The periodic table as we know it now seems complete, its current 118 elements nicely fitting in the seven familiar rows. How many more can be synthesized — and how will the table expand to accommodate them? The search for ever-heavier elements is pointing towards new periods, though perhaps not as neatly ordered as the first seven.
Just 150 years ago, Dmitri I. Mendeleev formulated the original version of what would become one of the most ubiquitous items in chemistry, the periodic table. He had ordered the 63 then-known elements on the basis of their atomic weights and chemical properties, placing elements that showed a similar behaviour into columns1. Others were also working on classifying the elements, but a particularly notable feature of Mendeleev’s table (an early version of which is pictured) is that he had left empty spaces where he believed an element that hadn’t yet been discovered should fit in. He temporarily named those elements after their above-neighbours in the periodic table — eka-aluminum, eka-silicon, eka-boron and so on — and predicted their properties based on their locations. Soon several of these were indeed discovered, starting with gallium, then germanium and scandium. The fact that their properties did match the predicted ones gave great credibility to Mendeleev’s table, and it was widely adopted. Since then, the periodic table has evolved with chemistry and undergone some adjustments. One thing hasn’t changed though — it has remained an irreplaceable chart not only to classify all the known elements but also to guide the search for new ones.
All elements beyond element 92, uranium, were artificially synthesized, either with particle accelerators, in nuclear reactors or, unexpectedly, in a hydrogen-bomb explosion. Fermium and einsteinium were first discovered in the debris of a thermonuclear weapon test, before being made more conventionally with accelerators. The year 2016 (give or take a couple of days) proved to be exciting, seeing interest for the periodic table spread well beyond the scientific community. The International Union of Pure and Applied Chemistry confirmed on 30 December 2015 the existence of no fewer than four new artificial elements and in November 2016 formally accepted the discoverers’ proposed names and symbols2. Elements 113, 115, 117 and 118 — nihonium (Nh), moscovium (Mc), tennessine (Ts) and oganesson (Og), respectively — complete the seventh period and as such the periodic table as we know it (Fig. 1, yellow background).
Even as these elements were being synthesized, the search for even-heavier elements had already begun. How many can be created and where exactly on the table will they be added?
Synthesis of the superheavy elements
All transactinide elements with atomic number (Z) ≥ 104, now typically called superheavy elements3, have artificially been synthesized one atom at a time in heavy-ion-induced nuclear fusion reactions at high-power accelerator facilities. A projectile nucleus is accelerated to very high speeds (using a heavy-ion accelerator) and used to bombard a target nucleus in the hope that they will fuse together in a heavy atom. This is highly challenging, and not just because of the technical equipment needed. For two nuclei to fuse into a single one, the projectile energy must be just right. If it is too low, the two nuclei, which both have a positive charge, cannot meet owing to the large repulsive Coulomb force. Conversely if it is too high, the fused nucleus possesses extra energy and breaks into light nuclei by nuclear fission. Even under the right conditions, the probability for the nuclei to fuse together and survive nuclear fission is exceedingly low, which means that the experiments have to be run for weeks or even months for one atom. Additionally, in the event of fusion, the heavy nucleus formed is so unstable that it decays rapidly, making it difficult to detect.
The period after the Second World War saw fierce competition in superheavy synthesis between the Lawrence Radiation Laboratory in the USA (now the Lawrence Berkeley National Laboratory) and the Joint Institute for Nuclear Research (JINR) in Dubna, Russia. Rutherfordium, dubnium and seaborgium were created in those years. By the 1980s–90s, a new player had emerged, the Gesellschaft für Schwerionenforschung (GSI, now the GSI Helmholtzzentrum fur Schwerionenforschung GmbH) in Darmstadt, Germany, where the six elements from bohrium to copernicium (107–112) were created4 by fusion reactions using the projectiles of 54Cr, 58Fe, 62,64Ni and 70Zn, the ‘doubly magic’ target of 208Pb and nearby 209Bi. In a manner somewhat similar to the effect of closed electron shells, certain numbers of protons and neutrons (Z = 2, 8, 20, 28, 50 or 82 and N = 2, 8, 20, 28, 50, 82 or 126) have been shown to exhibit particularly large binding energies, conferring particular stability to these nuclei. 208Pb, with its 82 protons and 126 neutrons, is doubly magic.
The apparatus required for these syntheses includes an accelerator for the projectile nucleus, a target, a separator to divide the desired superheavy nuclei formed from lighter by-products of the reaction and a detector to collect data on the superheavy nuclei formed — or rather, data on their decay chains by alpha-decay (the emission of a helium nucleus) and spontaneous fission into lighter nuclei. From the decay signature observed the existence of the nuclei that had been formed by nuclear fusion is inferred.
Those elements discovered at GSI were synthesized using the recoil separator SHIP (Separator for Heavy Ion Reaction Products), which can separate the fusion reaction products from the projectiles and the by-products, in flight, within microseconds. Alpha- and spontaneous-fission decays of the products can then be observed with a silicon semiconductor detector. Similar experimental techniques were used for all heavier elements. Nihonium was synthesized5 in the fusion reaction of 209Bi + 70Zn at RIKEN, Japan. The elements from flerovium to oganesson (114–118) were created6 at JINR in fusion reactions using the target nuclei plutonium, americium, curium, berkelium and californium, respectively, and the doubly magic projectile 48Ca (Z = 20 and N = 28).
The search for the superheavy elements beyond 118 is proving to be a great challenge. Man-made actinide targets such as the ones used to make elements 114–118 can only be produced at high-flux nuclear reactors such as that in Oak Ridge National Laboratory, USA7. Unfortunately, those target materials can only be prepared in sufficient quantities (at least around 1 mg) up to californium isotopes (Z = 98), which means that in combination with the 48Ca projectile (Z = 20) these reactions can only be used to make elements up to Z = 118.
The synthesis of heavier elements must therefore involve heavier projectiles than 48Ca which was used to make elements 114–118; those projectiles are not doubly magic nuclei though, which means that fusion probabilities decrease drastically. Attempts to create elements 119 and 120 in this manner are underway: the synthesis of element 120 was attempted through the reactions 244Pu + 58Fe at JINR8 and 238U + 64Ni, 248Cm + 54Cr and 249Cf + 50Ti at GSI4,9,10, while the synthesis of element 119 has been attempted through 249Bk + 50Ti at GSI10 and 248Cm + 51V at RIKEN11 — but those have remained unsuccessful. The production probabilities of elements 119 and 120 appear to be far lower (at least one order of magnitude) than those for flerovium and moscovium, which were formed in the 48Ca-induced reactions. Technological improvements are therefore required to produce, and detect, elements beyond 118 — for example, the development of beam acceleration able to handle heavier projectiles and that can also accelerate simultaneously higher numbers of ions, tough targets that resist the heat released by high-current beams, rapid and efficient separation of products as well as more-sensitive detectors.
The construction of new accelerator facilities and the development of new experimental devices have already started in a few laboratories. An accelerator complex called the ‘SHE-Factory’ (SHE stands for superheavy element) is being built at the JINR, where a new cyclotron (DC-280 cyclotron) is being developed alongside targets, separators and detectors6,12. At RIKEN, the heavy-ion linear accelerator is under upgrade, with a 28 GHz super-conducting electron cyclotron resonance ion source, a super-conducting quarter-wavelength resonator and a new gas-filled recoil ion separator being installed. Further undergoing constructions are the Super Separator Spectrometer (S3) in SPIRAL2 (Système de Production d’Ions Radioactifs en Ligne, phase II) at GANIL (Grand Accélérateur National d’Ions Lourds), France and a superconducting continuous wave linear accelerator at GSI.
Current theoretical calculations of the fusion probabilities suggest that, even if the experiments were conducted under optimal condition, it would take hundreds of days (at least) to synthesize only one atom of element 119 or 120. Yet, the existing experimental data and theoretical models do not allow for reliable predictions to be made on the fusion probabilities for the isotopes of elements beyond 118, or on their nuclear properties (such as their decay mode, half-life, mass and fission barrier). Thus, further studies on the known superheavy isotopes are necessary to enable more-reliable models to be developed, which will in turn guide the synthesis of these elements in year-long experiments using powerful accelerators.
How far can we go?
With increasing Z in the superheavy region, the electrons associated with such heavy nuclei move at relativistic speeds (approaching the speed of light, eventually increasing their masses), with very noticeable effects on electron orbitals13. The modern relativistic electronic structure theory suggests that the periodic table ends around element 173. There, the binding energy of the 1s electron (1.022 MeV) attains twice the value of the electron mass (which is equivalent to 0.511 MeV) so that a pair comprising an anti-electron (positron) and electron can be spontaneously created and break the atomic system14. Pekka Pyykkö explored15 the electronic configuration of all elements up to element 172; Fig. 1 shows his suggested extended periodic table.
At the bottom of the f-block, the actinide series runs from actinium to lawrencium by filling the 5f electron shell — though this series, and that of the lanthanides, which sits just above it, aren’t set in stone. The location of lutetium and lawrencium is currently being debated, as these elements can be seen as belonging either to the f-block or to group 3, below scandium and yttrium, owing to their similar electronic structures16. In the transactinide region, the first nine elements (from rutherfordium to copernicium) are predicted to belong to the d-block, as per their 6d configuration. They are followed by the elements from nihonium to oganesson, which belong to the p-block (with their 7p configuration).
Yet undiscovered, 119 and 120 are predicted to possess 8s1 and 8s2 electron configurations, respectively. Thus, they should be alkali and alkaline earth elements in groups 1 and 2, respectively, in an eighth row of the table. So far, just like the other periods. For heavier elements, however, the levels of 7d, 6f and 5g orbitals, and also those of the 9s, 9p1/2 and 8p3/2 orbitals are energetically so close to each other that the p, d, f and g blocks are no longer distinguishable13,15. The conventional classification on the basis of simple electronic configurations in the ground state can no longer be applied. In Pyykkö’s proposition15, elements 121–138 are classified as the 5g elements, followed by the 8p1/2 elements 139 and 140, the 6f elements 141–155, and the 7d elements 156−164, so that they would be located at the bottom of these respective groups (Fig. 1). The 9s1 and 9s2 elements 165 and 166 are in groups 1 and 2 of a ninth period of the table, respectively, followed by the 9p1/2 elements 167 and 168 and the 8p3/2 elements 169–172, by taking the energetic order 9s <9p1/2 <8p3/2.
This makes adding elements to the bottom of the periodic table a less orderly process than that of the first seven periods, neatly arranged by increasing atomic numbers.
Island of stability
The question of which element will be the heaviest, and thus the last one in the periodic table, is determined by the balance between the repulsive Coulomb forces of many positive protons in a tiny nucleus and the attractive nuclear forces among nucleons (protons and neutrons). With increasing Z, it becomes difficult to bind many protons due to the increasingly repulsive Coulomb forces and the nucleus becomes increasingly unstable and prone to breaking spontaneously by nuclear fission6.
In 1939, the macroscopic theory, which treats the atomic nucleus as a charged liquid drop, described the fission process and indicated that elements beyond Z = 100 would immediately break down by spontaneous fission6 — making fermium the last element. Heavier elements up to element 118 do exist however. This is because of the above-mentioned ‘shell energy’ at magic numbers of protons and neutrons, which stabilize nuclei against spontaneous fission.
In the 1960s, the shell effect was introduced in the macroscopic model (macroscopic–microscopic model), predicting a strong nuclear shell effect at Z = 114 and N = 184 (refs. 6,17). A large area of very heavy nuclei, predicted to be relatively stable, appeared around this nucleus, forming an ‘island of stability’. In further investigations, a variety of theoretical approaches have been devoted to determining the exact location of this island6,17. In almost all of the models, the strong neutron shell is predicted at N = 184. The effect of the proton shell however is relatively weak and model-dependent. In purely microscopic calculations, the magic proton number is expected to be Z = 120–126. Tantalizingly, calculations have indicated that the most stable superheavy nuclei could have long half-lives of hundreds, thousands and even millions of years.
Figure 2 shows the upper end of the chart of nuclides18, where known heavy nuclides are presented (in colour) against the proton and neutron numbers. In the heaviest element region, alpha and spontaneous fission decay modes are dominant. The shell energy of yet-unknown isotopes, calculated according to the modern Koura–Tachibaba–Uno–Yamada (KTUY) mass formula that combines the macroscopic and microscopic models19, is displayed as a grey scale. This model predicts masses, half-lives and several decay modes of the nuclei — darker shades represent more-stable isotopes and the island of stability appears in the upper right corner.
So far, although isotopes with (or near) Z = 114 protons have been produced, none have come close to comprising 184 neutrons. One possible route to approach this N = 184 shell is the use of radioactive ion beams that have a larger neutron excess than 48Ca. Unfortunately, the intensity of the radioactive ion beams at the most advanced accelerator complexes, and even at those currently being designed, is much lower than that required for performing such experiments.
Searching for new elements and investigating their chemical properties is one of the most fundamental research subjects in chemistry. Very low production yields (only one atom per minute for rutherfordium to per day for flerovium) and short half-lives (~1 min for rutherfordium to ~1 s for flerovium) of the superheavy element nuclides force us to perform rapid, efficient and repetitive one-atom-at-a-time experiments at accelerators to try and probe their behaviour. It is not conceivable that weighable quantities of any superheavy element will be produced in the near future and they will most likely have next-to-no immediate practical use.
Research on superheavy elements, however, is important from both a fundamental and technological perspective. Owing to the predicted strong influence of relativistic effect, any experimental investigation of their properties is fascinating. Recent chemical endeavours have demonstrated that the chemical characteristics of the superheavy elements can indeed no longer be predicted by simple extrapolations of the regularities of the periodic table3,13. The superheavy elements, which exist in extreme regions of atomic nuclei and atoms, are thus very good laboratories to benchmark theoretical models and refine them. This helps our understanding of atomic nuclei and atoms, which is in turn valuable for both basic and applied sciences.