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Oganesson, 118Og
Oganesson
Pronunciation
Appearancemetallic (predicted)
Mass number[294]
Oganesson in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Rn
↑
Og
↓
—
tennessine ← oganesson → ununennium
Atomic number (Z)118
Groupgroup 18 (noble gases)
Periodperiod 7
Block  p-block
Electron configuration[Rn] 5f14 6d10 7s2 7p6 (predicted)[3][4]
Electrons per shell2, 8, 18, 32, 32, 18, 8 (predicted)
Physical properties
Phase at STPsolid (predicted)[5]
Melting point325 ± 15 K ​(52 ± 15 °C, ​125 ± 27 °F) (predicted)[5]
Boiling point450 ± 10 K ​(177 ± 10 °C, ​350 ± 18 °F) (predicted)[5]
Density (near r.t.)7.2 g/cm3 (solid, 319 K, calculated)[5]
when liquid (at m.p.)6.6 g/cm3 (liquid, 327 K, calculated)[5]
Atomic properties
Oxidation statescommon: (none)
(−1),[4] (+1),[6] (+2),[7] (+4),[7] (+6)[4]
Ionization energies
  • 1st: 860 kJ/mol (calculated)[8]
  • 2nd: 1560 kJ/mol (calculated)[8]
Atomic radiusempirical: 152 pm (predicted)[9]
Covalent radius157 pm (predicted)[10]
Other properties
Natural occurrencesynthetic
Crystal structure ​face-centered cubic (fcc)
Face-centered cubic crystal structure for oganesson

(extrapolated)[11]
CAS Number54144-19-3
History
Namingafter Yuri Oganessian
PredictionHans Peter Jørgen Julius Thomsen (1895)
DiscoveryJoint Institute for Nuclear Research and Lawrence Livermore National Laboratory (2002)
Isotopes of oganesson
Main isotopes[12] Decay
Isotope abun­dance half-life (t1/2) mode pro­duct
294Og synth 0.7 ms[13][14] α 290Lv
SF –
 Category: Oganesson
| references

Oganesson is a synthetic chemical element; it has symbol Og and atomic number 118. It was first synthesized in 2002 at the Joint Institute for Nuclear Research (JINR) in Dubna, near Moscow, Russia, by a joint team of Russian and American scientists. In December 2015, it was recognized as one of four new elements by the Joint Working Party of the international scientific bodies IUPAC and IUPAP. It was formally named on 28 November 2016.[15][16] The name honors the nuclear physicist Yuri Oganessian, who played a leading role in the discovery of the heaviest elements in the periodic table.

Oganesson has the highest atomic number and highest atomic mass of all known elements. On the periodic table of the elements it is a p-block element, a member of group 18, and the last member of period 7. Its only known isotope, oganesson-294, is highly radioactive, with a half-life of 0.7 ms and, as of 2025, only five atoms have been successfully produced.[17] This has so far prevented any experimental studies of its chemistry. Because of relativistic effects, theoretical studies predict that it would be a solid at room temperature, and significantly reactive,[3][17] unlike the other members of group 18 (the noble gases).

Introduction

[edit]
This section is an excerpt from Superheavy element § Introduction.[edit]

Synthesis of superheavy nuclei

[edit]
A graphic depiction of a nuclear fusion reaction
A graphic depiction of a nuclear fusion reaction. Two nuclei fuse into one, emitting a neutron. Reactions that created new elements to this moment were similar, with the only possible difference that several singular neutrons sometimes were released, or none at all.

A superheavy[a] atomic nucleus is created in a nuclear reaction that combines two other nuclei of unequal size[b] into one; roughly, the more unequal the two nuclei in terms of mass, the greater the possibility that the two react.[23] The material made of the heavier nuclei is made into a target, which is then bombarded by the beam of lighter nuclei. Two nuclei can only fuse into one if they approach each other closely enough; normally, nuclei (all positively charged) repel each other due to electrostatic repulsion. The strong interaction can overcome this repulsion but only within a very short distance from a nucleus; beam nuclei are thus greatly accelerated in order to make such repulsion insignificant compared to the velocity of the beam nucleus.[24] The energy applied to the beam nuclei to accelerate them can cause them to reach speeds as high as one-tenth of the speed of light. However, if too much energy is applied, the beam nucleus can fall apart.[24]

Coming close enough alone is not enough for two nuclei to fuse: when two nuclei approach each other, they usually remain together for about 10−20 seconds and then part ways (not necessarily in the same composition as before the reaction) rather than form a single nucleus.[24][25] This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed.[24] Each pair of a target and a beam is characterized by its cross section—the probability that fusion will occur if two nuclei approach one another expressed in terms of the transverse area that the incident particle must hit in order for the fusion to occur.[c] This fusion may occur as a result of the quantum effect in which nuclei can tunnel through electrostatic repulsion. If the two nuclei can stay close past that phase, multiple nuclear interactions result in redistribution of energy and an energy equilibrium.[24]

External videos
video icon Visualization of unsuccessful nuclear fusion, based on calculations from the Australian National University[27]

The resulting merger is an excited state[28]—termed a compound nucleus—and thus it is very unstable.[24] To reach a more stable state, the temporary merger may fission without formation of a more stable nucleus.[29] Alternatively, the compound nucleus may eject a few neutrons, which would carry away the excitation energy; if the latter is not sufficient for a neutron expulsion, the merger would produce a gamma ray. This happens in about 10−16 seconds after the initial nuclear collision and results in creation of a more stable nucleus.[29] The definition by the IUPAC/IUPAP Joint Working Party (JWP) states that a chemical element can only be recognized as discovered if a nucleus of it has not decayed within 10−14 seconds. This value was chosen as an estimate of how long it takes a nucleus to acquire electrons and thus display its chemical properties.[30][d]

Decay and detection

[edit]

The beam passes through the target and reaches the next chamber, the separator; if a new nucleus is produced, it is carried with this beam.[32] In the separator, the newly produced nucleus is separated from other nuclides (that of the original beam and any other reaction products)[e] and transferred to a surface-barrier detector, which stops the nucleus. The exact location of the upcoming impact on the detector is marked; also marked are its energy and the time of the arrival.[32] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[35] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[32]

Stability of a nucleus is provided by the strong interaction. However, its range is very short; as nuclei become larger, its influence on the outermost nucleons (protons and neutrons) weakens. At the same time, the nucleus is torn apart by electrostatic repulsion between protons, and its range is not limited.[36] Total binding energy provided by the strong interaction increases linearly with the number of nucleons, whereas electrostatic repulsion increases with the square of the atomic number, i.e. the latter grows faster and becomes increasingly important for heavy and superheavy nuclei.[37][38] Superheavy nuclei are thus theoretically predicted[39] and have so far been observed[40] to predominantly decay via decay modes that are caused by such repulsion: alpha decay and spontaneous fission.[f] Almost all alpha emitters have over 210 nucleons,[42] and the lightest nuclide primarily undergoing spontaneous fission has 238.[43] In both decay modes, nuclei are inhibited from decaying by corresponding energy barriers for each mode, but they can be tunneled through.[37][38]

Apparatus for creation of superheavy elements
Scheme of an apparatus for creation of superheavy elements, based on the Dubna Gas-Filled Recoil Separator set up in the Flerov Laboratory of Nuclear Reactions in JINR. The trajectory within the detector and the beam focusing apparatus changes because of a dipole magnet in the former and quadrupole magnets in the latter.[44]

Alpha particles are commonly produced in radioactive decays because the mass of an alpha particle per nucleon is small enough to leave some energy for the alpha particle to be used as kinetic energy to leave the nucleus.[45] Spontaneous fission is caused by electrostatic repulsion tearing the nucleus apart and produces various nuclei in different instances of identical nuclei fissioning.[38] As the atomic number increases, spontaneous fission rapidly becomes more important: spontaneous fission partial half-lives decrease by 23 orders of magnitude from uranium (element 92) to nobelium (element 102),[46] and by 30 orders of magnitude from thorium (element 90) to fermium (element 100).[47] The earlier liquid drop model thus suggested that spontaneous fission would occur nearly instantly due to disappearance of the fission barrier for nuclei with about 280 nucleons.[38][48] The later nuclear shell model suggested that nuclei with about 300 nucleons would form an island of stability in which nuclei will be more resistant to spontaneous fission and will primarily undergo alpha decay with longer half-lives.[38][48] Subsequent discoveries suggested that the predicted island might be further than originally anticipated; they also showed that nuclei intermediate between the long-lived actinides and the predicted island are deformed, and gain additional stability from shell effects.[49] Experiments on lighter superheavy nuclei,[50] as well as those closer to the expected island,[46] have shown greater than previously anticipated stability against spontaneous fission, showing the importance of shell effects on nuclei.[g]

Alpha decays are registered by the emitted alpha particles, and the decay products are easy to determine before the actual decay; if such a decay or a series of consecutive decays produces a known nucleus, the original product of a reaction can be easily determined.[h] (That all decays within a decay chain were indeed related to each other is established by the location of these decays, which must be in the same place.)[32] The known nucleus can be recognized by the specific characteristics of decay it undergoes such as decay energy (or more specifically, the kinetic energy of the emitted particle).[i] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[j]

The information available to physicists aiming to synthesize a superheavy element is thus the information collected at the detectors: location, energy, and time of arrival of a particle to the detector, and those of its decay. The physicists analyze this data and seek to conclude that it was indeed caused by a new element and could not have been caused by a different nuclide than the one claimed. Often, provided data is insufficient for a conclusion that a new element was definitely created and there is no other explanation for the observed effects; errors in interpreting data have been made.[k]

History

[edit]
See also: Timeline of chemical element discoveries

Early speculation

[edit]

The possibility of a seventh noble gas, after helium, neon, argon, krypton, xenon, and radon, was considered almost as soon as the noble gas group was discovered. Danish chemist Hans Peter Jørgen Julius Thomsen predicted in April 1895, the year after the discovery of argon, that there was a whole series of chemically inert gases similar to argon that would bridge the halogen and alkali metal groups: he expected that the seventh of this series would end a 32-element period which contained thorium and uranium and have an atomic weight of 292, close to the 294 now known for the first and only confirmed isotope of oganesson.[61] Danish physicist Niels Bohr noted in 1922 that this seventh noble gas should have atomic number 118 and predicted its electronic structure as 2, 8, 18, 32, 32, 18, 8, matching modern predictions.[62] Following this, German chemist Aristid von Grosse wrote an article in 1965 predicting the likely properties of element 118.[11] It was 107 years from Thomsen's prediction before oganesson was successfully synthesized, although its chemical properties have not been investigated to determine if it behaves as the heavier congener of radon.[63] In a 1975 article, American chemist Kenneth Pitzer suggested that element 118 should be a gas or volatile liquid due to relativistic effects.[64]

Unconfirmed discovery claims

[edit]

In late 1998, Polish physicist Robert Smolańczuk published calculations on the fusion of atomic nuclei towards the synthesis of superheavy atoms, including oganesson.[65] His calculations suggested that it might be possible to make element 118 by fusing lead with krypton under carefully controlled conditions, and that the fusion probability (cross section) of that reaction would be close to the lead–chromium reaction that had produced element 106, seaborgium. This contradicted predictions that the cross sections for reactions with lead or bismuth targets would go down exponentially as the atomic number of the resulting elements increased.[65]

In 1999, researchers at Lawrence Berkeley National Laboratory made use of these predictions and announced the discovery of elements 118 and 116, in a paper published in Physical Review Letters,[66] and very soon after the results were reported in Science.[67] The researchers reported that they had performed the reaction

208
82Pb + 86
36Kr → 293
118Og + n.

In 2001, they published a retraction after researchers at other laboratories were unable to duplicate the results and the Berkeley lab could not duplicate them either.[68] In June 2002, the director of the lab announced that the original claim of the discovery of these two elements had been based on data fabricated by principal author Victor Ninov.[69][70] Newer experimental results and theoretical predictions have confirmed the exponential decrease in cross sections with lead and bismuth targets as the atomic number of the resulting nuclide increases.[71]

Discovery reports

[edit]
Schematic diagram of oganesson-294 alpha decay, with a half-life of 0.89 ms and a decay energy of 11.65 MeV. The resulting livermorium-290 decays by alpha decay, with a half-life of 10.0 ms and a decay energy of 10.80 MeV, to flerovium-286. Flerovium-286 has a half-life of 0.16 s and a decay energy of 10.16 MeV, and undergoes alpha decay to copernicium-282 with a 0.7 rate of spontaneous fission. Copernicium-282 itself has a half-life of only 1.9 ms and has a 1.0 rate of spontaneous fission.
Radioactive decay pathway of the isotope oganesson-294.[13] The decay energy and average half-life are given for the parent isotope and each daughter isotope. The fraction of atoms undergoing spontaneous fission (SF) is given in green.

The first genuine decay of atoms of oganesson was observed in 2002 at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, by a joint team of Russian and American scientists. Headed by Yuri Oganessian, a Russian nuclear physicist of Armenian ethnicity, the team included American scientists from the Lawrence Livermore National Laboratory in California.[72] The discovery was not announced immediately, because the decay energy of 294Og matched that of 212mPo, a common impurity produced in fusion reactions aimed at producing superheavy elements, and thus announcement was delayed until after a 2005 confirmatory experiment aimed at producing more oganesson atoms.[73] The 2005 experiment used a different beam energy (251 MeV instead of 245 MeV) and target thickness (0.34 mg/cm2 instead of 0.23 mg/cm2).[13] On 9 October 2006, the researchers announced[13] that they had indirectly detected a total of three (possibly four) nuclei of oganesson-294 (one or two in 2002[74] and two more in 2005) produced via collisions of californium-249 atoms and calcium-48 ions.[75][76][77][78][79]

249
98Cf + 48
20Ca → 294
118Og + 3 n.

In 2011, IUPAC evaluated the 2006 results of the Dubna–Livermore collaboration 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".[80]

Because of the very small fusion reaction probability (the fusion cross section is ~0.3–0.6 pb or (3–6)×10−41 m2) the experiment took four months and involved a beam dose of 2.5×1019 calcium ions that had to be shot at the californium target to produce the first recorded event believed to be the synthesis of oganesson.[81] Nevertheless, researchers were highly confident that the results were not a false positive, since the chance that the detections were random events was estimated to be less than one part in 100000.[82]

In the experiments, the alpha-decay of three atoms of oganesson was observed. A fourth decay by direct spontaneous fission was also proposed. A half-life of 0.89 ms was calculated: 294
Og decays into 290
Lv by alpha decay. Since there were only three nuclei, the half-life derived from observed lifetimes has a large uncertainty: 0.89+1.07
−0.31 ms.[13]

294
118Og → 290
116Lv + 4
2He

The identification of the 294
Og nuclei was verified by separately creating the putative daughter nucleus 290
Lv directly by means of a bombardment of 245
Cm with 48
Ca ions,

245
96Cm + 48
20Ca → 290
116Lv + 3 n,

and checking that the 290
Lv decay matched the decay chain of the 294
Og nuclei.[13] The daughter nucleus 290
Lv is very unstable, decaying with a lifetime of 14 milliseconds into 286
Fl, which may experience either spontaneous fission or alpha decay into 282
Cn, which will undergo spontaneous fission.[13]

Confirmation

[edit]

In December 2015, the Joint Working Party of international scientific bodies International Union of Pure and Applied Chemistry (IUPAC) and International Union of Pure and Applied Physics (IUPAP) recognized the element's discovery and assigned the priority of the discovery to the Dubna–Livermore collaboration.[83] This was on account of two 2009 and 2010 confirmations of the properties of the granddaughter of 294Og, 286Fl, at the Lawrence Berkeley National Laboratory, as well as the observation of another consistent decay chain of 294Og by the Dubna group in 2012. The goal of that experiment had been the synthesis of 294Ts via the reaction 249Bk(48Ca,3n), but the short half-life of 249Bk resulted in a significant quantity of the target having decayed to 249Cf, resulting in the synthesis of oganesson instead of tennessine.[84]

From 1 October 2015 to 6 April 2016, the Dubna team performed a similar experiment with 48Ca projectiles aimed at a mixed-isotope californium target containing 249Cf, 250Cf, and 251Cf, with the aim of producing the heavier oganesson isotopes 295Og and 296Og. Two beam energies at 252 MeV and 258 MeV were used. Only one atom was seen at the lower beam energy, whose decay chain fitted the previously known one of 294Og (terminating with spontaneous fission of 286Fl), and none were seen at the higher beam energy. The experiment was then halted, as the glue from the sector frames covered the target and blocked evaporation residues from escaping to the detectors.[85] The production of 293Og and its daughter 289Lv, as well as the even heavier isotope 297Og, is also possible using this reaction. The isotopes 295Og and 296Og may also be produced in the fusion of 248Cm with 50Ti projectiles.[85][86][87] A search beginning in summer 2016 at RIKEN for 295Og in the 3n channel of this reaction was unsuccessful, though the study is planned to resume; a detailed analysis and cross section limit were not provided. These heavier and likely more stable isotopes may be useful in probing the chemistry of oganesson.[88][89]

Naming

[edit]
Element 118 was named after Yuri Oganessian, a pioneer in the discovery of synthetic elements, with the name oganesson (Og). Oganessian and the decay chain of oganesson-294 were pictured on a stamp of Armenia issued on 28 December 2017.

Using Mendeleev's nomenclature for unnamed and undiscovered elements, oganesson is sometimes known as eka-radon (until the 1960s as eka-emanation, emanation being the old name for radon).[11] In 1979, IUPAC assigned the systematic placeholder name ununoctium to the undiscovered element, with the corresponding symbol of Uuo,[90] and recommended that it be used until after confirmed discovery of the element.[91] Although widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, the recommendations were mostly ignored among scientists in the field, who called it "element 118", with the symbol of E118, (118), or simply 118.[4]

Before the retraction in 2001, the researchers from Berkeley had intended to name the element ghiorsium (Gh), after Albert Ghiorso (a leading member of the research team).[92]

The Russian discoverers reported their synthesis in 2006. According to IUPAC recommendations, the discoverers of a new element have the right to suggest a name.[93] In 2007, the head of the Russian institute stated the team were considering two names for the new element: flyorium, in honor of Georgy Flyorov, the founder of the research laboratory in Dubna; and moskovium, in recognition of the Moscow Oblast where Dubna is located.[94] He also stated that although the element was discovered as an American collaboration, who provided the californium target, the element should rightly be named in honor of Russia since the Flyorov Laboratory of Nuclear Reactions at JINR was the only facility in the world which could achieve this result.[95] These names were later suggested for element 114 (flerovium) and element 116 (moscovium).[96] Flerovium became the name of element 114; the final name proposed for element 116 was instead livermorium,[97] with moscovium later being proposed and accepted for element 115 instead.[98]

Traditionally, the names of all noble gases end in "-on", with the exception of helium, which was not known to be a noble gas when discovered. The IUPAC guidelines valid at the moment of the discovery approval however required all new elements be named with the ending "-ium", even if they turned out to be halogens (traditionally ending in "-ine") or noble gases (traditionally ending in "-on").[99] While the provisional name ununoctium followed this convention, a new IUPAC recommendation published in 2016 recommended using the "-on" ending for new group 18 elements, regardless of whether they turn out to have the chemical properties of a noble gas.[100]

The scientists involved in the discovery of element 118, as well as those of 117 and 115, held a conference call on 23 March 2016 to decide their names. Element 118 was the last to be decided upon; after Oganessian was asked to leave the call, the remaining scientists unanimously decided to have the element "oganesson" after him. Oganessian was a pioneer in superheavy element research for sixty years reaching back to the field's foundation: his team and his proposed techniques had led directly to the synthesis of elements 107 through 118. Mark Stoyer, a nuclear chemist at the LLNL, 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."[101]

In internal discussions, IUPAC asked the JINR if they wanted the element to be spelled "oganeson" to match the Russian spelling more closely. Oganessian and the JINR refused this offer, citing the Soviet-era practice of transliterating names into the Latin alphabet under the rules of the French language ("Oganessian" is such a transliteration) and arguing that "oganesson" would be easier to link to the person.[102][l] In June 2016, IUPAC announced that the discoverers planned to give the element the name oganesson (symbol: Og). The name became official on 28 November 2016.[98] In 2017, Oganessian commented on the naming:[103]

For me, it is an honour. The discovery of element 118 was by scientists at the Joint Institute for Nuclear Research in Russia and at the Lawrence Livermore National Laboratory in the US, and it was my colleagues who proposed the name oganesson. My children and grandchildren have been living in the US for decades, but my daughter wrote to me to say that she did not sleep the night she heard because she was crying.[103]

— Yuri Oganessian

The naming ceremony for moscovium, tennessine, and oganesson was held on 2 March 2017 at the Russian Academy of Sciences in Moscow.[104]

In a 2019 interview, when asked what it was like to see his name in the periodic table next to Einstein, Mendeleev, the Curies, and Rutherford, Oganessian responded:[102]

Not like much! You see, not like much. It is customary in science to name something new after its discoverer. It's just that there are few elements, and this happens rarely. But look at how many equations and theorems in mathematics are named after somebody. And in medicine? Alzheimer, Parkinson. There's nothing special about it.

Characteristics

[edit]

Other than nuclear properties, no properties of oganesson or its compounds have been measured; this is due to its extremely limited and expensive production[105] and the fact that it decays very quickly. Thus only predictions are available.

Nuclear stability and isotopes

[edit]
Main article: Isotopes of oganesson
Oganesson (row 118) is slightly above the "Island of stability" (white ellipse) and thus its nuclei are slightly more stable than otherwise predicted.

The stability of nuclei quickly decreases with the increase in atomic number after curium, element 96, whose most stable isotope, 247Cm, has a half-life four orders of magnitude longer than that of any subsequent element. All nuclides with an atomic number above 101 undergo radioactive decay with half-lives shorter than 30 hours. No elements with atomic numbers above 82 (after lead) have stable isotopes.[106] This is because of the ever-increasing Coulomb repulsion of protons, so that the strong nuclear force cannot hold the nucleus together against spontaneous fission for long. Calculations suggest that in the absence of other stabilizing factors, elements with more than 104 protons should not exist.[107] However, researchers in the 1960s suggested that the closed nuclear shells around 114 protons and 184 neutrons should counteract this instability, creating an island of stability in which nuclides could have half-lives reaching thousands or millions of years. While scientists have still not reached the island, the mere existence of the superheavy elements (including oganesson) confirms that this stabilizing effect is real, and in general the known superheavy nuclides become exponentially longer-lived as they approach the predicted location of the island.[108][109] Oganesson is radioactive, decaying via alpha decay and spontaneous fission,[110][111] with a half-life that appears to be less than a millisecond. Nonetheless, this is still longer than some predicted values.[112][113]

Calculations using a quantum-tunneling model predict the existence of several heavier isotopes of oganesson with alpha-decay half-lives close to 1 ms.[114][115]

Theoretical calculations done on the synthetic pathways for, and the half-life of, other isotopes have shown that some could be slightly more stable than the synthesized isotope 294Og, most likely 293Og, 295Og, 296Og, 297Og, 298Og, 300Og and 302Og (the last reaching the N = 184 shell closure).[112][116] Of these, 297Og might provide the best chances for obtaining longer-lived nuclei,[112][116] and thus might become the focus of future work with this element. Some isotopes with many more neutrons, such as some located around 313Og, could also provide longer-lived nuclei.[117] The isotopes from 291Og to 295Og might be produced as daughters of element 120 isotopes that can be reached in the reactions 249–251Cf+50Ti, 245Cm+48Ca, and 248Cm+48Ca.[118]

In a quantum-tunneling model, the alpha decay half-life of 294
Og was predicted to be 0.66+0.23
−0.18 ms[112] with the experimental Q-value published in 2004.[119] Calculation with theoretical Q-values from the macroscopic-microscopic model of Muntian–Hofman–Patyk–Sobiczewski gives somewhat lower but comparable results.[120]

Calculated atomic and physical properties

[edit]

Oganesson is a member of group 18, the zero-valence elements. The members of this group are usually inert to most common chemical reactions (for example, combustion) because the outer valence shell is completely filled with eight electrons. This produces a stable, minimum energy configuration in which the outer electrons are tightly bound.[121] It is thought that similarly, oganesson has a closed outer valence shell in which its valence electrons are arranged in a 7s27p6 configuration.[3]

Consequently, some expect oganesson to have similar physical and chemical properties to other members of its group, most closely resembling the noble gas above it in the periodic table, radon.[122] Following the periodic trend, oganesson would be expected to be slightly more reactive than radon. However, theoretical calculations have shown that it could be significantly more reactive.[7] In addition to being far more reactive than radon, oganesson may be even more reactive than the elements flerovium and copernicium, which are heavier homologs of the more chemically active elements lead and mercury, respectively.[3] The reason for the possible enhancement of the chemical activity of oganesson relative to radon is an energetic destabilization and a radial expansion of the last occupied 7p-subshell.[3] More precisely, considerable spin–orbit interactions between the 7p electrons and the inert 7s electrons effectively lead to a second valence shell closing at flerovium, and a significant decrease in stabilization of the closed shell of oganesson.[3] It has also been calculated that oganesson, unlike the other noble gases, binds an electron with release of energy, or in other words, it exhibits positive electron affinity,[123][124] due to the relativistically stabilized 8s energy level and the destabilized 7p3/2 level,[125] whereas copernicium and flerovium are predicted to have no electron affinity.[126][127] Nevertheless, quantum electrodynamic corrections have been shown to be quite significant in reducing this affinity by decreasing the binding in the anion Og− by 9%, thus confirming the importance of these corrections in superheavy elements.[123] 2022 calculations expect the electron affinity of oganesson to be 0.080(6) eV.[8]

Monte Carlo simulations of oganesson's molecular dynamics predict it has a melting point of 325±15 K and a boiling point of 450±10 K due to relativistic effects (if these effects are ignored, oganesson would melt at ≈220 K). Thus oganesson would probably be a solid rather than a gas under standard conditions, though still with a rather low melting point.[5][17]

Oganesson is expected to have an extremely broad polarizability, almost double that of radon.[3] Because of its tremendous polarizability, oganesson is expected to have an anomalously low first ionization energy of about 860 kJ/mol, similar to that of cadmium and less than those of iridium, platinum, and gold. This is significantly smaller than the values predicted for darmstadtium, roentgenium, and copernicium, although it is greater than that predicted for flerovium.[128] Its second ionization energy should be around 1560 kJ/mol.[8] Even the shell structure in the nucleus and electron cloud of oganesson is strongly impacted by relativistic effects: the valence and core electron subshells in oganesson are expected to be "smeared out" in a homogeneous Fermi gas of electrons, unlike those of the "less relativistic" radon and xenon (although there is some incipient delocalisation in radon), due to the very strong spin–orbit splitting of the 7p orbital in oganesson.[129] A similar effect for nucleons, particularly neutrons, is incipient in the closed-neutron-shell nucleus 302Og and is strongly in force at the hypothetical superheavy closed-shell nucleus 472164, with 164 protons and 308 neutrons.[129] Studies have also predicted that due to increasing electrostatic forces, oganesson may have a semibubble structure in proton density, having few protons at the center of its nucleus.[130][131] Moreover, spin–orbit effects may cause bulk oganesson to be a semiconductor, with a band gap of 1.5±0.6 eV predicted. All the lighter noble gases are insulators instead: for example, the band gap of bulk radon is expected to be 7.1±0.5 eV.[132]

Predicted compounds

[edit]
Skeletal model of a planar molecule with a central atom symmetrically bonded to four peripheral (fluorine) atoms.
XeF
4 has a square planar molecular geometry.
Skeletal model of a terahedral molecule with a central atom (oganesson) symmetrically bonded to four peripheral (fluorine) atoms.
OgF
4 is predicted to have a tetrahedral molecular geometry.

The only confirmed isotope of oganesson, 294Og, has much too short a half-life to be chemically investigated experimentally. Therefore, no compounds of oganesson have been synthesized yet.[73] Nevertheless, calculations on theoretical compounds have been performed since 1964.[11] It is expected that if the ionization energy of the element is high enough, it will be difficult to oxidize and therefore, the most common oxidation state would be 0 (as for the noble gases);[133] nevertheless, this appears not to be the case.[63]

Calculations on the diatomic molecule Og
2 showed a bonding interaction roughly equivalent to that calculated for Hg
2, and a dissociation energy of 6 kJ/mol, roughly 4 times of that of Rn
2.[3] Most strikingly, it was calculated to have a bond length shorter than in Rn
2 by 0.16 Å, which would be indicative of a significant bonding interaction.[3] On the other hand, the compound OgH+ exhibits a dissociation energy (in other words proton affinity of oganesson) that is smaller than that of RnH+.[3]

The bonding between oganesson and hydrogen in OgH is predicted to be very weak and can be regarded as a pure van der Waals interaction rather than a true chemical bond.[6] On the other hand, with highly electronegative elements, oganesson seems to form more stable compounds than for example copernicium or flerovium.[6] The stable oxidation states +2 and +4 have been predicted to exist in the fluorides OgF
2 and OgF
4.[134] The +6 state would be less stable due to the strong binding of the 7p1/2 subshell.[63] This is a result of the same spin–orbit interactions that make oganesson unusually reactive. For example, it was shown that the reaction of oganesson with F
2 to form the compound OgF
2 would release an energy of 106 kcal/mol of which about 46 kcal/mol come from these interactions.[6] For comparison, the spin–orbit interaction for the similar molecule RnF
2 is about 10 kcal/mol out of a formation energy of 49 kcal/mol.[6] The same interaction stabilizes the tetrahedral Td configuration for OgF
4, as distinct from the square planar D4h one of XeF
4, which RnF
4 is also expected to have;[134] this is because OgF4 is expected to have two inert electron pairs (7s and 7p1/2). As such, OgF6 is expected to be unbound, continuing an expected trend in the destabilisation of the +6 oxidation state (RnF6 is likewise expected to be much less stable than XeF6).[135][136] The Og–F bond will most probably be ionic rather than covalent, rendering the oganesson fluorides non-volatile.[7][137] OgF2 is predicted to be partially ionic due to oganesson's high electropositivity.[138] Oganesson is predicted to be sufficiently electropositive[138] to form an Og–Cl bond with chlorine.[7]

A compound of oganesson and tennessine, OgTs4, has been predicted to be potentially stable chemically.[139]

See also

[edit]

Notes

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References

[edit]

Bibliography

[edit]

Further reading

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Oganesson

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Oganesson (symbol Og) is a synthetic superheavy chemical element with atomic number 118, making it the heaviest known element and completing the seventh row of the periodic table.[1] As a member of group 18 (the noble gases), it is expected to exhibit properties similar to radon but influenced by strong relativistic effects that may alter its electronic structure, potentially making it more reactive and solid at room temperature rather than gaseous.[2] Only five atoms have ever been produced as of 2025, with its most stable isotope, ^{294}Og, possessing an extremely short half-life of approximately 0.7 milliseconds and decaying via alpha emission into livermorium-290.[3] The element was first synthesized in experiments conducted in 2005 and reported in 2006 at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, through a collaborative effort involving scientists from JINR and the Lawrence Livermore National Laboratory (LLNL) in the United States. This achievement involved accelerating calcium-48 ions to bombard a californium-249 target, resulting in the fusion reaction ^{48}Ca + ^{249}Cf → ^{294}Og + 3n, though initial attempts in 2002 were not immediately verified due to data issues in a prior unconfirmed claim.[4] The discovery was officially confirmed in 2006 after additional experiments produced three decay chains attributable to oganesson, leading to IUPAC validation in 2015.[3] In 2016, the International Union of Pure and Applied Chemistry (IUPAC) approved the name "oganesson" in honor of Russian nuclear physicist Yuri Tsolakovich Oganessian, who led the transactinide research efforts at JINR and contributed significantly to superheavy element synthesis.[1] Due to its fleeting existence and radioactivity, oganesson's chemical properties remain largely theoretical and uncharacterized experimentally, though computational models suggest it may form weak bonds and deviate from noble gas inertness.[5] Ongoing research focuses on producing more atoms to study its behavior and explore the "island of stability" for potentially longer-lived superheavy isotopes beyond atomic number 118.[6]

Introduction

Element overview

Oganesson is a synthetic superheavy chemical element with the symbol Og and atomic number 118. It belongs to group 18 of the periodic table, classified among the noble gases, and occupies the final position in period 7, thereby completing the seventh row of the table.[1][2] As the heaviest element currently known, oganesson represents the culmination of efforts to extend the periodic table beyond naturally occurring elements.[7] Superheavy elements such as oganesson are artificially produced in minute quantities using high-energy particle accelerators, exhibiting extreme instability due to their large atomic nuclei. Theoretical nuclear physics models suggest the existence of an "island of stability" in the superheavy realm, where isotopes with specific proton and neutron numbers—potentially including those near oganesson's atomic number of 118—could possess enhanced stability and longer half-lives compared to neighboring isotopes.[8][9] This concept drives ongoing research into superheavy nuclei, though oganesson's confirmed isotopes remain highly radioactive with sub-millisecond half-lives. The only confirmed isotope of oganesson is ^{294}Og, with a mass number of 294, produced through nuclear fusion reactions in laboratory settings.[2][7] This isotope underscores oganesson's position at the frontier of synthetic element production, highlighting both the achievements and challenges in exploring the limits of nuclear matter.

Initial synthesis and detection

Oganesson was first synthesized in 2002 through the hot fusion reaction between a beam of calcium-48 ions and a target of californium-249, producing the isotope oganesson-294 along with three neutrons: $ ^{249}\mathrm{Cf} + ^{48}\mathrm{Ca} \to ^{294}\mathrm{Og} + 3n $. The experiment was carried out by a collaborative team from the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, and the Lawrence Livermore National Laboratory. The $ ^{48}\mathrm{Ca} $ beam was accelerated to an energy of 245 MeV in the laboratory frame, corresponding to an excitation energy of the compound nucleus in the range 26.6–31.7 MeV to favor the 3n evaporation channel. The target consisted of $ ^{249}\mathrm{Cf} $ enriched to greater than 98% purity, deposited as a thin layer with an areal density of 0.23 mg/cm² on a titanium backing.[10] The heavy fusion-evaporation residues were isolated from the primary beam particles and lighter reaction products using the Dubna gas-filled recoil separator (DGFRS) at the Flerov Laboratory of Nuclear Reactions, JINR. This device employs a helium gas medium at low pressure to slow down and separate the recoiling oganesson nuclei based on their charge-to-mass ratio, achieving a transmission efficiency of approximately 35% for such superheavy recoils. The separated recoils were implanted into a 12-strip silicon detector array positioned at the focal plane of the separator, which had an overall efficiency of 87% for detecting alpha particles.[10] Detection of the synthesized $ ^{294}\mathrm{Og} $ atoms relied on registering time- and position-correlated alpha decay sequences from the implanted recoils, characteristic of the decay chain $ ^{294}\mathrm{Og} \to ^{290}\mathrm{Lv} \to ^{286}\mathrm{Fl} \to ^{282}\mathrm{Cn} \to ^{278}\mathrm{Ds} \to ^{274}\mathrm{Hs} $. The initial alpha decay from oganesson was identified by energies in the range of 10.9–11.7 MeV, for example, 11.32 MeV in one observed event, followed by subsequent alpha decays and often terminating in spontaneous fission of $ ^{286}\mathrm{Fl} $. These genetic links between decays confirmed the production of element 118.[10] The 2002 experiment yielded one confirmed decay chain after irradiating the target with $ 2.5 \times 10^{19} $ $ ^{48}\mathrm{Ca} $ ions, corresponding to a production cross section of about 1 pb for the 3n channel. A follow-up experiment in 2005, using a higher beam energy of 251 MeV (excitation energy 32.1–36.6 MeV) and a thicker target of 0.34 mg/cm², detected two additional correlated decay chains after a dose of $ 1.6 \times 10^{19} $ ions. These three events established the initial synthesis and detection of oganesson, with subsequent efforts producing a total of five confirmed atoms as of 2025.[10][11][12]

History

Theoretical predictions

In the late 19th and early 20th centuries, Dmitri Mendeleev and contemporaries envisioned the periodic table as a finite structure with positions for undiscovered elements, including those that would complete the seventh row up to atomic number 118, based on recurring periodicity and shell-filling patterns.[13] These predictions emphasized the table's natural endpoint around Z=118, anticipating a noble gas-like element in group 18, though without detailed nuclear stability considerations.[14] By the 1960s, Glenn T. Seaborg advanced theoretical frameworks for superheavy elements beyond Z=100, proposing an "island of stability" centered around atomic numbers Z=114 to 126 and neutron numbers N≈184, where closed nuclear shells could confer enhanced stability against fission and decay, potentially allowing isotopes of element 118 to exhibit relatively long half-lives. Seaborg's model, grounded in shell-model extrapolations and empirical trends from transuranic elements, suggested that superheavies like Z=118 might bridge actinide-like and novel electronic behaviors, though experimental verification remained distant. In the 1970s, detailed macroscopic-microscopic calculations by E.O. Fiset and J.R. Nix refined these ideas, predicting fission barriers and half-lives for superheavy nuclei, including those near Z=118 with N=184, where closed neutron and proton shells (N=184 and Z=114 or nearby) could yield half-lives up to 10^9 years for certain isotopes near the island's center, primarily limited by alpha decay rather than spontaneous fission.[15] These computations, using the liquid-drop model augmented by shell corrections, highlighted the potential stability of superheavy nuclides in this region while underscoring challenges from proton drip-line proximity. Pre-2000 theoretical speculations on element 118's chemistry focused on its expected noble gas character, tempered by relativistic effects from high nuclear charge. Kenneth S. Pitzer's 1975 Dirac-Hartree-Fock calculations indicated that relativistic stabilization of the 8s electrons in Z=118 would enhance volatility, suggesting a gaseous or low-boiling state akin to radon, while maintaining relative inertness due to closed-shell configuration, though with possible deviations from lighter group 18 trends. Subsequent pre-2000 models, incorporating spin-orbit coupling, predicted that these effects might slightly polarize the electron cloud, potentially allowing weak interactions, but affirmed overall noble gas-like behavior without significant reactivity.

Experimental discovery efforts

Early efforts to synthesize element 118 faced significant challenges and unconfirmed claims. In 1999, a team at Lawrence Berkeley National Laboratory (LBNL) reported the observation of three decay chains attributed to the production of element 118 through the fusion reaction ^{208}Pb + ^{86}Kr, claiming a cross-section of approximately 2 pb. However, subsequent reanalysis revealed that the data were fabricated, leading to the retraction of the claim in 2002 after investigations confirmed irregularities in the experimental records. This incident underscored the difficulties in detecting rare superheavy nuclei and heightened scrutiny on verification processes for such discoveries. The successful synthesis of element 118 was achieved by a collaboration between the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, and the Lawrence Livermore National Laboratory (LLNL) in the United States. In 2002, using the U400 cyclotron at JINR, researchers bombarded a ^{249}Cf target with a beam of ^{48}Ca ions, producing one atom of ^{294}Og, which decayed through a chain involving alpha emissions to known isotopes. These events were initially tentative due to the low statistics, but the decay characteristics aligned with theoretical expectations for superheavy nuclei.[16] To confirm the 2002 results, the team conducted further experiments in 2005 at the same facility, accelerating ^{48}Ca ions to energies around 247 MeV onto another ^{249}Cf target, yielding two additional ^{294}Og atoms with consistent decay sequences. The combined data from 2002 and 2005, totaling three events, demonstrated a production cross-section of about 0.5 pb, providing robust evidence for the isotope ^{294}Og. These findings were published in 2006, establishing the initial synthesis of element 118. A 2012 experiment at JINR produced one additional decay chain, further corroborating the results.[3] The discovery received official validation through a joint IUPAC/IUPAP working party, which in 2015 reviewed the experimental data and confirmed that the JINR-LLNL collaboration met the criteria for the identification of element 118, completing the seventh row of the periodic table.[17] Independent efforts to replicate the synthesis using alternative methods have been limited by the need for specialized heavy targets like californium, but the decay chain observations have been corroborated in related superheavy element studies.[18]

Official recognition and naming

Following the initial reports of element 118's synthesis by the Joint Institute for Nuclear Research (JINR) in Dubna, Russia, and the Lawrence Livermore National Laboratory (LLNL) in the United States, the International Union of Pure and Applied Chemistry (IUPAC) included the temporary systematic name ununoctium (symbol: Uuo) in its 2011 report on atomic weights of the elements, as a placeholder pending formal verification of the discovery.[19] In December 2015, IUPAC, in collaboration with the International Union of Pure and Applied Physics (IUPAP), verified the priority of discovery for element 118 to the JINR-LLNL collaboration based on their experimental evidence from 2002 to 2006, completing the seventh row of the periodic table and initiating the official naming process.[18] The discoverers proposed the name oganesson (symbol: Og) in June 2016, honoring Russian nuclear physicist Yuri Oganessian for his pioneering contributions to superheavy element research, including his leadership in the JINR efforts; this proposal underwent a five-month public review period as per IUPAC guidelines.[1] On November 28, 2016, the IUPAC Bureau formally approved oganesson as the official name, replacing ununoctium, with the announcement published on November 30, 2016, and the full recommendations detailed in the IUPAC journal Pure and Applied Chemistry.[20]

Nuclear properties

Known isotopes

Oganesson has only one confirmed isotope, ^{294}Og, which was first synthesized in 2006 through the fusion-evaporation reaction ^{249}Cf + ^{48}Ca, yielding three atoms with a half-life of 0.58^{+0.44}_{-0.18} ms. Additional experiments using the same reaction have produced a total of five atoms of ^{294}Og as of 2025. This isotope decays predominantly via alpha emission to ^{290}Lv.[10][21] Lighter isotopes such as ^{293}Og have been theoretically predicted and searched for using alternative fusion reactions, such as those involving lighter actinide targets, but no atoms have been observed. Heavier isotopes including ^{295}Og and ^{296}Og lie within the predicted island of stability, where shell effects might confer relatively longer half-lives on the order of seconds or more, though none have been synthesized experimentally. Due to the sole observation of ^{294}Og, the standard atomic weight of oganesson is assigned as [294].[22]

Stability and decay modes

Oganesson isotopes are extremely unstable due to their high atomic number, leading to short half-lives dominated by alpha decay and spontaneous fission. The only experimentally observed isotope, ^{294}Og, decays primarily via alpha emission to ^{290}Lv, releasing an alpha particle with an energy of 11.70 \pm 0.03 MeV and exhibiting a half-life of 0.58^{+0.44}_{-0.18} ms.[21] This decay chain continues with ^{290}Lv undergoing alpha decay to ^{286}Fl at an energy of 10.86 \pm 0.14 MeV and a half-life of approximately 14 ms, followed by ^{286}Fl decaying mainly through spontaneous fission (with a partial alpha decay branch) at a half-life of about 0.11 s, and further proceeding through alpha decays and fissions to lighter nuclei.[21] Theoretical models incorporating shell corrections within the macroscopic-microscopic framework predict an alpha decay half-life for ^{294}Og of approximately 0.58 ms, closely matching the experimental value and highlighting the influence of nuclear shell effects on stability in this region. For heavier oganesson isotopes (A > 294), calculations indicate that spontaneous fission becomes the dominant decay mode, as the fission barrier height decreases due to reduced shell stabilization, resulting in half-lives shorter than 1 \mu s. Shell corrections play a crucial role in determining fission barriers for superheavy elements like oganesson, where they enhance stability against fission near magic neutron numbers. In particular, isotopes approaching N = 184, such as ^{302}Og, are predicted to exhibit significantly longer half-lives—potentially on the order of minutes or more—due to heightened shell effects forming part of the island of stability, though such nuclei remain unsynthesized.

Atomic and physical properties

Electronic configuration and atomic structure

Oganesson's atomic mass is [294] u for its only known isotope, ^{294}Og.[7] Its ground-state electron configuration is predicted to be [Rn] 5f14 6d10 7s2 7p6, consistent with its position as the heaviest element in group 18 of the periodic table.[7][23] However, relativistic effects profoundly alter this arrangement at the spinor level, yielding [Rn] 5f14 6d10 7s2 7p1/22 7p3/24, where the 7p subshell is split into distinct j=1/2 and j=3/2 components due to strong spin-orbit coupling.[24] This spin-orbit interaction inverts the energy ordering of the 7p orbitals compared to lighter homologues, with the 7p3/2 spinors destabilized and raised in energy relative to the stabilized 7p1/2 spinors, resulting in a large splitting of approximately 10.1 eV. The relativistic destabilization of the 7p3/2 orbitals enhances their spatial extension, while the 7s2 electrons experience stabilization through contraction, making the overall valence shell more polarizable than expected for a noble gas.[25][24] Dirac-Fock calculations, incorporating relativistic effects via the Dirac equation, demonstrate these orbital distortions quantitatively: the 7s orbital contracts significantly (radial maximum reduced by over 20% compared to scalar relativistic approximations), whereas the 7p3/2 orbitals expand, leading to a diffuse outer electron density. These computations, often extended with many-body correlation methods like coupled-cluster theory, underscore the dominance of relativity in shaping oganesson's atomic structure.[26][25] The first ionization potential of oganesson, corresponding to removal of an electron from the outermost 7p3/2 spinor, is predicted at approximately 860 kJ/mol (8.9 eV), markedly lower than that of radon due to the relativistic weakening of 7p-7p electron repulsion in the expanded orbitals.[26][27] Higher ionization potentials, such as the second from 7p3/2, exceed 1500 kJ/mol, reflecting the stability of the inner valence electrons.[26][27]

Predicted physical characteristics

Due to strong relativistic effects on its electronic structure, oganesson is predicted to exist as a solid at room temperature and standard pressure, diverging markedly from the gaseous states of lighter group 18 elements like xenon and radon.[28] These effects destabilize the 7p orbitals, leading to increased electron delocalization and enhanced interatomic interactions that favor a condensed phase. Computational studies using density functional theory (DFT) and perturbation theory for molecular clusters (PTMC) indicate a face-centered cubic lattice structure (predicted) for solid oganesson, with cohesive energies comparable to those of solid radon but augmented by relativistic stabilization.[28][7] The predicted density of solid oganesson ranges from 6.6 to 7.4 g/cm³ near room temperature, reflecting its relatively compact atomic size despite the high atomic number; liquid densities are estimated slightly lower at around 6.6–7.1 g/cm³.[28] The calculated atomic radius is approximately 157 pm (predicted, covalent), while the van der Waals radius is predicted to be about 239 pm based on dipole polarizability-derived models.[7] Static dipole polarizability values from relativistic DFT calculations are estimated at 58 ± 6 atomic units, significantly higher than for radon (37.2 a.u.), which contributes to stronger van der Waals forces and the observed solid state.[28] Thermodynamic predictions suggest a melting point of 325 ± 15 K (predicted), derived from combined PTMC and thermodynamic integration methods, indicating oganesson would melt just above 0°C under standard pressure.[28] The boiling point is estimated at 450 ± 10 K (predicted), implying relatively low volatility for a group 18 element, though still lower than typical metals; these values highlight the transitional nature of oganesson's physical behavior influenced by its predicted [Rn] 5f¹⁴ 6d¹⁰ 7s² 7p⁶ electronic configuration.[28][29]

Chemical properties

Relativistic effects on chemistry

In superheavy elements like oganesson (Og, Z=118), relativistic effects arise primarily from the high velocities of inner-shell electrons, which approach a significant fraction of the speed of light, approximately 0.86c for the 1s orbital due to the intense nuclear attraction.[30] This leads to an increase in effective electron mass (γ ≈ 1.95), causing a direct relativistic contraction of s and p_{1/2} orbitals, while indirect effects from the altered nuclear potential further stabilize these orbitals and destabilize higher-angular-momentum ones.[30] As a result, the 7s and 7p_{1/2} orbitals in oganesson contract significantly, shifting their energies downward by several eV compared to non-relativistic predictions.[31] These orbital changes profoundly impact oganesson's position in group 18, deviating from the inertness of lighter noble gases like xenon. Relativistic stabilization of the core reduces the first ionization potential to approximately 8.89 eV, lower than xenon's 12.13 eV and even radon's 10.75 eV, thereby enhancing potential reactivity.[32] Due to the element's short half-life and limited production, experimental values for electronegativity, oxidation states, and typical ionic radius remain unknown; theoretical predictions exist, including those based on Shannon radii for the ionic radius.[33][34] The large spin-orbit splitting in the 7p shell (~10 eV) destabilizes the 7p_{3/2} subshell, expanding its radial extent and promoting electron participation in bonding, contrary to the closed-shell stability expected for noble gases.[35] This destabilization suggests oganesson could form diatomic compounds such as OgF_2 or OgCl_2 with halogens, where the 7p_{3/2} electrons engage in weak covalent interactions.[35] Theoretical comparisons with lighter group 18 elements employ scalar relativistic pseudopotentials to isolate these effects, revealing that while xenon and radon exhibit minimal deviations, oganesson's valence electrons show delocalized behavior akin to a semiconductor, with a narrowed band gap of ~1.5 eV in its solid form. Such methods, often combined with density functional theory, confirm the relativistic enhancement of interatomic cohesion and polarizability, underscoring oganesson's anomalous chemical profile.[36]

Predicted compounds and reactivity

Theoretical studies using coupled-cluster methods with spin-orbit coupling predict that oganesson can form fluorides such as OgF₂ and OgF₄, with the latter potentially adopting a tetrahedral geometry under relativistic conditions, and bond energies estimated around 200 kJ/mol for these compounds.[37] These bonds exhibit a mixed ionic-covalent character, with spin-orbit effects enhancing stability by increasing dissociation energies compared to scalar-relativistic approximations.[37] Higher fluorides like OgF₆ may also be feasible due to oganesson's predicted electropositivity, though detailed geometries favor lower coordination in most models.[37] Interhalogen compounds, such as OgCl₄, are theoretically possible as analogs to known xenon and krypton chlorides, but their instability arises from oganesson's short half-life, limiting any practical formation or observation.[37] The only confirmed isotope, ²⁹⁴Og, has a half-life of approximately 0.7 ms, decaying primarily via alpha emission, which severely restricts chemical investigations. Relativistic calculations indicate that the oganesson dimer, Og₂, forms a weakly bound van der Waals complex with a binding energy of about 7 kJ/mol and a bond length of 4.33 Å, driven by enhanced polarizability from spin-orbit splitting in the 7p shell—this contrasts with the even weaker bonding in He₂, highlighting oganesson's anomalous reactivity for a noble gas. Experimental verification of these predicted compounds remains challenging due to the sub-millisecond lifetime of oganesson isotopes, necessitating reliance on advanced theoretical models or speculative techniques like matrix isolation in noble gas matrices or gas-phase chromatography simulations to infer reactivity patterns.

References

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  20. [PDF] IUPAC Periodic Table of the Elements
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  24. [PDF] Ionization potentials and electron affinity of oganesson - arXiv
  25. https://doi.org/10.1103/PhysRevLett.120.053001
  26. https://doi.org/10.1002/anie.202011976
  27. Oganesson - Los Alamos National Laboratory
  28. Relativistic effects on the electronic structure of the heaviest ...
  29. Oganesson Is a Semiconductor: On the Relativistic Band‐Gap ... - NIH
  30. [2107.02164] Ionization potentials and electron affinity of oganesson
  31. Colloquium: Superheavy elements: Oganesson and beyond
  32. Oganesson: A Noble Gas Element That Is Neither Noble Nor a Gas
  33. https://doi.org/10.1063/5.0294269
  34. Oganesson - Element information, properties and uses | Periodic Table
  35. Atomic Spectra Database
  36. WebElements Periodic Table » Oganesson » the essentials
  37. Oganesson - Element information, properties and uses | Periodic Table
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