Oganesson

• 1. Introduction

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  Oganesson, element 118 with the symbol Og, is the heaviest atom known, and its namesake was unimpressed by the honor. In a 2019 interview, the physicist Yuri Oganessian was asked how it felt to see his name in the periodic table beside Einstein, Mendeleev, the Curies and Rutherford. He answered, "Not like much!" Naming a discovery after its finder is customary in science, he explained, and mathematics and medicine are full of such names, from theorems to Alzheimer and Parkinson.

  The element itself is just as hard to pin down. Its only known isotope, oganesson-294, has a half-life of 0.7 milliseconds, far too brief for chemical studies. Apart from its nuclear properties, nothing about oganesson has ever been measured. Making it is extremely limited and expensive, and the atoms vanish almost at once.

  So how do scientists find an atom that exists for less than a thousandth of a second? Who first imagined such an element, and who wrongly claimed to have found it? And what do theorists expect it to be like, if anyone could ever hold it?

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• 2. Predicted Before Discovered

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  Helium, neon, argon, krypton, xenon and radon were the known noble gases, and chemists began wondering about a seventh almost as soon as the family was recognized. In April 1895, a year after argon was discovered, the Danish chemist Hans Peter Jørgen Julius Thomsen forecast a whole series of inert gases that would bridge the halogens and the alkali metals. He expected the seventh gas of the series to end a 32-element period containing thorium and uranium. He gave it an atomic weight of 292, close to the 294 of the isotope eventually made.

  In 1922, the Danish physicist Niels Bohr noted that this seventh noble gas should have atomic number 118. He also predicted its electron arrangement as 2, 8, 18, 32, 32, 18, 8, which matches modern predictions.

  The German chemist Aristid von Grosse followed in 1965 with an article on the likely properties of element 118. Ten years later, in 1975, the American chemist Kenneth Pitzer suggested that element 118 should be a gas or a volatile liquid, because of relativistic effects.

  From Thomsen's forecast to the first successful synthesis of the element would pass 107 years.

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• 3. The Fabricated Discovery

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  In late 1998, the Polish physicist Robert Smolańczuk published calculations on fusing atomic nuclei to build superheavy atoms. His numbers suggested that fusing lead with krypton under carefully controlled conditions could produce element 118. The fusion probability, known as the cross section, would be close to that of the lead and chromium reaction that had made element 106, seaborgium. That ran against the expectation that cross sections for lead or bismuth targets fall exponentially as the product's atomic number rises.

  In 1999, researchers at Lawrence Berkeley National Laboratory relied on those predictions and announced the discovery of elements 118 and 116. The claim appeared in a paper in Physical Review Letters, and the results were reported in Science very soon after.

  By 2001, the Berkeley team had published a retraction. Researchers at other laboratories could not duplicate the results, and the Berkeley lab could not either. In June 2002, the lab's director announced that the original claim rested on data fabricated by the principal author, Victor Ninov. Newer experiments and calculations have since confirmed that cross sections do fall exponentially with lead and bismuth targets.

  In that same year, 2002, a different team recorded the first genuine decays of element 118.

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• 4. The Dubna Experiments

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  At the Joint Institute for Nuclear Research in Dubna, near Moscow, a team headed by Yuri Oganessian watched atoms of element 118 decay. Oganessian is a Russian nuclear physicist of Armenian ethnicity. His group included American scientists from Lawrence Livermore National Laboratory in California. The reaction fired calcium-48 ions at a target of californium-249. Fusion was so unlikely that the first event believed to be oganesson took four months and an enormous dose of calcium ions.

  A coincidence of energies kept the result quiet. The decay energy of oganesson-294 matched that of polonium-212m, a common impurity in experiments aimed at superheavy elements. The announcement waited for a confirmatory experiment in 2005. That run used a beam energy of 251 MeV instead of 245 MeV, and a target thickness of 0.34 instead of 0.23 milligrams per square centimeter.

  On the 9th of October 2006, the researchers announced that they had indirectly detected three, possibly four, nuclei of oganesson-294. One or two came in 2002 and two more in 2005. Alpha decay was observed in three atoms, and a fourth decay by spontaneous fission was also proposed. From these events a half-life of 0.89 milliseconds was calculated, with a large uncertainty because so few atoms were seen.

  The team was highly confident that the detections were not a false positive. They estimated the chance that the events were random at less than one part in 100000. To check the identification, they created the putative daughter nucleus directly in a separate bombardment and confirmed that its decay matched the chain. That daughter is very unstable, decaying with a lifetime of 14 milliseconds.

  In 2011, IUPAC evaluated the Dubna-Livermore results and concluded: "The three events reported for the Z = 118 isotope have very good internal redundancy but with no anchor to known nuclei do not satisfy the criteria for discovery."

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• 5. Recognition and Heavier Cousins

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  In December 2015, the Joint Working Party of IUPAC and IUPAP recognized element 118 and gave priority to the Dubna-Livermore collaboration. Two things had changed since the cautious verdict of 2011. In 2009 and 2010, Lawrence Berkeley National Laboratory confirmed properties of the granddaughter nucleus, flerovium-286. In 2012, the Dubna group saw another consistent decay chain of oganesson-294.

  That 2012 experiment was not even aiming for oganesson. Its goal was tennessine-294, made from berkelium-249 and calcium-48. The short half-life of berkelium-249 meant that much of the target had decayed to californium-249, so oganesson appeared instead of tennessine.

  From the 1st of October 2015 to the 6th of April 2016, the Dubna team fired calcium-48 projectiles at a mixed californium target holding californium-249, californium-250 and californium-251. They hoped to make the heavier isotopes oganesson-295 and oganesson-296, and they tried beam energies of 252 and 258 MeV. Only one atom appeared, at the lower energy, and its decay chain fit the known one for oganesson-294. The experiment stopped when glue from the sector frames covered the target and blocked reaction products from reaching the detectors.

  The same reaction could also yield oganesson-293 and its daughter livermorium-289, and even oganesson-297. Fusing curium-248 with titanium-50 might produce oganesson-295 and oganesson-296. Researchers hope such heavier, likely more stable isotopes could be used to probe oganesson's chemistry. A 2017 search at RIKEN using the curium and titanium reaction was unsuccessful.

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• 6. Naming the Element

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  Following Mendeleev's habit for undiscovered elements, element 118 was long called eka-radon, and until the 1960s eka-emanation, since emanation was the old name for radon. In 1979, IUPAC assigned the placeholder name ununoctium, with the symbol Uuo, to be used until a discovery was confirmed. Chemistry classrooms and advanced textbooks adopted it. Working scientists in the field mostly ignored the recommendation and said "element 118", writing E118, (118), or simply 118.

  Berkeley had planned a name of its own. Before the 2001 retraction, its researchers intended to call the element ghiorsium, with the symbol Gh, after Albert Ghiorso, a leading member of their team.

  In 2007, the head of the Russian institute said the Dubna team was weighing two names. One was flyorium, for Georgy Flyorov, the founder of the Dubna laboratory. The other was moskovium, for the Moscow Oblast where Dubna is located. He argued that although American collaborators supplied the californium target, the Flyorov Laboratory of Nuclear Reactions was the only facility in the world that could achieve the result. Those names landed elsewhere. Flerovium became element 114. Livermorium was the final proposal for element 116, and moscovium was later proposed and accepted for element 115.

  Helium is the one noble gas whose name does not end in "-on", because it was not known to be a noble gas when discovered. The IUPAC guidelines in force at the time of approval required every new element to end in "-ium", even a halogen or a noble gas. The placeholder ununoctium followed that rule. A new IUPAC recommendation in 2016 called for the "-on" ending on new group 18 elements, whatever their chemistry turns out to be.

  On the 23rd of March 2016, the scientists behind elements 115, 117 and 118 held a conference call to decide their names. Element 118 was settled last. After Oganessian was asked to leave the call, the others unanimously chose "oganesson" in his honor. He had been a pioneer of superheavy element research for sixty years, reaching back to the field's foundation. His team and his proposed techniques had led directly to elements 107 through 118.

  Mark Stoyer, a nuclear chemist at Lawrence Livermore, later recalled: "We had intended to propose that name from Livermore, and things kind of got proposed at the same time from multiple places. I don't know if we can claim that we actually proposed the name, but we had intended it."

  In internal discussions, IUPAC asked the Dubna institute whether it wanted the spelling "oganeson", closer to the Russian. Oganessian and the institute refused. They cited the Soviet-era practice of transliterating names into the Latin alphabet under French rules, which produced "Oganessian". They also argued that "oganesson" would be easier to link to the person.

  In June 2016, IUPAC announced that the discoverers planned to name the element oganesson, with the symbol Og. The name became official on the 28th of November 2016. In 2017, Oganessian said, "For me, it is an honour." He noted that his colleagues proposed the name. His daughter, whose family had lived in the United States for decades, wrote that she had not slept the night she heard, because she was crying.

  The naming ceremony for moscovium, tennessine and oganesson was held on the 2nd of March 2017 at the Russian Academy of Sciences in Moscow.

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• 7. The Edge of Stability

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  Curium, element 96, marks a cliff in nuclear stability. Its most stable isotope, curium-247, has a half-life four orders of magnitude longer than that of any later element. Every nuclide above atomic number 101 decays with a half-life shorter than 30 hours. No element above lead, number 82, has a stable isotope. The electrical repulsion between protons keeps growing, and the strong nuclear force cannot hold such a nucleus together against spontaneous fission for long.

  In the 1960s, researchers proposed a rescue. Calculations suggested that elements with more than 104 protons should not exist at all. But closed nuclear shells around 114 protons and 184 neutrons might counter that instability. The result would be an island of stability, where nuclides could have half-lives of thousands or millions of years. Nobody has reached the island. Yet the existence of oganesson and its fellow superheavy elements confirms the stabilizing effect is real. Known superheavy nuclides grow exponentially longer-lived as they approach the island's predicted location.

  Oganesson-294 decays by alpha decay and spontaneous fission, and its lifespan still exceeds some theoretical predictions. A quantum-tunneling model predicts several heavier isotopes with alpha-decay half-lives close to 1 millisecond.

  Theoretical work on synthetic pathways points to isotopes slightly more stable than the one made so far. The likeliest are oganesson-293, 295, 296, 297, 298, 300 and 302. The last of these, oganesson-302, reaches the 184-neutron shell closure. Oganesson-297 may offer the best chance of longer-lived nuclei, and it might become the focus of future work. Some isotopes with many more neutrons, around oganesson-313, could also be longer-lived.

  Isotopes from oganesson-291 to oganesson-295 might also turn up as daughters of element 120. Those parents could be reached by fusing californium-249 to californium-251 with titanium-50, or curium-245 or curium-248 with calcium-48.

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• 8. Predicted Chemistry

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  The electron arrangement 7s2 7p6 would give oganesson a completely filled outer shell of eight electrons, the signature of group 18. Members of this group usually resist common reactions such as combustion, because a filled outer shell binds its electrons tightly. Some therefore expect oganesson to resemble radon, the noble gas directly above it. Following the periodic trend, it should be slightly more reactive than radon.

  Calculations say it could be far more reactive. It may even exceed flerovium and copernicium, the heavier relatives of the chemically active lead and mercury. The cause is an energetic destabilization and radial expansion of the last occupied 7p subshell, driven by strong spin-orbit interactions between the 7p electrons and the inert 7s electrons.

  Unlike the other noble gases, oganesson is predicted to bind an extra electron and release energy. Copernicium and flerovium are predicted to have no such electron affinity. Quantum electrodynamic corrections cut the binding in the oganesson anion by 9 percent. Calculations from 2022 expect an electron affinity of about 0.08 electronvolts.

  Monte Carlo simulations of oganesson's molecular dynamics predict that relativistic effects raise its melting and boiling points considerably. Without them it would melt at a much lower temperature. Oganesson would probably be a solid under standard conditions, though with a rather low melting point.

  Its polarizability is expected to be almost double that of radon. That produces an anomalously low first ionization energy of about 860 kJ/mol, similar to cadmium and lower than iridium, platinum and gold. The second ionization energy should be around 1560 kJ/mol. Very strong spin-orbit splitting of the 7p orbital is expected to smear its electron subshells into a homogeneous Fermi gas of electrons, unlike those of radon and xenon. Studies also suggest a semibubble structure in proton density, with few protons at the center of the nucleus.

  Bulk oganesson might even be a semiconductor, with a predicted band gap of 1.5 eV. All the lighter noble gases are insulators. The band gap of bulk radon, for example, is expected to be 7.1 eV.

  No compound of oganesson has been synthesized, since oganesson-294 lives far too briefly for experiments. Calculations on theoretical compounds have been performed since 1964. A high ionization energy would suggest the most common oxidation state is 0, as for the noble gases, but that appears not to hold. The bond between oganesson and hydrogen is predicted to be very weak, a pure van der Waals interaction rather than a true chemical bond. The proton affinity of oganesson is predicted to be smaller than that of radon.

  With highly electronegative elements, oganesson seems to form more stable compounds than copernicium or flerovium. Oxidation states of +2 and +4 are predicted in the fluorides OgF2 and OgF4. The +6 state would be less stable because of the strong binding of the 7p1/2 subshell, so OgF6 is expected to be unbound. Radon hexafluoride is likewise expected to be much less stable than xenon hexafluoride. OgF4 is expected to be tetrahedral, with two inert electron pairs. The oganesson-fluorine bond will most probably be ionic, which would make the fluorides non-volatile. Oganesson is also predicted to be electropositive enough to form a bond with chlorine.

  Spin-orbit interactions matter here. In one calculated reaction, about 46 of the 106 kcal/mol released comes from them. For a similar molecule, the figure is about 10 of 49 kcal/mol.

  A compound of oganesson and tennessine, OgTs4, has been predicted to be potentially stable chemically.

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