. It might be that chemistry will die, and then suddenly, something may appear with extra stability. And if it lives a long time, and it attracts electrons … Hey! Chemistry is reborn! It’s so fantastic that one is afraid to think about it. But it might be there!’
Don’t worry just yet: the periodic table isn’t going anywhere. These models are predictions far ahead of where we are in terms of element hunting and might not even apply – at the moment, they are just theory. Eric Scerri, a chemist and philosopher of science at the University of California, Los Angeles, thinks the periodic table is safe for now. ‘It’s still going to be relevant,’ he argues. ‘I’m reluctant to make changes. Relativistic effects change things, but when it comes to how to organise the periodic table, I don’t think we should build in any major contortions. It’s an approximation.’
The periodic table, as Scerri points out, bases itself on more than just an element’s properties. Modern chemists follow something called the Madelung rule, which uses energy levels to predict which subshells orbiting an atom an electron will join. The rule has its exceptions, but is still the most general way we have to deduce how the periodic table as a whole should be organised.
For elements 119 and 120, it’s obvious where they should go: at the bottom of group 1 and group 2, kicking off a new row of the periodic table. The problem is element 121 and beyond. Here, in the rows above, are where the lanthanides and actinides start – those elements sat in their own section at the bottom of the periodic table. If you follow the standard pattern of how the elements are laid out, 121 would sit under the actinides – a ‘super actinide’ group. The table would then follow the pattern it’s always done. According to the latest definitions, anything heavier than element 126 will be a ‘beyond superheavy’ element.
It’s not the only option, however. Pekka Pyykkö, from the University of Helsinki, is one of the leading theorists on where the periodic table will end. His own model has 121, starting another box cast off from the main periodic table, split away from even the lanthanides and actinides. This would involve an entirely new electron shell type – very compact and deep inside the atom. For Pyykkö, elements 139 and 140 would be part of the main table, while element 141 onwards would form a new row placed under the actinides. ‘It’s going to be a mess,’ Pyykkö admits. ‘But this will give you, from a chemical point of view, a possible periodic table.’
The problem with any of these models is that nobody really knows what’s going to happen. We don’t know where the periodic table will end. We don’t know how it should be arranged. We don’t even know if our current table is right. Currently, the best models say that there are 172 elements. Some argue for 173; others fewer. Some physicists don’t see why we should even stop at all. Ask three theorists, and you’ll get three different theories.
‘Element 172 is a chemist’s calculation,’ points out Mark Stoyer. ‘But once you reach the massive size of an atom that element 172 would be, the innermost electrons are very relativistic. They gain mass, so are heavier, so the orbit contracts so much those electrons spend a significant fraction of time actually inside the nucleus. Amazing when you think about it – electrons whizzing around inside the nucleus.’
Stoyer’s right – it would be loosely the equivalent of the Earth orbiting through the Sun. This is a problem even the world’s fas
test computers, such as Oak Ridge’s Summit, can’t solve. ‘We can completely calculate effects only for very light nuclei,’ Stoyer continues. ‘After carbon [element 6] we start to run out of steam. Anything larger is intractable, even given the biggest supercomputer. This makes superheavy elements so fascinating!’
For now, the debates will remain theoretical. Already, we’ve come a long way from Antoine Lavoisier’s list of elements, or the table as put together by Dmitri Mendeleev or Henry Moseley. As Nancy Stoyer pointed out earlier, the periodic table is a living construct. It is constantly made, broken and made again. While we think of it as relatively static because it sits on our wall, in truth it’s a malleable tool: it’s a guidebook to our chemical universe – and guidebooks get updated.
‘And what if Nazarewicz is right?’ I ask Scerri. What if there is a region of the periodic table where elements, as we understand them, simply don’t exist?
‘Practically speaking?’ he sighs, thinking of the potential arguments among chemists and physicists. ‘All hell would break loose.’
* * *
This theoretical work is more than just a thought experiment. It’s explaining the fundamental rules of our universe. These, in turn, help us to make breakthroughs in astrophysics, computing, nanomachines, energy and medicine. But the only way we can really show what’s going on is by getting there. Making these elements. And that’s where Yuri Oganessian and the other element hunters come in.
It’s why I’ve come back to Dubna. Back, one last time, to JINR. Oganessian has no plan to stop making elements. As with Seaborg, Flerov and Ghiorso, there is something that drives him to still pick at the edges of the periodic table into his mid-eighties. Oganessian’s search takes many forms. He’s still obtaining meteorites to check for traces of superheavy elements, hoping to find the telltale marks in olivine crystals that prove an impact from something heavier than uranium. ‘They’re like a flying lab,’ he muses. ‘For detecting superheavies, they are ideal.’ Now the superheavy elements are better understood, he’s also able to make better predictions about where they may exist (or may have existed) in nature. Looking in hot springs, it turns out, was all wrong – if their traces are anywhere on Earth, it will likely be at the poles, falling from the sky as cosmic rays that became trapped in ice. But Oganessian’s real weapon is a short walk from his office, down the snowy boulevards of Dubna.
‘Titanium-50 has two more protons, but the same quantity of neutrons, as calcium-48,’ Sergei Dmitriev explains, outlining the Russian plan. ‘The stability of the compound nucleus will be lower, and the cross section decreases at least one order of magnitude. When we produce element 118 [at the moment], we produce about one nucleus a month. If it’s an order of magnitude less, it would be one nucleus every 10 months. We’re not so rich as to be able to spend that much time with a cyclotron.’ That’s why, while RIKEN have chosen to upgrade their equipment to hunt for elements 119 and 120, the JINR team have decided to build an entirely new cyclotron. We’re standing in the room I last saw on a monitor in Dmitriev’s office. Here, standing in the room as machine parts are pressure-hosed down with acetone, I can appreciate what this will become: the DC-280 cyclotron. The Russians call it the Superheavy Element Factory. When it comes online, it will be the most formidable element-hunting machine the world has ever seen.
Oganessian walks up to it, places his hand on the metal and gives it an affectionate pat. His new toy has had quite the journey. Parts have come from Russia, the US, the Czech Republic, Bulgaria, Romania and Slovakia. The main magnet came from Ukraine, just as the eastern fringes of the country became embroiled in a civil war: the 1,100t prize had to be transported on the back of a train through the conflict zone to make it to JINR. Nikolay Aksenov tells me that he remembers phoning up the Ukrainian factory to check the progress and hearing gunfire down the end of the line. He can laugh about it today; neither he nor the factory staff were laughing at the time.
‘This is like Pandora’s box,’ Oganessian says. ‘A new facility. A new accelerator. It’s 10 times more powerful than the existing [machine]. The intensity will be 10 times more. Another technical achievement is the energy variation. I mean … I designed it for superheavy element research.’
As with any new piece of scientific equipment, the cyclotron will be tested first, going over known nuclides and explored decay chains to make sure it works. Then, gradually, it will ramp up the quantity of production. ‘The first step will be a factor of 10,’ Oganessian says above the pressure hose. ‘Then a factor of 100. So, if the species [elements we are making] is 114 or 115, and we’re producing 1 atom a day at the moment, this will produce 100 atoms a day. It’s not much, but there’s some mass. We can really check and find out the properties. With the new set-up, we can do so much more!’
The new machine is why the Russians have so much faith in their titanium-50 beam. Even with the drop-off in cross sections compared with calcium-48, you’ll still be able to increase your chance of finding a new element by a factor of 10. It is arguably the world’s best hope for making elements 119 and 120. It’s also far more. If (or when) the Superheavy Element Factory works, the elements will finally be produced on a scale that will allow proper chemistry. Can you truly call an element that only exists for atoms at a time, that vanishes in less than a thousandth of a second, something you’ve discovered? The Superheavy Element Factory will end those debates.
In the next 10 years a whole new collection of elements will be open to investigation. Who knows what we will find. Perhaps the superheavy elements will find their place in the world; perhaps they will just help us understand it. Either way, they will stop being just ‘that weird bit at the bottom of the periodic table’.
I turn to Oganessian. Despite his discoveries, despite the advances the heaviest elements have brought in decoding our universe, there’s still the greatest question of all. After 60 years in superheavy research, what drives him to continue?
Oganessian shrugs. ‘If you have a device that can do this, why not?’
Epilogue
The Royal Society sits at the heart of London, UK, a stone’s throw from Buckingham Palace. It’s one of the most prestigious scientific institutions in the world. Its Charter Book, containing the signatures of every fellow since its foundation in 1660, is a formidable list of names: Isaac Newton, Michael Faraday, Stephen Hawking. The atomic and elemental discoverers also grace its rolls. Look through its pages and you’ll find the signatures of Ernest Rutherford, Enrico Fermi, Lise Meitner and Glenn Seaborg (women weren’t allowed to be made fellows until 1945, so Marie Curie isn’t present). The Society’s main reception halls are upstairs; here, the greatest minds of each generation gather to pay their respects to the giants of their age.
Tonight, the room has filled to honour Yuri Oganessian.1 It’s an inauspicious time for such a gathering: only a week earlier, former spy Sergei Skripal and his daughter Yulia were poisoned in Salisbury. The international community has pointed the finger squarely at the Russian government, and the UK Foreign Office has been advised to keep the meeting as low-key as possible. Trouble is also brewing in the superheavy community, this time over whether there was enough evidence to confirm that the Dubna–Livermore team discovered element 117. Nobody really doubts that they found tennessine; it’s an argument about the semantics of what counts when it comes to element discovery. Even so, bad blood stirs once again. When the Russian team were confronted with the doubts at a conference, they rose in unison and walked out.
But this isn’t the time. It’s a moment to breathe, reflect and celebrate.
The element hunters’ creations shaped our world. Their names have become legend. It’s time for a Charter Book of their own.
Edwin McMillan went on to lead Berkeley Lab until 1974. In 1984 he suffered the first of a series of strokes and eventually passed away from complications of diabetes in 1991, aged 83. You can see his Nobel Prize medal at the National Museum of American History in Washington, DC.
Phil Ab
elson became interested in naval power. In 1946 he published a report advocating for nuclear-powered submarines – now a standard branch of navies across the world. He died in 2004.
The leading element discoverer of all time remains Al Ghiorso. Never willing to sit back and enjoy retirement, Ghiorso continued to work at Berkeley Lab into old age. The last of the original element hunters, he died in 2010, aged 95. His wife Wilma, who had conspired with Helen Seaborg to get Al on the team, died in 1995.
Kenneth Street returned to Berkeley and eventually became the lab’s deputy director. He died in 2006.
Gregory Choppin went on to teach at Florida State University, where the chemistry professorship is named in his honour. He died in 2015.
Bernard Harvey had a long and successful career at Berkeley. He died in 2016.