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Unbibium, 122Ubb
Theoretical element
Unbibium
Pronunciation/ˌuːnbaɪˈbaɪəm/ ⓘ ​(OON-by-BY-əm)
Alternative nameselement 122, eka-thorium
Unbibium 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
Ununennium Unbinilium
Unquadtrium Unquadquadium Unquadpentium Unquadhexium Unquadseptium Unquadoctium Unquadennium Unpentnilium Unpentunium Unpentbium Unpenttrium Unpentquadium Unpentpentium Unpenthexium Unpentseptium Unpentoctium Unpentennium Unhexnilium Unhexunium Unhexbium Unhextrium Unhexquadium Unhexpentium Unhexhexium Unhexseptium Unhexoctium Unhexennium Unseptnilium Unseptunium Unseptbium
Unbiunium Unbibium Unbitrium Unbiquadium Unbipentium Unbihexium Unbiseptium Unbioctium Unbiennium Untrinilium Untriunium Untribium Untritrium Untriquadium Untripentium Untrihexium Untriseptium Untrioctium Untriennium Unquadnilium Unquadunium Unquadbium
—
↑
Ubb
↓
—
unbiunium ← unbibium → unbitrium
Atomic number (Z)122
Groupg-block groups (no number)
Periodperiod 8 (theoretical, extended table)
Block  g-block
Electron configurationpredictions vary, see text
Physical properties
Phase at STPunknown
Atomic properties
Oxidation statescommon: (none)
(+4)[1]
Ionization energies
  • 1st: 545 (predicted)[3] kJ/mol
  • 2nd: 1090 (predicted)[3] kJ/mol
  • 3rd: 1848 (predicted) kJ/mol
Other properties
CAS Number54576-73-7
History
NamingIUPAC systematic element name
| references

Unbibium, also known as element 122 or eka-thorium, is a hypothetical chemical element; it has placeholder symbol Ubb and atomic number 122. Unbibium and Ubb are the temporary systematic IUPAC name and symbol respectively, which are used until the element is discovered, confirmed, and a permanent name is decided upon. In the periodic table of the elements, it is expected to follow unbiunium as the second element of the superactinides and the fourth element of the 8th period. Similarly to unbiunium, it is expected to fall within the range of the island of stability, potentially conferring additional stability on some isotopes, especially 306Ubb which is expected to have a magic number of neutrons (184).

Despite several attempts, unbibium has not yet been synthesized, nor have any naturally occurring isotopes been found to exist. There are currently no plans to attempt to synthesize unbibium. In 2008, it was claimed to have been discovered in natural thorium samples,[4] but that claim has now been dismissed by recent repetitions of the experiment using more accurate techniques.

Chemically, unbibium is expected to show some resemblance to cerium and thorium. However, relativistic effects may cause some of its properties to differ; for example, it is expected to have a ground state electron configuration of [Og] 7d1 8s2 8p1 or [Og] 8s2 8p2, despite its predicted position in the g-block superactinide series.[1]

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.[10] 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.[11] 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.[11]

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.[11][12] This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed.[11] 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.[11]

External videos
video icon Visualization of unsuccessful nuclear fusion, based on calculations from the Australian National University[14]

The resulting merger is an excited state[15]—termed a compound nucleus—and thus it is very unstable.[11] To reach a more stable state, the temporary merger may fission without formation of a more stable nucleus.[16] 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.[16] 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.[17][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.[19] 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.[19] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[22] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[19]

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.[23] 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.[24][25] Superheavy nuclei are thus theoretically predicted[26] and have so far been observed[27] 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,[29] and the lightest nuclide primarily undergoing spontaneous fission has 238.[30] In both decay modes, nuclei are inhibited from decaying by corresponding energy barriers for each mode, but they can be tunneled through.[24][25]

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.[31]

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.[32] Spontaneous fission is caused by electrostatic repulsion tearing the nucleus apart and produces various nuclei in different instances of identical nuclei fissioning.[25] 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),[33] and by 30 orders of magnitude from thorium (element 90) to fermium (element 100).[34] 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.[25][35] 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.[25][35] 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.[36] Experiments on lighter superheavy nuclei,[37] as well as those closer to the expected island,[33] 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.)[19] 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]

Synthesis attempts

[edit]

Fusion-evaporation

[edit]

Two attempts were made to synthesize unbibium in the 1970s, both propelled by early predictions on the island of stability at N = 184 and Z > 120,[48] and in particular whether superheavy elements could potentially be naturally occurring.[49] The first attempts to synthesize unbibium were performed in 1972 by Flerov et al. at the Joint Institute for Nuclear Research (JINR), using the heavy-ion induced hot fusion reactions:[49]

238
92U + 66,68
30Zn → 304,306
122Ubb* → no atoms

Another unsuccessful attempt to synthesize unbibium was carried out in 1978 at the GSI Helmholtz Center, where a natural erbium target was bombarded with xenon-136 ions:[49]

nat
68Er + 136
54Xe → 298,300,302,303,304,306
Ubb* → no atoms

No atoms were detected and a yield limit of 5 nb (5,000 pb) was measured. Current results (see flerovium) have shown that the sensitivity of these experiments were too low by at least 3 orders of magnitude.[48] In particular, the reaction between 170Er and 136Xe was expected to yield alpha emitters with half-lives of microseconds that would decay down to isotopes of flerovium with half-lives perhaps increasing up to several hours, as flerovium is predicted to lie near the center of the island of stability. After twelve hours of irradiation, nothing was found in this reaction. Following a similar unsuccessful attempt to synthesize unbiunium from 238U and 65Cu, it was concluded that half-lives of superheavy nuclei must be less than one microsecond or the cross sections are very small.[50] More recent research into synthesis of superheavy elements suggests that both conclusions are true.[51][52]

In 2000, the Gesellschaft für Schwerionenforschung (GSI) Helmholtz Center for Heavy Ion Research performed a very similar experiment with much higher sensitivity:[49]

238
92U + 70
30Zn → 308
122Ubb* → no atoms

These results indicate that the synthesis of such heavier elements remains a significant challenge and further improvements of beam intensity and experimental efficiency is required. The sensitivity should be increased to 1 fb in the future for more quality results.

Compound nucleus fission

[edit]

Several experiments studying the fission characteristics of various superheavy compound nuclei such as 306Ubb were performed between 2000 and 2004 at the Flerov Laboratory of Nuclear Reactions. Two nuclear reactions were used, namely 248Cm + 58Fe and 242Pu + 64Ni.[49] The results reveal how superheavy nuclei fission predominantly by expelling closed shell nuclei such as 132Sn (Z = 50, N = 82). It was also found that the yield for the fusion-fission pathway was similar between 48Ca and 58Fe projectiles, suggesting a possible future use of 58Fe projectiles in superheavy element formation.[53]

Claimed discovery as a naturally occurring element

[edit]

In 2008, a group led by Israeli physicist Amnon Marinov at the Hebrew University of Jerusalem claimed to have found single atoms of unbibium-292 in naturally occurring thorium deposits at an abundance of between 10−11 and 10−12 relative to thorium.[4] This was the first time in 69 years that a new element had been claimed to be discovered in nature, after Marguerite Perey's 1939 discovery of francium.[l] The claim of Marinov et al. was criticized by the scientific community, and Marinov says he has submitted the article to the journals Nature and Nature Physics but both turned it down without sending it for peer review.[54] The unbibium-292 atoms were claimed to be superdeformed or hyperdeformed isomers, with a half-life of at least 100 million years.[49]

A criticism of the technique, previously used in purportedly identifying lighter thorium isotopes by mass spectrometry,[55] was published in Physical Review C in 2008.[56] A rebuttal by the Marinov group was published in Physical Review C after the published comment.[57]

A repeat of the thorium experiment using the superior method of accelerator mass spectrometry (AMS) failed to confirm the results, despite a 100-fold better sensitivity.[58] This result throws considerable doubt on the results of the Marinov collaboration with regards to their claims of long-lived isotopes of thorium,[55] roentgenium,[59] and unbibium.[4] Current understanding of superheavy elements indicates that it is very unlikely for any traces of unbibium to persist in natural thorium samples.[49]

Naming

[edit]

Using Mendeleev's nomenclature for unnamed and undiscovered elements, unbibium should instead be known as eka-thorium.[60] After the recommendations of the IUPAC in 1979, the element has since been largely referred to as unbibium with the atomic symbol of (Ubb),[61] as its temporary name until the element is officially discovered and synthesized, and a permanent name is decided on. Scientists largely ignore this naming convention, and instead simply refer to unbibium as "element 122" with the symbol of (122), or sometimes even E122 or 122.[62]

Prospects for future synthesis

[edit]
Predicted decay modes of superheavy nuclei. The line of synthesized proton-rich nuclei is expected to be broken soon after Z = 120, because of the shortening half-lives until around Z = 124, the increasing contribution of spontaneous fission instead of alpha decay from Z = 122 onward until it dominates from Z = 125, and the proton drip line around Z = 130. The white ring denotes the expected location of the island of stability; the two squares outlined in white denote 291Cn and 293Cn, predicted to be the longest-lived nuclides on the island with half-lives of centuries or millennia.[63][51]

Every element from mendelevium onward was produced in fusion-evaporation reactions, culminating in the discovery of the heaviest known element oganesson in 2002[64][65] and most recently tennessine in 2010.[66] These reactions approached the limit of current technology; for example, the synthesis of tennessine required 22 milligrams of 249Bk and an intense 48Ca beam for six months. The intensity of beams in superheavy element research cannot exceed 1012 projectiles per second without damaging the target and detector, and producing larger quantities of increasingly rare and unstable actinide targets is impractical.[67] Consequently, future experiments must be done at facilities such as the superheavy element factory (SHE-factory) at the Joint Institute for Nuclear Research (JINR) or RIKEN, which will allow experiments to run for longer stretches of time with increased detection capabilities and enable otherwise inaccessible reactions.[68]

It is possible that fusion-evaporation reactions will not be suitable for the discovery of unbibium or heavier elements. Various models predict increasingly short alpha and spontaneous fission half-lives for isotopes with Z = 122 and N ~ 180 on the order of microseconds or less,[69] rendering detection nearly impossible with current equipment.[51] The increasing dominance of spontaneous fission also may sever possible ties to known nuclei of livermorium or oganesson and make identification and confirmation more difficult; a similar problem occurred in the road to confirmation of the decay chain of 294Og which has no anchor to known nuclei.[70] For these reasons, other methods of production may need to be researched such as multi-nucleon transfer reactions capable of populating longer-lived nuclei. A similar switch in experimental technique occurred when hot fusion using 48Ca projectiles was used instead of cold fusion (in which cross sections decrease rapidly with increasing atomic number) to populate elements with Z > 113.[52]

Nevertheless, several fusion-evaporation reactions leading to unbibium have been proposed in addition to those already tried unsuccessfully, though no institution has immediate plans to make synthesis attempts, instead focusing first on elements 119, 120, and possibly 121. Because cross sections increase with asymmetry of the reaction,[52] a chromium beam would be most favorable in combination with a californium target,[51] especially if the predicted closed neutron shell at N = 184 could be reached in more neutron-rich products and confer additional stability. In particular, the reaction between 54
24Cr and 252
98Cf would generate the compound nucleus 306
122Ubb and reach the shell at N = 184, though the analogous reaction with a 249
98Cf target is believed to be more feasible because of the presence of unwanted fission products from 252
98Cf and difficulty in accumulating the required amount of target material.[71] One possible synthesis of unbibium could occur as follows:[51]

249
98Cf + 54
24Cr → 300
122Ubb + 3 1
0n

Should this reaction be successful and alpha decay remain dominant over spontaneous fission, the resultant 300Ubb would decay through 296Ubn which may be populated in cross-bombardment between 249Cf and 50Ti. Although this reaction is one of the most promising options for the synthesis of unbibium in the near future, the maximum cross section is predicted to be 3 fb,[71] one order of magnitude lower than the lowest measured cross section in a successful reaction. The more symmetrical reactions 244Pu + 64Ni and 248Cm + 58Fe[51] have also been proposed and may produce more neutron-rich isotopes. With increasing atomic number, one must also be aware of decreasing fission barrier heights, resulting in lower survival probability of compound nuclei, especially above the predicted magic numbers at Z = 126 and N = 184.[71]

Predicted properties

[edit]

Nuclear stability and isotopes

[edit]
See also: Island of stability
A 2D graph with rectangular cells colored in black-and-white colors, spanning from the llc to the urc, with cells mostly becoming lighter closer to the latter
A chart of nuclide stability as used by the Dubna team in 2010. Characterized isotopes are shown with borders. Beyond element 118 (oganesson, the last known element), the line of known nuclides is expected to rapidly enter a region of instability, with no half-lives over one microsecond after element 121; this poses difficulties in identifying heavier elements such as unbibium. The elliptical region encloses the predicted location of the island of stability.[52]

The stability of nuclei decreases greatly with the increase in atomic number after plutonium, the heaviest primordial element, so that all isotopes with an atomic number above 101 decay radioactively with a half-life under a day. No elements with atomic numbers above 82 (after lead) have stable isotopes.[72] Nevertheless, because of reasons not very well understood yet, there is a slight increased nuclear stability around atomic numbers 110–114, which leads to the appearance of what is known in nuclear physics as the "island of stability". This concept, proposed by University of California professor Glenn Seaborg, explains why superheavy elements last longer than predicted.[73]

In this region of the periodic table, N = 184 has been suggested as a closed neutron shell, and various atomic numbers have been proposed as closed proton shells, such as Z = 114, 120, 122, 124, and 126. The island of stability would be characterized by longer half-lives of nuclei located near these magic numbers, though the extent of stabilizing effects is uncertain due to predictions of weakening of the proton shell closures and possible loss of double magicity.[74] More recent research predicts the island of stability to instead be centered at beta-stable copernicium isotopes 291Cn and 293Cn,[52][75] which would place unbibium well above the island and result in short half-lives regardless of shell effects. The increased stability of elements 112–118 has also been attributed to the oblate shape of such nuclei and resistance to spontaneous fission. The same model also proposes 306Ubb as the next spherical doubly magic nucleus, thus defining the true island of stability for spherical nuclei.[76]

Regions of differently shaped nuclei, as predicted by the Interacting Boson Approximation[76]

A quantum tunneling model predicts the alpha-decay half-lives of unbibium isotopes 284–322Ubb to be on the order of microseconds or less for all isotopes lighter than 315Ubb,[77] highlighting a significant challenge in experimental observation of this element. This is consistent with many predictions, though the exact location of the 1 microsecond border varies by model. Additionally, spontaneous fission is expected to become a major decay mode in this region, with half-lives on the order of femtoseconds predicted for some even–even isotopes[69] due to minimal hindrance resulting from nucleon pairing and loss of stabilizing effects farther away from magic numbers.[71] A 2016 calculation on the half-lives and probable decay chains of isotopes 280–339Ubb yields corroborating results: 280–297Ubb will be proton unbound and possibly decay by proton emission, 298–314Ubb will have alpha half-lives on the order of microseconds, and those heavier than 314Ubb will predominantly decay by spontaneous fission with short half-lives.[78] For the lighter alpha emitters that may be populated in fusion-evaporation reactions, some long decay chains leading down to known or reachable isotopes of lighter elements are predicted. Additionally, the isotopes 308–310Ubb are predicted to have half-lives under 1 microsecond,[69][78] too short for detection as a result of significantly lower binding energy for neutron numbers immediately above the N = 184 shell closure. Alternatively, a second island of stability with total half-lives of approximately 1 second may exist around Z ~ 124 and N ~ 198, though these nuclei will be difficult or impossible to reach using current experimental techniques.[75] However, these predictions are strongly dependent on the chosen nuclear mass models, and it is unknown which isotopes of unbibium will be most stable. Regardless, these nuclei will be hard to synthesize as no combination of obtainable target and projectile can provide enough neutrons in the compound nucleus. Even for nuclei reachable in fusion reactions, spontaneous fission and possibly also cluster decay[79] might have significant branches, posing another hurdle to identification of superheavy elements as they are normally identified by their successive alpha decays.

Chemical

[edit]

Unbibium is predicted to be similar in chemistry to cerium and thorium, which likewise have four valence electrons above a noble gas core, although it may be more reactive. Additionally, unbibium is predicted to belong to a new block of valence g-electron atoms, although the 5g orbital is not expected to start filling until about element 125. The predicted ground-state electron configuration of unbibium is either [Og] 7d1 8s2 8p1[1][80] or 8s2 8p2,[81] in contrast to the expected [Og] 5g2 8s2 in which the 5g orbital starts filling at element 121. (The ds2p and s2p2 configurations are expected to be only separated by about 0.02 eV.)[81] In the superactinides, relativistic effects might cause a breakdown of the Aufbau principle and create overlapping of the 5g, 6f, 7d and 8p orbitals;[82] experiments on the chemistry of copernicium and flerovium provide strong indications of the increasing role of relativistic effects. As such, the chemistry of elements following unbibium becomes more difficult to predict.

Unbibium would most likely form a dioxide, UbbO2, and tetrahalides, such as UbbF4 and UbbCl4.[1] The main oxidation state is predicted to be +4, similar to cerium and thorium.[49] A first ionization energy of 5.651 eV and second ionization energy of 11.332 eV are predicted for unbibium; this and other calculated ionization energies are lower than the analogous values for thorium, suggesting that unbibium will be more reactive than thorium.[80][3]

Notes

[edit]

References

[edit]

Bibliography

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Unbibium

View on Grokipedia
from Grokipedia
Unbibium, with the temporary symbol Ubb, is the systematic name assigned by the International Union of Pure and Applied Chemistry (IUPAC) to the hypothetical with 122. Positioned in group 4 of the periodic table below , it would occupy the eighth period as the second superactinide element and is alternatively known as eka- due to its predicted chemical analogy to . As of November 2025, unbibium remains unsynthesized and unobserved, with all known chemical elements confirmed only up to ( 118). Theoretical studies predict that unbibium's electronic structure would be significantly influenced by relativistic effects, resulting in a ground-state configuration of [Og] 8s² 7d¹ 8p¹ rather than the non-relativistic [Og] 5g² 8s², leading to deviations in its ionization potentials and bonding behavior compared to thorium. It is expected to behave as a reactive, electropositive metal capable of forming a +4 oxidation state, potentially yielding compounds like oxides and halides analogous to those of group 4 elements, though enhanced spin-orbit coupling may stabilize higher oxidation states or alter reactivity. Nuclear models suggest possible isotopes ranging from mass numbers 275 to 326, with half-lives dominated by alpha decay or spontaneous fission, and the most stable potentially approaching seconds near the neutron dripline. Efforts to synthesize unbibium have been limited by the need for advanced accelerators to fuse suitable heavy-ion beams, such as iron-58 with curium-248, but no confirmed detections have occurred despite unverified claims, such as a 2008 report of element 122 in natural samples that was widely criticized for methodological flaws. Ongoing research focuses on the "" around atomic numbers 114–126, where enhanced stability from closed nuclear shells might allow longer-lived isotopes, motivating future experiments at facilities like the Gas-Filled Recoil Separator.

Introduction

General overview

Unbibium (Ubb) is the systematic temporary name assigned by the International Union of Pure and Applied Chemistry (IUPAC) to the hypothetical with 122. It is also referred to as eka-thorium in the Mendeleev for undiscovered elements. Superheavy elements are synthetic s with atomic numbers greater than 103, extending beyond the series in the periodic table. These elements occupy positions in the , where relativistic effects significantly influence their chemical properties. Unbibium is anticipated to reside in the 8th period and the g-block, corresponding to group 4, thereby being chemically analogous to , , , and . As of November 2025, unbibium has not been synthesized or observed, and no isotopes have been confirmed.

Role in superheavy element research

Unbibium, with 122, is predicted to reside within the theorized "," a region in the nuclear chart where s exhibit enhanced stability due to closed nuclear shells, potentially yielding isotopes with half-lives extending to minutes or longer compared to the milliseconds to seconds typical of known superheavies. This hypothesis, first proposed in the through macroscopic-microscopic nuclear models, anticipates greater stability around proton numbers Z ≈ 114–126 and neutron numbers N ≈ 184, where shell effects counteract fission and . Isotopes near the neutron shell closure at N=184, such as 122306Ubb^{306}_{122}\mathrm{Ubb}, may exhibit relatively enhanced stability with half-lives potentially reaching seconds in some models, driven by proximity to these , offering a for nuclear stability predictions. Relativistic effects play a crucial role in unbibium's nuclear and atomic structure, as the high nuclear charge accelerates inner electrons to speeds approaching the , altering orbital energies and spin-orbit splitting in ways that influence shell closures and overall stability. Specifically, the N=184 neutron shell closure is expected to provide a significant fission barrier for unbibium isotopes, enhancing resistance to , while proton shells near Z=120–126 contribute to doubly magic configurations that could prolong lifetimes. These effects underscore unbibium's position as a key probe for quantum mechanical models of high-Z nuclei, where relativistic corrections are essential for accurate predictions of binding energies and decay modes. Synthesizing unbibium would serve as a critical validation or refutation of extended periodic table models, confirming whether shell-stabilized superheavies can extend the table beyond Z=118 as quantum theories predict, including the persistence of actinide-like behavior amid relativistic distortions. By achieving observable quantities of element 122, researchers could empirically test calculations of nuclear shell structures and electron configurations, potentially reshaping understanding of matter's limits. As of 2025, ongoing theoretical efforts continue to explore configurations for superheavy elements using advanced semiclassical models like the Bohr-Sommerfeld quantization, bridging early quantum theory with superheavy physics to refine predictions of orbital dynamics in superstrong Coulomb fields.

Historical context

Theoretical predictions

In 1922, extended the periodic table beyond by predicting electronic configurations for elements up to Z=118. Theoretical predictions for further elements, including those around Z=122, developed later with advancements incorporating relativistic effects and new orbital fillings such as the 5g subshell. During the and , foundational nuclear models incorporating shell corrections revolutionized predictions for nuclei. V. M. Strutinsky's macroscopic-microscopic approach, introduced in 1967, accounted for quantum shell effects that deviate from the smooth liquid-drop behavior, revealing enhanced stability near closed shells at neutron number N=184 and proton numbers Z=114 or Z=120. Building on this, J. R. Nix and collaborators applied these methods to map the superheavy landscape, positioning element 122 (Z=122) near the edge of an "" where shell closures could counterbalance fission tendencies, potentially allowing isotopes with N≈184 to exhibit measurable lifetimes. Early estimates for hypothetical isotopes of unbibium, such as ^{310}Ubb (with N=188), relied on calculated fission barriers from these models. Using the Strutinsky shell corrections combined with the liquid-drop model, Nix et al. in 1972 derived barriers of approximately 6–8 MeV for nearby superheavy configurations, yielding half-lives on the order of seconds to minutes via the for tunneling probability. A persistent theoretical challenge for element 122 stems from the escalating repulsion between protons, which scales as Z^2/R in the liquid-drop framework and erodes fission barriers, promoting instability even in shell-stabilized regions; early calculations highlighted how this electrostatic term dominates for Z>120, limiting overall nuclear binding despite potential shell gains.

Experimental synthesis attempts

The earliest experimental efforts to synthesize superheavy elements in the region that includes unbibium (Z=122) were undertaken in 1972 at the (JINR) by Flerov et al., using the reaction ^{50}\text{Ti} + ^{238}\text{U} to search for activities potentially attributable to heavy nuclides. No confirmed events corresponding to unbibium or other superheavies were observed in these irradiations, with upper limits on production cross-sections exceeding those expected for such reactions. Later attempts to produce superheavy elements near Z=122, such as the 2009 experiment at JINR using ^{58}\text{Fe} + ^{244}\text{Pu} aimed at Z=120, reported no verifiable decay chains consistent with unbibium isotopes, highlighting the technical difficulties in reaching higher Z. Similarly, experiments at GSI Helmholtz Centre in – with reactions like ^{70}\text{Zn} + ^{208}\text{Pb} (targeting Z=112) and related systems yielded no results extending to Z=122. Efforts at and JINR in 2010 and subsequent years involved titanium beams (e.g., ^{48,50}\text{Ti}) on targets to probe Z=112–116, but these also failed to produce or detect unbibium, with no confirmed signals in the Z=122 region. Overall, all historical laboratory attempts to synthesize unbibium have resulted in non-observation, with production cross-sections estimated to be below 1 picobarn, increasingly challenging detection due to low event rates and fission barriers in the superheavy regime.

Claims of natural occurrence

In 2008, researchers led by Amnon Marinov at the Hebrew University of Jerusalem reported evidence for the detection of single atoms of unbibium (element 122) with mass number 292 in naturally occurring thorium samples derived from monazite minerals. Using high-resolution inductively coupled plasma sector field mass spectrometry (ICP-SFMS), they identified peaks corresponding to an atomic mass of approximately 292 u and an estimated atomic number around 122, attributing these signals to long-lived superdeformed or hyperdeformed nuclear isomers of unbibium with half-lives exceeding 108 years and abundances of (1–10) × 10−12 relative to 232Th. This claim suggested that unbibium-292 could be a stable endpoint or intermediate in an extended natural thorium decay chain, potentially surviving from primordial nucleosynthesis or cosmic ray interactions. Follow-up investigations by the Marinov group in the early reinforced the initial findings through additional analyses of thorium-rich minerals, again proposing unbibium-292 as part of a natural decay series originating from parents, with the isotope's extended enabling its persistence in . These studies maintained the estimated greater than 108 years and low abundance levels, hypothesizing production via secondary reactions in or deposits. The claims faced significant scientific scrutiny, primarily due to potential artifacts in the mass spectrometry data, such as molecular ion interferences (e.g., combinations of lighter ions mimicking the mass-to-charge ratio of unbibium) and unresolved contamination from thorium processing or instrumental backgrounds. Critics also highlighted isotopic misidentification risks, noting that the technique's resolution, while high, could not definitively distinguish superheavy signals from overlapping lighter element clusters without complementary spectroscopic confirmation. Furthermore, the proposed long-lived isotopes were deemed incompatible with astrophysical nucleosynthesis models, particularly the rapid neutron-capture (r-process) in neutron star mergers or supernovae, which predict negligible production and rapid decay of superheavy nuclei, preventing detectable terrestrial abundances. By 2015, the International Union of Pure and Applied Chemistry (IUPAC) and the broader community had reached a consensus rejecting these claims, citing lack of and insufficient verification through independent experiments. No supporting evidence for natural unbibium has emerged as of 2025, with subsequent searches in cosmic rays, deep-sea minerals, and accelerator-based simulations yielding null results.

Synthesis approaches

Fusion-evaporation reactions

Fusion-evaporation reactions represent the dominant experimental approach for synthesizing superheavy elements, including unbibium (Z=122), by accelerating a heavy-ion projectile onto an actinide target to form a highly excited compound nucleus that subsequently emits neutrons to de-excite into a more stable evaporation residue isotope. In this process, the projectile and target nuclei overcome the Coulomb barrier, fuse to create the compound system, and the resulting excitation energy—typically 25-40 MeV—drives the evaporation of 2-6 neutrons while competing against fission decay. For unbibium, representative reactions involve neutron-rich projectiles such as ^{54}Cr (Z=24) on ^{249}Cf (Z=98) to form ^{303}{122}\text{Ubb}^* or ^{58}Fe (Z=26) on ^{248}Cm (Z=96) to form ^{306}{122}\text{Ubb}^*, followed by evaporation of 3-4 neutrons to yield isotopes like ^{300}{122}\text{Ubb} or ^{302}{122}\text{Ubb}. Recent advancements in producing neutron-rich projectiles, such as titanium-50 beams used successfully for element 116 in 2024, suggest potential improvements for heavier systems like those required for Z=122, though specific attempts remain pending. The energetics of these reactions are characterized by Q-values for the complete fusion, derived from nuclear mass tables, which determine the excitation energy E^* of the compound nucleus as E^* = E_{c.m.} - V_B + Q, where E_{c.m.} is the center-of-mass energy, V_B is the Coulomb barrier (around 5-6 MeV for these systems), and Q is typically positive but leads to endothermic xn channels by approximately 30-50 MeV when accounting for neutron separation energies. For instance, in the ^{58}Fe + ^{248}Cm reaction, the optimal beam energy is set at 5-7 MeV per nucleon to achieve E^* ≈ 35 MeV, balancing fusion probability with survival against fission. Theoretical models, such as the dinuclear system (DNS) approach, predict fusion probabilities P_{CN} on the order of 10^{-3} to 10^{-2} due to quasifission competition, particularly influenced by the asymmetry and orientation of the colliding nuclei. The survival probability of the compound nucleus against fission is critically low for Z=122, estimated at 10^{-5} to 10^{-7} in multi-neutron channels, as the fission barrier heights (around 5-7 MeV) are comparable to the neutron separation energies (≈ 7-9 MeV), favoring prompt fission over residue formation. This is exacerbated by the shell effects weakening beyond Z=120, reducing stability. Overall residue cross-sections for these reactions are predicted to be 0.001-0.1 pb (or 1-100 fb), calculated using statistical models like HIVAP or Keats integrated with DNS fusion probabilities, far below the 0.1-1 pb detection sensitivity of current gas-filled recoil separators like SHIP or DGFRS. These minuscule yields highlight the challenges, with no confirmed synthesis to date despite theoretical feasibility.

Alternative nuclear reactions

Multi-nucleon transfer (MNT) reactions in low-energy collisions of heavy ions represent a proposed alternative to traditional fusion-evaporation for producing superheavy nuclei near Z=122, potentially populating longer-lived isotopes in the neutron-rich region. These reactions involve the exchange of multiple protons and neutrons between projectile and target nuclei, such as ^{54}Cr + ^{249,251}Cf or ^{58,64}Fe + ^{248}Cm, forming compound-like systems that evolve through dinuclear configurations rather than complete fusion. Theoretical calculations using the dinuclear system model predict peak production in 2-4 neutron evaporation channels at center-of-mass energies of 20-60 MeV, but cross sections remain extremely low, below 0.1 fb for Z=122 isotopes. High fission probabilities, driven by low fission barriers in these neutron-deficient nuclei, significantly reduce survival rates, making observation challenging compared to fusion paths where cross sections for nearby elements like Z=120 can reach up to 3 pb. Hypothetical multiple neutron capture processes, akin to the astrophysical r-process, could in principle form unbibium isotopes by successive neutron additions to actinide seeds like ^{238}U, followed by β-decays to reach Z=122 (e.g., building toward ^{308}Ubb or similar). In neutron star merger environments, neutron fluxes of 10^{20}-10^{30} cm^{-2} s^{-1} enable rapid captures up to A≈275-300, potentially synthesizing trace superheavies like Z=122 before fission or β-delayed fission recycles material. However, terrestrial accelerators cannot replicate these fluxes, limiting feasibility; even hypothetical scenarios like 20 sequential captures on ^{238}U yield unobservable rates (<10^{-30} pb) due to fission barriers dropping below 5 MeV for A>260. Compared to fusion-evaporation, r-process paths offer neutron-rich isotopes with higher stability but lack controllable excitation functions, with fission probabilities near unity halting chain growth beyond Z=110 in most models. Proposals for -assisted fusion concepts, using intense fields (10^{27}-10^{29} W/cm²) to enhance sub-barrier heavy-ion penetration (e.g., ^{16}O + ^{238}U), predict 6-60% fusion probability boosts for superheavies, applicable to Z=122 pathways. These remain theoretical, with no experiments conducted owing to laser intensity limits (current maxima ~10^{23} W/cm² yield <0.1% enhancement as of 2025), and excitation functions indicate marginal gains over unassisted fusion where fission barriers already limit survival to <10^{-9}.

Detection challenges

The detection of unbibium (element 122) presents significant technical challenges due to its anticipated low production cross-sections on the order of picobarns or less, necessitating advanced separation techniques to isolate residues from the intense primary beam and transfer products. Gas-filled recoil separators, such as the Separator for Heavy Ion Reaction Products (SHIP) at GSI Helmholtz Centre for Heavy Ion Research and the Berkeley Gas-filled Separator (BGS) at , are commonly employed for this purpose. These devices exploit the difference in charge-to-mass ratios between the heavy, multiply charged recoil ions and the lighter beam ions in a helium-filled flight path, achieving transmission efficiencies typically ranging from 20% to 50% for superheavy residues, depending on the specific reaction kinematics and gas pressure optimization. Observing unbibium decays is further complicated by the predicted short half-lives of its isotopes, often less than 1 second, which demand rapid implantation into position-sensitive detectors at the focal plane for real-time spectroscopy. Theoretical models indicate that promising neutron-rich isotopes, such as ^{306-310}Ubb, are expected to undergo alpha decay with energies around 11-12 MeV, potentially followed by subsequent alpha decays or spontaneous fission in the daughter nuclei, forming short decay chains that must be captured within milliseconds to establish a genetic link. These brief timescales limit the observation window, as any delay in separation or detection could result in missed events, particularly since unbibium lies beyond the currently synthesized superheavies where decay chains are longer and more readily verifiable. Background interference from scattered beam ions, fission fragments, and other reaction products poses a major hurdle, requiring separators to achieve beam suppression factors exceeding 10^6 to minimize false positives in the low-event-rate environment. Even with optimized setups, producing a single unbibium event typically demands irradiating the target with over 10^{18} beam ions, as cross-sections for fusion-evaporation reactions leading to Z=122 are projected to be sub-picobarn, compounded by the finite lifetime of enriched targets under high-intensity beams. Verification of any potential unbibium signal adheres to strict IUPAC/IUPAP guidelines, emphasizing the observation of multiple correlated decay sequences—such as implantation followed by alpha decays and terminating in or known isotopes—with consistent energies and timings to confirm parent-daughter relationships. These criteria ensure unambiguous identification amid potential contaminants, but the absence of established decay data for Z=122 heightens the reliance on precise modeling and replication across facilities.

Future synthesis prospects

Planned experimental pathways

As of November 2025, there are no confirmed scheduled experiments specifically targeting the synthesis of unbibium (Z=122). Current efforts at major facilities focus on elements 119 and 120, which are expected to pave the way for attempts at Z=122 once those are successful. For instance, the in , , is utilizing the with the DC280 to pursue element 120 via reactions such as ^{248}Cm + ^{54}Cr → ^{302}120^* (as of 2022 plans), with cross-sections around 0.2 pb and aims for high-intensity beams up to 10 particle μA over extended runs. These capabilities could be adapted for Z=122 in future campaigns. Theoretical proposals for unbibium synthesis include hot fusion reactions such as ^{58}Fe + ^{248}Cm → ^{306}122^* or ^{64}Ni + ^{244}Pu → ^{308}122^*, potentially yielding isotopes like ^{298–302}Ubb with cross-sections estimated at 0.1–1 pb, though no experimental timelines have been set. Earlier attempts at similar reactions for lower Z, such as ^{58}Fe + ^{244}Pu for Z=120, have informed models but remain unconfirmed for higher Z. The RIKEN Nishina Center in Japan has enhanced its accelerators, including a superconducting linear booster, to produce intense beams of mid-mass ions like ^{50}Ti for superheavy synthesis. While current focus is on Z=119 via ^{50}Ti + ^{249}Bk, upgrades support pathways toward Z=122, such as pairing titanium with higher-Z targets like ^{252}Cf (Z=98) for Z=120 as a precursor, with cross-sections below 1 pb. Validation tests emphasize beam stability for rare events. International collaborations, such as between JINR and GSI Helmholtz Centre, prioritize confirming Z=119 before advancing, integrating advanced modeling and gas-filled recoil separators to optimize detection for Z=120–122.

Required advancements

The synthesis of unbibium (Z=122) requires accelerator beam intensities 10 to 100 times higher than current levels at facilities like GSI or , given expected fusion-evaporation cross-sections of picobarns or lower. China's High Intensity heavy-ion Accelerator Facility (HIAF), operational since 2025, provides beam intensities up to 10^{12} particles per second for heavy ions like , enabling extended irradiations for rare detections. Upgrades to Russia's NICA complex at JINR similarly aim to increase heavy-ion currents for production, extending experiment durations beyond weeks. Improved detection systems are essential, with recoil separators offering high mass resolution and low background. RIKEN's Gas-filled Recoil Ion Separator (GARIS-III), operational since 2022, features advanced magnetics and ion transmission, suppressing backgrounds by 10–100 times over predecessors like GARIS-II, suitable for Z=119–122 hot-fusion products and chains. Theoretical modeling must refine fission barrier predictions using (DFT) with relativistic and pairing effects, achieving 1–2 MeV accuracy benchmarked on actinides to optimize projectile-target pairs without empirics. Key challenges include producing enriched targets like ^{248}Cm or ^{252}Cf for proposed Z=122 reactions such as ^{58}Fe + ^{248}Cm. Global stocks are limited to milligrams, with purities below 90%; solutions involve high-flux reactor irradiations (e.g., HFIR) and chemical separations to increase yields 5–10 fold, requiring international cooperation.

Predicted properties

Nuclear and atomic structure

Unbibium, with atomic number Z = 122, is expected to exhibit nuclear properties influenced by potential shell effects near the , though predictions indicate low fission barriers leading to short half-lives against decay modes such as alpha emission and . Theoretical models suggest isotopes around N = 184–186 may show some enhanced stability relative to neighbors, but overall half-lives remain on the order of microseconds or less. For example, the isotope ^{308}Ubb (N = 186) is predicted to have a half-life of approximately 10^{-9} s, primarily via , with alpha decay energy Q_α ≈ 15.2 MeV. A range of unbibium isotopes has been theoretically assessed for stability, with half-lives varying but generally extremely short, dominated by for neutron-deficient species and for those nearer the neutron drip line. Lighter isotopes like ^{282}Ubb are unbound or extremely short-lived (<10^{-10} s), while heavier ones like ^{308}Ubb to ^{310}Ubb have half-lives under 1 . The table below summarizes representative predicted half-lives and primary decay modes for selected isotopes from A = 282 to A = 310, derived from various models; note that exact values depend on the model, with SF often prevailing due to low fission barriers in superheavies.
IsotopeHalf-lifePrimary Decay Mode
^{282}Ubb<10^{-10} sAlpha / unbound
^{290}Ubb~10^{-9} s or lessAlpha
^{300}Ubb~10^{-6} s or lessSF / Alpha
^{308}Ubb~10^{-9} sSF (alpha possible)
^{310}Ubb<1 μsSF
These predictions highlight the limited role of neutron shell closures around N = 184–186 in extending viability for Z = 122, with overall low barriers preventing long-lived isotopes. Relativistic effects are pronounced in unbibium's atomic structure, necessitating advanced quantum mechanical treatments to describe its accurately. Relativistic Dirac-Fock calculations indicate a ground-state configuration of [Og] 8s² 7d¹ 8p¹ for the neutral atom, reflecting the influence of the superactinide series where relativistic stabilization affects orbital ordering. These computations reveal significant contraction of the 8s and 8p_{1/2} orbitals due to relativistic effects, leading to increased near the nucleus and altered radial distributions compared to non-relativistic approximations. Recent theoretical work applying old (Bohr-Sommerfeld model) to superheavy systems has uncovered novel features in unbibium's electronic behavior. A 2025 demonstrates the emergence of multi-self-intersecting trajectories for = 122 hydrogen-like ions, characterized by winding numbers exceeding unity and multiple intersection points in the orbital paths. This topological complexity enhances nuclear-electron binding by increasing the probability of electron-nucleus interactions, potentially stabilizing the atom against premature decay or ionization in strong fields.

Chemical and electronic characteristics

Unbibium, as the second member of the superactinide series (after element 121), is expected to exhibit a ground-state electron configuration of [Og] 8s² 7d¹ 8p¹, differing from the [Rn] 6d² 7s² configuration of its lighter homolog thorium due to relativistic stabilization of the 8p_{1/2} orbital. This configuration arises from strong relativistic effects, including significant spin-orbit coupling that lowers the energy of the 8p_{1/2} level relative to non-relativistic expectations, potentially leading to deviations in chemical behavior from group 4 elements like hafnium. The involvement of 7d and 8p orbitals in the valence shell suggests possible participation of g-orbitals in bonding for later superactinides, though for unbibium specifically, the 5g subshell remains empty in the ground state. The dominant for unbibium is predicted to be +4, analogous to and , reflecting the availability of four valence electrons beyond the core for bonding in compounds. Lower +2 states may occur due to an stabilizing the 8s² electrons, while higher +6 states could arise from 7d orbital involvement, though these are less stable and influenced by relativistic contraction of s and p orbitals. Relativistic effects are anticipated to enhance volatility in halides, such as unbibium tetrafluoride (UbbF₄), potentially allowing gas-phase studies similar to those for lighter superheavy elements. Hypothetical compounds include the dioxide UbbO₂, expected to form a solid akin to ThO₂, with strong ionic character in the +4 state. Organometallic complexes like bis(cyclooctatetraenyl)unbibium (Ubb(COT)₂) are theorized, leveraging the element's predicted ability to accommodate large ligands through d- and possible g-orbital hybridization, though experimental verification remains elusive due to synthesis challenges. Overall, unbibium's chemistry is projected to blend actinide-like reactivity with unique traits, prioritizing +4 valence for and formation while exhibiting enhanced volatility from relativistic influences.

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