Case Study 11.1: Oganesson — Element 118 and the End (for Now) of the Periodic Table

The Synthesis

On the morning of October 9, 2006, a silicon detector at the focal plane of the Dubna Gas-Filled Recoil Separator recorded an unusual event: a high-energy implantation signal followed, 0.5 milliseconds later, by an alpha particle with an energy of 11.65 MeV. The alpha was followed by a second alpha at 10.84 MeV (after 7.1 ms), then a third at 10.19 MeV (after 0.12 s), and finally a burst of high-energy fragments characteristic of spontaneous fission (after 0.8 ms). The experimenters at JINR, led by Yuri Oganessian, had been bombarding a $^{249}$Cf target with $^{48}$Ca ions for months. What they were seeing was the decay chain of the heaviest element ever created: element 118.

The reaction was:

$$^{48}\text{Ca} + ^{249}\text{Cf} \rightarrow ^{297}\text{Og}^* \rightarrow ^{294}\text{Og} + 3n$$

The compound nucleus $^{297}$Og$^*$ was formed with an excitation energy of approximately 35 MeV and cooled by evaporating three neutrons to produce $^{294}$Og in (or near) its ground state. The production cross section was approximately 0.5 pb — about $5 \times 10^{-37}$ cm$^2$. At the beam intensity and target thickness available, this corresponded to a production rate of roughly one atom every two months of continuous bombardment.

By the end of the experiment, three decay chains attributable to $^{294}$Og had been observed. Three atoms — that was the entirety of the evidence for the existence of the heaviest element known to science.

The Target: $^{249}$Cf

The $^{249}$Cf target was itself a scientific achievement. Californium-249 is produced by prolonged neutron irradiation of lighter actinides in high-flux nuclear reactors. The material used at Dubna was produced at Oak Ridge National Laboratory's High Flux Isotope Reactor (HFIR), where curium targets were irradiated for months. After chemical separation — a painstaking process involving ion-exchange chromatography in hot cells to isolate microgram quantities of Cf from a complex mixture of actinides and fission products — approximately 10 mg of $^{249}$Cf was obtained. This material was electroplated onto a titanium backing to form the rotating target used in the experiment.

$^{249}$Cf has a half-life of 351 years, making it stable enough for use as a target over the duration of the experiment. But it is also radioactive and self-heating (it produces about 3 milliwatts of thermal power per milligram from alpha decay), which constrains the target design.

The Decay Chain

The observed decay chain of $^{294}$Og provides a wealth of information:

Several features are noteworthy:

Alpha decay dominates over spontaneous fission for the first three members of the chain ($^{294}$Og, $^{290}$Lv, $^{286}$Fl). In the pure liquid drop model, spontaneous fission should be overwhelmingly faster than alpha decay for all of these nuclei. The fact that alpha decay wins demonstrates that the fission barriers are enhanced by shell effects — exactly as predicted by the island of stability theory.

The alpha energies decrease along the chain (11.65 $\rightarrow$ 10.84 $\rightarrow$ 10.19 MeV), corresponding to decreasing Q$_\alpha$ values as the chain moves toward more neutron-rich (relative to the daughter Z) nuclei. The trend is consistent with theoretical mass predictions.

Spontaneous fission terminates the chain at $^{282}$Cn (Z = 112, N = 170). Here the neutron number is far from N = 184, and the shell stabilization of the fission barrier is weaker. This is consistent with the picture that the island of stability's influence weakens as N decreases.

What Oganesson Tells Us About Nuclear Structure

The very existence of $^{294}$Og — with a measurable half-life of 0.7 ms — is a confirmation of the shell model in the superheavy region. The liquid drop model predicts that this nucleus should fission in less than $10^{-18}$ seconds. The 15 orders of magnitude difference between the predicted (liquid drop) and observed half-lives is entirely attributable to shell effects.

The Q$_\alpha$ value of $^{294}$Og (approximately 11.81 MeV, calculated from the measured alpha energy) constrains the nuclear mass surface. When combined with the Q$_\alpha$ values of the daughter nuclei, the complete chain provides mass differences that can be compared to theoretical predictions from macroscopic-microscopic models, Skyrme-Hartree-Fock calculations, and relativistic mean-field theories. The data are broadly consistent with models that place a shell gap near Z = 114, though the uncertainties are still too large to definitively distinguish between Z = 114 and Z = 120 as the proton magic number.

Oganesson's Predicted Chemical Properties

Oganesson sits in Group 18 of the periodic table, directly below radon. Its electron configuration is predicted to be [Rn]5f$^{14}$6d$^{10}$7s$^2$7p$^4_{1/2}$7p$^2_{3/2}$. If oganesson followed the trend of lighter noble gases, it would be expected to be a chemically inert gas at room temperature, with a closed-shell electronic structure.

However, relativistic four-component Dirac-Fock calculations tell a profoundly different story. The key results from the theoretical work of Jerabek, Schuetrumpf, Schwerdtfeger, and others include:

1. Smeared-out electron shells. Unlike lighter elements, where electrons occupy well-defined shells with distinct radial probability distributions, the electron density in Og shows no clear shell structure. The electrons form a nearly homogeneous cloud — more like a Thomas-Fermi electron gas than a shell-model atom.

2. Positive electron affinity. All noble gases from helium through radon have negative or zero electron affinities (they do not bind an extra electron). Oganesson is predicted to have a positive electron affinity of approximately 0.056 eV — small, but qualitatively different from zero.

3. Extreme polarizability. The static dipole polarizability of Og is predicted to be about 58 atomic units, more than twice that of radon (27.3 a.u.). This makes Og far more susceptible to van der Waals interactions than a typical noble gas.

4. Solid at room temperature. The high polarizability leads to strong van der Waals interactions between Og atoms. Band-structure calculations predict that oganesson would crystallize into a face-centered cubic solid at room temperature and atmospheric pressure — the first solid noble gas at STP. (Radon, by comparison, condenses at $-62$ C.)

5. Semiconductor. Calculations by Smits and Schwerdtfeger (2020) using relativistic coupled-cluster theory and solid-state DFT predict that solid oganesson would have a band gap of approximately 1.5 eV — making it a semiconductor. A semiconducting noble gas is an extraordinary prediction, one that highlights how relativistic effects can fundamentally reshape the periodic table.

The Experimental Frontier

None of these predictions have been verified experimentally. The reasons are clear: only about a dozen atoms of oganesson have ever been produced, each with a half-life of less than 1 millisecond, at a rate of a few atoms per year. Chemistry experiments on superheavy elements (which have been performed for elements up to Z = 114) require at least several atoms per hour and half-lives of seconds or longer.

The SHE Factory at JINR aims to increase the production rate of $^{294}$Og by approximately an order of magnitude through higher beam intensities. Even so, chemistry experiments on oganesson remain a distant goal. The more likely near-term experimental advance would be the production of more neutron-rich Og isotopes (closer to N = 184), which are predicted to have longer half-lives.

One indirect experimental test of oganesson's electronic structure could come from measuring its first ionization energy or its adsorption behavior on gold or silicon detector surfaces during in-flight transport experiments. If oganesson's electron cloud is truly as diffuse as predicted, its interaction with surfaces should differ measurably from radon's. Such measurements, while extremely challenging at current production rates, might become feasible at the SHE Factory.

Oganesson in the Context of the Nuclear Chart

It is instructive to place $^{294}$Og on the chart of nuclides and consider its position relative to the predicted island of stability. With Z = 118 and N = 176, oganesson sits 8 neutrons short of the N = 184 shell closure and either 4 protons above the Z = 114 closure or 2 below the Z = 120 closure, depending on which theoretical prediction is correct.

In either case, oganesson is on the proton-rich slope of the island — not at the summit. This is significant because the measured half-life of 0.7 ms, while impressive (the liquid drop model would predict femtoseconds or less), is still short. If the same element could be produced with 8 additional neutrons ($^{302}$Og, N = 184), the half-life could increase by many orders of magnitude — potentially to seconds or minutes. This would transform oganesson from an ephemeral laboratory artifact into a nucleus stable enough for at least some chemical characterization.

The challenge of reaching $^{302}$Og is formidable. It would require a reaction that produces a compound nucleus with approximately 8 more neutrons than $^{48}$Ca + $^{249}$Cf provides, and no obvious projectile-target combination exists. Multi-nucleon transfer reactions, or the use of radioactive beams of very neutron-rich isotopes, may eventually provide a path.

Comparison with the Noble Gas Family

The noble gases form one of the most distinctive families on the periodic table. Their properties evolve systematically as Z increases:

The trend is clear: boiling point and polarizability increase monotonically with Z, while ionization energy decreases. Oganesson fits these trends in one sense — its polarizability and boiling point are the largest, and its ionization energy is the smallest. But the predicted magnitude of its polarizability and the possible solid state at room temperature represent a qualitative departure from noble gas behavior, not just a quantitative extension of the trend.

The lesson is that the periodic table's vertical trends, which work beautifully for the first six periods, cannot be trusted to extrapolate into the seventh period and beyond without relativistic corrections. Oganesson is the most dramatic example, but similar anomalies are predicted for flerovium (too noble for Group 14) and copernicium (too volatile for Group 12).

Significance

Oganesson occupies a unique position in science. It is the heaviest element ever created. It sits at the terminus of the periodic table's seventh row. Its predicted properties challenge the very framework — the periodic table — that organizes all of chemistry. And its existence, measured in fractions of a millisecond in a detector at a Russian laboratory, confirms one of the boldest predictions of nuclear shell theory: that quantum mechanics can stabilize nuclei against the overwhelming classical force of Coulomb repulsion.

The story of oganesson is not finished. The next chapters will be written by the physicists who produce its more neutron-rich isotopes, by the chemists who devise methods to study its properties atom by atom, and by the theorists who continue to map the uncharted territory where nuclear physics, relativistic quantum mechanics, and chemistry converge.

Discussion Questions

1. The three observed decay chains of $^{294}$Og are the sole experimental basis for the existence of element 118. Is this sufficient evidence? What additional measurements would strengthen the case? Compare to the standards used for particle physics discoveries (the "5-sigma" criterion).

2. If solid oganesson were a semiconductor with a band gap of 1.5 eV, how would this compare to silicon (1.1 eV) and germanium (0.67 eV)? What are the practical implications — could you ever make an oganesson transistor? Why or why not?

3. The prediction that oganesson's electron shells are "smeared out" challenges the fundamental assumption of the periodic table: that elements in the same group have similar valence electron configurations. At what point does the periodic table cease to be a useful organizing principle? Is this a failure of the framework or a triumph of the physics?