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Unbihexium, 126Ubh
Theoretical element
Unbihexium
Pronunciation/ˌuːnbaɪˈhɛksiəm/ ⓘ ​(OON-by-HEK-see-əm)
Alternative nameselement 126, eka-plutonium
Unbihexium 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
—
↑
Ubh
↓
—
unbipentium ← unbihexium → unbiseptium
Atomic number (Z)126
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), (+6), (+8)[1]
Other properties
CAS Number54500-77-5
History
NamingIUPAC systematic element name
| references

Unbihexium, also known as element 126 or eka-plutonium, is a hypothetical chemical element; it has atomic number 126 and placeholder symbol Ubh. Unbihexium and Ubh are the temporary IUPAC name and symbol, respectively, until the element is discovered, confirmed, and a permanent name is decided upon. In the periodic table, unbihexium is expected to be a g-block superactinide and the eighth element in the 8th period. Unbihexium has attracted attention among nuclear physicists, especially in early predictions targeting properties of superheavy elements, for 126 may be a magic number of protons near the center of an island of stability, leading to longer half-lives, especially for 310Ubh or 354Ubh which may also have magic numbers of neutrons.[2]

Early interest in possible increased stability led to the first attempted synthesis of unbihexium in 1971 and searches for it in nature in subsequent years. Despite several reported observations, more recent studies suggest that these experiments were insufficiently sensitive; hence, no unbihexium has been found naturally or artificially. Predictions of the stability of unbihexium vary greatly among different models; some suggest the island of stability may instead lie at a lower atomic number, closer to copernicium and flerovium.

Unbihexium is predicted to be a chemically active superactinide, exhibiting a variety of oxidation states from +1 to +8, and possibly being a heavier congener of plutonium. An overlap in energy levels of the 5g, 6f, 7d, and 8p orbitals is also expected, which complicates predictions of chemical properties for this element.

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.[8] 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.[9] 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.[9]

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.[9][10] This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed.[9] 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.[9]

External videos
video icon Visualization of unsuccessful nuclear fusion, based on calculations from the Australian National University[12]

The resulting merger is an excited state[13]—termed a compound nucleus—and thus it is very unstable.[9] To reach a more stable state, the temporary merger may fission without formation of a more stable nucleus.[14] 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.[14] 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.[15][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.[17] 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.[17] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[20] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[17]

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.[21] 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.[22][23] Superheavy nuclei are thus theoretically predicted[24] and have so far been observed[25] 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,[27] and the lightest nuclide primarily undergoing spontaneous fission has 238.[28] In both decay modes, nuclei are inhibited from decaying by corresponding energy barriers for each mode, but they can be tunneled through.[22][23]

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.[29]

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.[30] Spontaneous fission is caused by electrostatic repulsion tearing the nucleus apart and produces various nuclei in different instances of identical nuclei fissioning.[23] 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),[31] and by 30 orders of magnitude from thorium (element 90) to fermium (element 100).[32] 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.[23][33] 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.[23][33] 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.[34] Experiments on lighter superheavy nuclei,[35] as well as those closer to the expected island,[31] 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.)[17] 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]

The first and only attempt to synthesize unbihexium, which was unsuccessful, was performed in 1971 at CERN (European Organization for Nuclear Research) by René Bimbot and John M. Alexander using the hot fusion reaction:[2][46]

232
90Th + 84
36Kr → 316
126Ubh* → no atoms

High-energy (13-15 MeV) alpha particles were observed and taken as possible evidence for the synthesis of unbihexium. Subsequent unsuccessful experiments with higher sensitivity suggest that the 10 mb sensitivity of this experiment was too low; hence, the formation of unbihexium nuclei in this reaction was deemed highly unlikely.[47]

Possible natural occurrence

[edit]

A study in 1976 by a group of American researchers from several universities proposed that primordial superheavy elements, mainly livermorium, unbiquadium, unbihexium, and unbiseptium, with half-lives exceeding 500 million years[48] could be a cause of unexplained radiation damage (particularly radiohalos) in minerals.[49] This prompted many researchers to search for them in nature from 1976 to 1983. A group led by Tom Cahill, a professor at the University of California at Davis, claimed in 1976 that they had detected alpha particles and X-rays with the right energies to cause the damage observed, supporting the presence of these elements, especially unbihexium. Others claimed that none had been detected, and questioned the proposed characteristics of primordial superheavy nuclei.[50] In particular, they cited that the magic number N = 228 necessary for enhanced stability would create a neutron-excessive nucleus in unbihexium that might not be beta-stable, although several calculations suggest that 354Ubh may indeed be stable against beta decay.[51] This activity was also proposed to be caused by nuclear transmutations in natural cerium, raising further ambiguity upon this claimed observation of superheavy elements.[52]

Unbihexium has received particular attention in these investigations, for its speculated location in the island of stability may increase its abundance relative to other superheavy elements.[48] Any naturally occurring unbihexium is predicted to be chemically similar to plutonium and may exist with primordial 244Pu in the rare earth mineral bastnäsite.[48] In particular, plutonium and unbihexium are predicted to have similar valence configurations, leading to the existence of unbihexium in the +4 oxidation state. Therefore, should unbihexium occur naturally, it may be possible to extract it using similar techniques for the accumulation of cerium and plutonium.[48] Likewise, unbihexium could also exist in monazite with other lanthanides and actinides that would be chemically similar.[52] Recent doubt on the existence of primordial 244Pu casts uncertainty on these predictions, however,[53] as the nonexistence (or minimal existence) of plutonium in bastnäsite will inhibit possible identification of unbihexium as its heavier congener.

The possible extent of primordial superheavy elements on Earth today is uncertain. Even if they are confirmed to have caused the radiation damage long ago, they might now have decayed to mere traces, or even be completely gone.[54] It is also uncertain if such superheavy nuclei may be produced naturally at all, as spontaneous fission is expected to terminate the r-process responsible for heavy element formation between mass number 270 and 290, well before elements such as unbihexium may be formed.[55]

A recent hypothesis tries to explain the spectrum of Przybylski's Star by naturally occurring flerovium, unbinilium, and unbihexium.[56][57]

Naming

[edit]

Using the 1979 IUPAC recommendations, the element should be temporarily called unbihexium (symbol Ubh) until it is discovered, the discovery is confirmed, and a permanent name chosen.[58] Although widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, the recommendations are mostly ignored among scientists who work theoretically or experimentally on superheavy elements, who call it "element 126", with the symbol E126, (126), or 126.[59] Some researchers have also referred to unbihexium as eka-plutonium,[60][61] a name derived from the system Dmitri Mendeleev used to predict unknown elements, though such an extrapolation might not work for g-block elements with no known congeners, and eka-plutonium would instead refer to element 146[62] or 148[63] when the term is meant to denote the element directly below plutonium.

Prospects for future synthesis

[edit]

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 time periods with increased detection capabilities and enable otherwise inaccessible reactions.[68] Even so, it will likely be a great challenge to synthesize elements beyond unbinilium (120) or unbiunium (121), given their short predicted half-lives and low predicted cross sections.[69]

It has been suggested that fusion-evaporation will not be feasible to reach unbihexium. As 48Ca cannot be used for synthesis of elements beyond atomic number 118 or possibly 119, the only alternatives are increasing the atomic number of the projectile or studying symmetric or near-symmetric reactions.[70] One calculation suggests that the cross section for producing unbihexium from 249Cf and 64Ni may be as low as nine orders of magnitude lower than the detection limit; such results are also suggested by the non-observation of unbinilium and unbibium in reactions with heavier projectiles and experimental cross section limits.[71] If Z = 126 represents a closed proton shell, compound nuclei may have greater survival probability and the use of 64Ni may be more feasible for producing nuclei with 122 < Z < 126, especially for compound nuclei near the closed shell at N = 184.[72] However, the cross section still might not exceed 1 fb, posing an obstacle that may only be overcome with more sensitive equipment.[73]

Predicted properties

[edit]

Nuclear stability and isotopes

[edit]
This nuclear chart used by the Japan Atomic Energy Agency predicts the decay modes of nuclei up to Z = 149 and N = 256. At Z = 126 (top right), the beta-stability line passes through a region of instability towards spontaneous fission (half-lives less than 1 nanosecond) and extends into a "cape" of stability near the N = 228 shell closure, where an island of stability centered at the possibly doubly magic isotope 354Ubh may exist.[74]
This diagram depicts shell gaps in the nuclear shell model. Shell gaps are created when more energy is required to reach the shell at the next higher energy level, thus resulting in a particularly stable configuration. For protons, the shell gap at Z = 82 corresponds to the peak of stability at lead, and while there is disagreement of the magicity of Z = 114 and Z = 120, a shell gap appears at Z = 126, thus suggesting that there may be a proton shell closure at unbihexium.[75]

Extensions of the nuclear shell model predicted that the next magic numbers after Z = 82 and N = 126 (corresponding to 208Pb, the heaviest stable nucleus) were Z = 126 and N = 184, making 310Ubh the next candidate for a doubly magic nucleus. These speculations led to interest in the stability of unbihexium as early as 1957; Gertrude Scharff Goldhaber was one of the first physicists to predict a region of increased stability in the vicinity of, and possibly centered at, unbihexium.[2] This notion of an "island of stability" comprising longer-lived superheavy nuclei was popularized by University of California professor Glenn Seaborg in the 1960s.[76]

In this region of the periodic table, N = 184 and N = 228 have been suggested as closed neutron shells,[77] and various atomic numbers, including Z = 126, have been proposed as closed proton shells.[l] The extent of stabilizing effects in the region of unbihexium is uncertain, however, due to predictions of shifting or weakening of the proton shell closure and possible loss of double magicity.[77] More recent research predicts the island of stability to instead be centered at beta-stable isotopes of copernicium (291Cn and 293Cn)[70][78] or flerovium (Z = 114), which would place unbihexium well above the island and result in short half-lives regardless of shell effects.

Earlier models suggested the existence of long-lived nuclear isomers resistant to spontaneous fission in the region near 310Ubh, with half-lives on the order of millions or billions of years.[79] However, more rigorous calculations as early as the 1970s yielded contradictory results; it is now believed that the island of stability is not centered at 310Ubh, and thus will not enhance the stability of this nuclide. Instead, 310Ubh is thought to be very neutron-deficient and susceptible to alpha decay and spontaneous fission in less than a microsecond, and it may even lie at or beyond the proton drip line.[2][69][74] A 2016 calculation on the decay properties of 288–339Ubh upholds these predictions; the isotopes lighter than 313Ubh (including 310Ubh) may indeed lie beyond the drip line and decay by proton emission, 313–327Ubh will alpha decay, possibly reaching flerovium and livermorium isotopes, and heavier isotopes will decay by spontaneous fission.[80] This study and a quantum tunneling model predict alpha-decay half-lives under a microsecond for isotopes lighter than 318Ubh, rendering them impossible to identify experimentally.[80][81][m] Hence, the isotopes 318–327Ubh may be synthesized and detected, and may even constitute a region of increased stability against fission around N ~ 198 with half-lives up to several seconds, though such a region of increased stability is completely absent in other models.[78]

A "sea of instability" defined by very low fission barriers (caused by greatly increasing Coulomb repulsion in superheavy elements) and consequently fission half-lives on the order of 10−18 seconds is predicted across various models. Although the exact limit of stability for half-lives over one microsecond varies, stability against fission is strongly dependent on the N = 184 and N = 228 shell closures and rapidly drops off immediately beyond the influence of the shell closure.[69][74] Such an effect may be reduced, however, if nuclear deformation in intermediate isotopes may lead to a shift in magic numbers;[82] a similar phenomenon was observed in the deformed doubly magic nucleus 270Hs.[83] This shift could then lead to longer half-lives, perhaps on the order of days, for isotopes such as 342Ubh that would also lie on the beta-stability line.[82] A second island of stability for spherical nuclei may exist in unbihexium isotopes with many more neutrons, centered at 354Ubh and conferring additional stability in N = 228 isotones near the beta-stability line.[74] Originally, a short half-life of 39 milliseconds was predicted for 354Ubh toward spontaneous fission, though a partial alpha half-life for this isotope was predicted to be 18 years.[2] More recent analysis suggests that this isotope may have a half-life on the order of 100 years should the closed shells have strong stabilizing effects, placing it at the peak of an island of stability.[74] It may also be possible that 354Ubh is not doubly magic, as the Z = 126 shell is predicted to be relatively weak, or in some calculations, completely nonexistent. This suggests that any relative stability in unbihexium isotopes would be only due to neutron shell closures that may or may not have a stabilizing effect at Z = 126.[51][77]

Chemical

[edit]

Unbihexium is expected to be the sixth member of a superactinide series. It may have similarities to plutonium, as both elements have eight valence electrons over a noble gas core. In the superactinide series, the Aufbau principle is expected to break down due to relativistic effects, and an overlap of the energy levels of the 7d, 8p, and especially 5g and 6f orbitals is expected, which renders predictions of chemical and atomic properties of these elements very difficult.[84] The ground state electron configuration of unbihexium is thus predicted to be [Og] 5g2 6f2 7d1 8s2 8p1[85] or 5g1 6f4 8s2 8p1,[86] in contrast to [Og] 5g6 8s2 derived from Aufbau.

As with the other early superactinides, it is predicted that unbihexium will be able to lose all eight valence electrons in chemical reactions, rendering a variety of oxidation states up to +8 possible.[1] The +4 oxidation state is predicted to be most common, in addition to +2 and +6.[85][62] Unbihexium should be able to form the tetroxide UbhO4 and hexahalides UbhF6 and UbhCl6, the latter with a fairly strong bond dissociation energy of 2.68 eV.[87] Calculations suggest that a diatomic UbhF molecule will feature a bond between the 5g orbital in unbihexium and the 2p orbital in fluorine, thus characterizing unbihexium as an element whose 5g electrons should actively participate in bonding.[60][61] It is also predicted that the Ubh6+ (in particular, in UbhF6) and Ubh7+ ions will have the electron configurations [Og] 5g2 and [Og] 5g1, respectively, in contrast to the [Og] 6f1 configuration seen in Ubt4+ and Ubq5+ that bears more resemblance to their actinide homologs.[1] The activity of 5g electrons may influence the chemistry of superactinides such as unbihexium in new ways that are difficult to predict, as no known elements have electrons in a g orbital in the ground state.[62]

See also

[edit]

Notes

[edit]

References

[edit]

Bibliography

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Unbihexium

View on Grokipedia
from Grokipedia
Unbihexium (Ubh) is a hypothetical with 126, belonging to the g-block superactinides in the eighth period of the . It remains unsynthesized and unobserved, but is theorized to potentially achieve relative nuclear stability through closed proton and neutron shells at Z = 126 and N = 184, positioning it as a candidate within the predicted for superheavy nuclei. The systematic name "unbihexium" follows IUPAC conventions for unnamed elements, combining Latin roots "un-" (one), "bi-" (two), and "hex-" (six) to reflect the 126, with the placeholder symbol Ubh. Predicted nuclear properties include a range of isotopes from ^{288}Ubh to ^{326}Ubh, with longer half-lives (up to potentially detectable durations) for neutron-rich variants like ^{318}Ubh, ^{319}Ubh, and ^{320}Ubh, primarily decaying via alpha emission, while heavier isotopes may favor . These stability predictions arise from relativistic mean-field models and quantum mechanical fragmentation theories emphasizing shell effects. Chemically, unbihexium is expected to exhibit strong relativistic effects due to its high , leading to contracted orbitals that could enable unique bonding behaviors as a superactinide. Computational studies predict it may form stable diatomic molecules, such as unbihexium fluoride (UbhF), with a dissociation of about 7.5 eV when accounting for relativistic influences, distinguishing it from lighter homologues. Prospects for synthesis involve asymmetric heavy-ion fusion-evaporation reactions, such as ^{132}Sn + ^{194}Os or ^{70}Ni + ^{256}Cf, optimized at "hot" orientations to maximize cross-sections via fusion-like valleys, though estimated production rates remain below 10^{-36} cm², far beyond current accelerator capabilities. Recent studies using the dynamical cluster-decay model confirm cross-sections on the order of 10^{-36} cm² for proposed reactions, as of 2025. Theoretical interest in unbihexium extends to validating nuclear shell models and exploring the upper limits of matter stability, potentially informing broader understandings of and fundamental forces.

Introduction

Overview and nomenclature

Unbihexium (Ubh) is the systematic temporary name for the hypothetical superheavy with 126. This nomenclature follows the International Union of Pure and Applied Chemistry (IUPAC) conventions for undiscovered elements, where the name is constructed from numerical roots derived from Latin and Greek: "un-" for one (100s digit), "bi-" for two (10s digit), and "hex-" for six (units digit), yielding 100 + 20 + 6 = 126, terminated with the suffix "-ium" for neutrality. The placeholder symbol Ubh is formed from the initial letters of these roots. Upon potential synthesis and verification, IUPAC would oversee the assignment of a permanent name, adhering to guidelines established in 2016 that allow references to mythology, places, properties, or scientists while prohibiting offensive or eponymous names for living individuals. Unbihexium is positioned in the 8th period of the , within the superactinide series, and is theorized to extend group 16 (the chalcogens) beyond (Z=84), potentially behaving as eka-polonium in relativistic models, though its exact group assignment varies across theoretical frameworks due to g-block configurations. As a post-oganesson (Z=118) element, it represents a frontier in extending the periodic table, with its properties anticipated to deviate significantly from lighter homologues owing to relativistic effects and nuclear instability.

Role in superheavy element research

Unbihexium, with 126, serves as a critical target in research for probing the limits of nuclear stability, as its proton number aligns closely with predicted in the range of 114 to 126. These arise from shell-like structures in the nucleus, where filled proton shells enhance and resist fission, allowing researchers to test the boundaries of how far the periodic table can be extended beyond currently synthesized elements. Studying unbihexium would provide empirical data on whether such closures indeed confer greater stability to superheavy nuclei, informing the feasibility of even heavier elements. Central to unbihexium's significance is its potential position within the hypothesized island of stability, where isotopes with approximately 184 neutrons could exhibit significantly longer half-lives—potentially seconds to minutes or more—compared to the microseconds typical of known superheavies. This island stems from theoretical predictions of closed neutron shells at N=184, combined with proton magic numbers like Z=126, forming doubly magic configurations that minimize decay probabilities. Such stability would enable detailed studies of superheavy nuclear properties, advancing understanding of shell effects in extreme proton-rich environments. The pursuit of unbihexium builds on the successful synthesis of elements from (Z=104) to (Z=118), all confirmed by international consensus, with ongoing efforts in 2025 targeting elements 119 and 120 using advanced accelerators like those at and Berkeley Lab. Achieving unbihexium would validate quantum mechanical models, such as the extended to relativistic regimes, by confirming predicted shell closures and quantifying relativistic effects on nuclear orbitals in highly charged atoms. These insights are essential for refining theoretical frameworks that guide future synthesis strategies toward the .

History

Early theoretical predictions

The , independently developed by and J. Hans D. Jensen in the late 1940s and early 1950s, explained the stability of certain atomic nuclei through the concept of closed shells, analogous to electron shells in atoms. This model identified specific "" of protons (Z) or neutrons (N)—such as 2, 8, 20, 28, 50, 82, and 126—where nuclei exhibit enhanced and stability due to completed subshells. For superheavy elements, the model suggested Z=126 as a potential magic number corresponding to proton shell closure at the 3h_{11/2} orbital, implying greater nuclear stability in that region. In the 1960s, extensions of the further explored this proton magic number, incorporating effects like interactions and single-particle levels to predict possible isotopes near Z=126, particularly when paired with neutron magic numbers like N=184. These early calculations laid the groundwork for the hypothesis, where superheavy nuclei might exhibit longer half-lives due to doubly magic configurations. During the , advanced predictions for the , proposing an eighth period that included a g-block (l=4 orbitals) following the actinides, with elements extending up to Z=126. Seaborg's model envisioned the 8s and 8p orbitals initiating the period, followed by filling of 5g (18 electrons), 6f (14 electrons), and 7d (10 electrons) subshells, placing element 126 within the g-block and suggesting unique chemical behaviors influenced by these inner shells. Theoretical models evolved significantly in the late and with the incorporation of relativistic effects, as non-relativistic approximations failed to capture the strong spin-orbit coupling in heavy atoms. Pioneering Dirac-Hartree-Fock-Slater calculations by Bernd Fricke and John T. Waber demonstrated that relativistic corrections dramatically alter atomic radii, ionization potentials, and orbital energies for superheavy elements, shifting properties away from simple extrapolation of lighter homologs. By the and , more refined relativistic Dirac-Fock methods refined electronic structure predictions, accounting for quantum electrodynamic corrections and extended nuclear charge distributions. These calculations forecasted configurations for eighth-period elements like [Og] 8s² 8p^{1/2} 5g^{18} 6f^{14} 7d^{10} or similar variants, highlighting the destabilization of s and p orbitals due to relativistic contraction of inner shells and expansion of outer ones.

Synthesis attempts and challenges

As of 2025, no beyond (Z=118), first synthesized in 2006 via the reaction ^{48}Ca + ^{244}Pu at the (JINR), has been successfully produced, leaving unbihexium (Z=126) unsynthesized. The sole documented experimental attempt dates to the 1970s, when researchers at the Orsay Institute bombarded a ^{232}Th target with ^{84}Kr ions in a hot fusion reaction aimed at forming ^{316}Ubh; high-energy alpha particles (13–15 MeV) were observed and initially interpreted as evidence of superheavy residue formation, but subsequent analysis failed to confirm the presence of element 126 due to ambiguous decay signatures and lack of reproducibility. In the 2000s and 2010s, major laboratories including GSI Helmholtz Centre (Germany), (Japan), and JINR () focused on elements up to Z=118, with no dedicated runs reported for Z=126 owing to prohibitive technical barriers; however, theoretical investigations proposed viable beam-target combinations for future experiments, such as ^{68}Ni + ^{246}Cf and ^{70}Zn + ^{256}Cf, targeting isotopes like ^{314}Ubh and ^{326}Ubh in hot fusion channels with calculated evaporation residue cross sections around 10^{-13} to 10^{-9} mb (0.1 fb to 1 pb). More recent modeling in 2025 has highlighted asymmetric hot fusion reactions like ^{60}Fe + ^{250}Fm and ^{64}Ni + ^{249}Cf as potentially optimal for accessible Ubh isotopes, predicting peak cross sections below 0.1 pb near the barrier but emphasizing the need for advanced separators to isolate rare events. Key challenges stem from minuscule fusion probabilities driven by high Coulomb barriers in heavy-ion collisions, yielding cross sections typically under 1 pb—often as low as femt obarns—for proposed channels, necessitating ultra-intense beams (>10^{18} ions/s) and periods spanning months to years for even a single detection. Compounding this are the fleeting half-lives of targets, such as ^{256}Cf (∼13 minutes) and ^{246}Cf (<36 hours), which demand on-site production or rapid transport, while the anticipated Ubh isotopes exhibit predicted half-lives of microseconds to milliseconds, resulting in ultra-short chains (1–3 steps) that evade standard detection systems reliant on longer sequences for . Despite extended theoretical efforts and facility upgrades at JINR's Superheavy Element Factory and RIKEN's planned upgrades, no confirmed Ubh events have emerged by late , underscoring the frontier's reliance on next-generation accelerators.

Speculated natural occurrence

In the 1970s, researchers reported potential evidence for superheavy elements, including element 126 (unbihexium), in microscopic monazite inclusions within biotite mica samples from Madagascar, based on proton-induced X-ray emission spectroscopy that suggested characteristic X-ray lines consistent with Z=126. These inclusions were surrounded by giant radioactive halos, interpreted as decay products from primordial superheavy nuclei formed via rapid neutron capture (r-process) in ancient supernovae, with speculated traces persisting in uranium-rich ores or monazite due to hypothetical extended half-lives. However, follow-up analyses using mass spectrometry and refined X-ray techniques on similar samples found no such superheavy signatures, attributing the initial observations to measurement artifacts, such as overlapping spectral lines from lighter elements like thorium and uranium. During the 1980s, additional fringe claims emerged regarding anomalous heavy elements in the natural in , where isotopic anomalies in fission products were sometimes misinterpreted as evidence for transuranic or contributions beyond , potentially from events within the reactor zones. These reports suggested possible survival of short-lived heavy isotopes in the reactor's billion-year-old deposits, but subsequent geochemical studies confirmed the anomalies as resulting from standard fission yields of and lighter actinides, with no verifiable involvement due to instrumental resolution limits and lack of reproducible spectra. Post-2000 assessments conclude that no viable mechanism exists for natural unbihexium production detectable on , as the r-process in core-collapse supernovae or mergers generates superheavy nuclei transiently under extreme fluxes, but predicted half-lives for unbihexium isotopes are typically less than 1 second, far too brief for incorporation into planetary material or survival over geological timescales. Even in the hypothetical "" near Z=126 and N=184, the longest estimated half-lives reach only minutes to hours for doubly magic isotopes like ^{310}Ubh, insufficient for primordial retention or cosmic-ray propagation to . Such speculations contradict the observed endpoint of natural nucleosynthesis at Z=92 () in geochemical records, with no confirmed superheavy traces in meteorites, lunar samples, or terrestrial ores despite sensitive searches using . The absence of viable long-lived isotopes and the extreme astrophysical conditions required reinforce that unbihexium remains purely hypothetical and laboratory-dependent for study.

Predicted properties

Nuclear stability and isotopes

Theoretical predictions indicate that unbihexium (Z=126) could have isotopes ranging from ^{288}Ubh to ^{339}Ubh, with enhanced nuclear stability anticipated near the predicted neutron magic number N=184, corresponding to isotopes such as ^{310}Ubh. Models suggest that isotopes around A=310 to 326 exhibit relatively longer half-lives compared to neutron-deficient ones, potentially on the order of seconds to minutes for , enabling detection if synthesized, though exact values vary by model. Nuclear stability of unbihexium isotopes is assessed using the macroscopic-microscopic approach, which incorporates shell corrections to the liquid-drop model to account for quantum effects from closed shells. These corrections significantly influence fission barriers, estimated at 5-10 MeV for even-Z nuclei like unbihexium, with higher barriers (up to several MeV more) near proton (Z=126) and configurations due to increased . The dominant decay modes for unbihexium isotopes depend on neutron content: prevails in neutron-poor isotopes (e.g., A<310), while is favored for more neutron-rich ones approaching the . A representative process is 310Ubh→306Uuq+4He,^{310}\mathrm{Ubh} \to ^{306}\mathrm{Uuq} + ^{4}\mathrm{He}, with a Q-value of approximately 12 MeV, leading to half-lives potentially exceeding microseconds in favorable cases. Spontaneous fission half-lives are approximated by the formula t1/2SF≈10−21exp⁡(2πEbℏω) seconds,t_{1/2}^{\mathrm{SF}} \approx 10^{-21} \exp\left( \frac{2\pi E_b}{\hbar \omega} \right) \ \mathrm{seconds}, where EbE_b is the fission barrier height and ω\omega is the frequency of nuclear deformations (typically ~0.5-1 MeV/ℏ\hbar); this yields longer lifetimes for higher barriers in shell-stabilized isotopes.

Atomic and chemical characteristics

Unbihexium (Ubh, element 126) is predicted to be a g-block superactinide in the eighth period of the . Relativistic Dirac-Fock calculations predict the ground-state electronic configuration of neutral unbihexium as [Og] 5g² 6f³ 8s² 8p_{1/2}¹ or close variants, featuring significant contraction of the 5g orbitals due to spin-orbit coupling. These calculations incorporate direct relativistic effects from the and indirect effects from orbital contraction and stabilization, which dominate the atomic structure at such high atomic numbers. Strong relativistic influences are expected to induce an on the 8s electrons, stabilizing them and potentially rendering unbihexium more metallic in character compared to the volatile , with reduced volatility relative to (element 118). This shift arises from the large spin-orbit splitting in the 8p subshell (8p_{1/2} lower than 8p_{3/2}) and overall lanthanide-like contraction of inner orbitals, weakening interatomic bonding. Predicted oxidation states include +2 and +4, with possible access to higher states up to +6 or +8 under specific conditions, though +2 may predominate due to the inert pair. Hypothetical compounds such as UbhO₂ are anticipated to exhibit high stability, potentially resembling dioxide in structure but with altered bonding due to the contracted and 6f orbitals contributing minimally to covalency. The first potential is estimated at approximately 8–10 eV, lower than expected for lighter group 16 elements owing to relativistic stabilization of the 8s electrons, while subsequent potentials increase progressively.

Prospects for synthesis

Experimental methods and obstacles

The synthesis of unbihexium (Z=126) relies primarily on fusion-evaporation reactions involving medium-mass projectiles and targets to form a compound nucleus that subsequently evaporates 4-5 neutrons. Preferred reaction channels include combinations such as ^{60}Fe + ^{250}Fm or ^{64}Ni + ^{249}Cf, leading to excited states like ^{310}Ubh^* or ^{313}Ubh^*, though cross sections are predicted to be extremely low, on the order of 10^{-5} fb or less, necessitating extended irradiation times. Heavier projectiles like ^{64}Ni + ^{249}Cf have also been evaluated for producing ^{313}Ubh, with evaporation residue cross sections estimated via systematic analyses of prior data. These reactions are conducted at beam energies near or above the to maximize fusion probability while minimizing quasifission. Current facilities for such experiments include the U-400 at the (JINR) in , which has successfully produced elements up to Z=118 using ^{48}Ca beams at intensities of 6-8 \times 10^{12} particles per second (up to 1.2 pμA), and the GSI Helmholtz Centre in with its UNILAC and SIS accelerators for heavier ion beams. Future upgrades, such as the Super Heavy Element (SHE) Factory at featuring the DC280 capable of delivering medium-mass beams (A=20-60) at 5-10 pμA and energies up to 10 MeV/u, or the facility at GSI, are essential to achieve the required intensities for viable production rates of unbihexium isotopes. Detection employs gas-filled recoil separators, such as the GARIS or Dubna Gas-Filled Recoil Separator (DGFRS), which kinematically separate evaporation residues from the intense primary beam and transfer products, followed by implantation into silicon detectors at the focal plane for identification via correlated or events. Major obstacles include Q-value limitations for reactions with heavier projectiles, which result in higher excitation energies (typically 15-25 MeV) for the compound nucleus at optimal beam energies, thereby reducing the survival probability against fission. The fission width Γ_f of the compound nucleus is approximately 1 MeV at these excitation energies, comparable to the evaporation width, leading to predominant fission decay over the desired channel and survival probabilities below 10^{-3}. Additionally, detection challenges arise from high due to scattered beam ions, quasifission fragments, and transfer reaction products, compounded by the one-atom-at-a-time production yields, which demand beam times of months or years to observe even a single event, with false-positive risks mitigated only through of decay chains.

Theoretical models and future directions

Theoretical models for the synthesis of unbihexium (Z=126) primarily rely on the dynamical cluster-decay model (DCM), which calculates fusion probabilities and decay pathways in heavy-ion collisions by treating the compound nucleus as a dinuclear system that evolves through cluster emission. The DCM incorporates deformation effects and barrier penetrability to predict evaporation residue formation, showing that fusion hindrance due to quasifission dominates at Z=126, with survival probabilities dropping below 10^{-3} for multi-neutron evaporation channels. A 2025 study published in Physical Review C applied the DCM to assess Z=126 production via neutron-rich beams, evaluating channels such as ^{60}Fe + ^{250}Fm and ^{64}Ni + ^{249}Cf to approach the N=184 neutron shell; predicted cross-sections for viable isotopes range from 0.1 to 1 femtobarn (fb), highlighting the need for beams exceeding 10^{19} ions to detect events. Hot-fusion reactions, using neutron-rich projectiles like ^{48}Ca on actinide targets, are favored over approaches with lead targets to produce more neutron-excessive compound nuclei near N=184, as cold fusion yields proton-richer residues with lower stability. Additional studies have explored cold valley paths in optimum orientations, such as ^{70}Ni + ^{256}Cf, predicting enhanced preformation probabilities. The evaporation residue cross-section is modeled as σER=σfus⋅Wsur/⟨ρ⟩\sigma_{ER} = \sigma_{fus} \cdot W_{sur} / \langle \rho \rangle, where σfus\sigma_{fus} is the fusion cross-section, WsurW_{sur} the survival probability against fission, and ⟨ρ⟩\langle \rho \rangle the average multiplicity, emphasizing the interplay of fusion efficiency and deexcitation dynamics. Future directions include exploring multi-nucleon transfer (MNT) reactions in collisions of heavy pairs, such as ^{238}U + ^{248}Cm, to populate neutron-rich Z=126 isotopes beyond fusion limits, with model predictions indicating cross-sections up to 10^{-36} cm² for N≈184 nuclei. Laser-assisted heavy-ion fusion offers potential enhancement by modulating the through intense fields, increasing fusion rates by factors of 10-100 in simulations for systems. Facilities like the NICA at JINR, operational post-2030, could enable high-luminosity MNT experiments with accelerated heavy ions, targeting the around Z=126 and N=184.

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