Unbihexium
Theoretical element | ||||||
---|---|---|---|---|---|---|
Unbihexium | ||||||
Pronunciation | /ˌuːnbaɪˈhɛksiəm/ | |||||
Alternative names | element 126, eka-plutonium | |||||
Unbihexium in the periodic table | ||||||
| ||||||
period 8 (theoretical, extended table) | ||||||
Block | g-block | |||||
Electron configuration | predictions vary, see text | |||||
Physical properties | ||||||
Phase at STP | unknown | |||||
Atomic properties | ||||||
Oxidation states | (+1), (+2), (+4), (+6), (+8) (predicted)[1] | |||||
Other properties | ||||||
CAS Number | 54500-77-5 | |||||
History | ||||||
Naming | IUPAC systematic element name | |||||
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
Synthesis of superheavy nuclei
![A graphic depiction of a nuclear fusion reaction](http://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Deuterium-tritium_fusion.svg/220px-Deuterium-tritium_fusion.svg.png)
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 second 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 for past that phase, multiple nuclear interactions result in redistribution of energy and an energy equilibrium.[9]
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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 second 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 its outer electrons and thus display its chemical properties.[15][d]
Decay and detection
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 second; 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]
Alpha particles are commonly produced in radioactive decays because 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.
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
Synthesis attempts
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
Possible natural occurrence
A study in 1976 by a group of American researchers from several universities proposed that
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.
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
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
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
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.
Predicted properties
Nuclear stability and isotopes
![](http://upload.wikimedia.org/wikipedia/commons/thumb/9/9e/Nuclear_chart_from_KTUY_model.svg/400px-Nuclear_chart_from_KTUY_model.svg.png)
![](http://upload.wikimedia.org/wikipedia/commons/thumb/7/70/Next_proton_shell.svg/260px-Next_proton_shell.svg.png)
Extensions of the
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
A "sea of instability" defined by very low
Chemical
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
See also
- Island of stability: flerovium–unbinilium–unbihexium
Notes
- superactinide series).[5]Terms "heavy isotopes" (of a given element) and "heavy nuclei" mean what could be understood in the common language—isotopes of high mass (for the given element) and nuclei of high mass, respectively.
- ^ The amount of energy applied to the beam particle to accelerate it can also influence the value of cross section. For example, in the 28
14Si
+ 1
0n
→ 28
13Al
+ 1
1p
reaction, cross section changes smoothly from 370 mb at 12.3 MeV to 160 mb at 18.3 MeV, with a broad peak at 13.5 MeV with the maximum value of 380 mb.[11] - ^ This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[16]
- ^ This separation is based on that the resulting nuclei move past the target more slowly then the unreacted beam nuclei. The separator contains electric and magnetic fields whose effects on a moving particle cancel out for a specific velocity of a particle.[18] Such separation can also be aided by a time-of-flight measurement and a recoil energy measurement; a combination of the two may allow to estimate the mass of a nucleus.[19]
- ^ Not all decay modes are caused by electrostatic repulsion. For example, beta decay is caused by the weak interaction.[26]
- ^ It was already known by the 1960s that ground states of nuclei differed in energy and shape as well as that certain magic numbers of nucleons corresponded to greater stability of a nucleus. However, it was assumed that there was no nuclear structure in superheavy nuclei as they were too deformed to form one.[31]
- ^ Since mass of a nucleus is not measured directly but is rather calculated from that of another nucleus, such measurement is called indirect. Direct measurements are also possible, but for the most part they have remained unavailable for superheavy nuclei.[36] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[37] Mass was determined from the location of a nucleus after the transfer (the location helps determine its trajectory, which is linked to the mass-to-charge ratio of the nucleus, since the transfer was done in presence of a magnet).[38]
- ^ If the decay occurred in a vacuum, then since total momentum of an isolated system before and after the decay must be preserved, the daughter nucleus would also receive a small velocity. The ratio of the two velocities, and accordingly the ratio of the kinetic energies, would thus be inverse to the ratio of the two masses. The decay energy equals the sum of the known kinetic energy of the alpha particle and that of the daughter nucleus (an exact fraction of the former).[27] The calculations hold for an experiment as well, but the difference is that the nucleus does not move after the decay because it is tied to the detector.
- Georgy Flerov,[39] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[40] In contrast, the LBL scientists believed fission information was not sufficient for a claim of synthesis of an element. They believed spontaneous fission had not been studied enough to use it for identification of a new element, since there was a difficulty of establishing that a compound nucleus had only ejected neutrons and not charged particles like protons or alpha particles.[16] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[39]
- ^ For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics in Stockholm, Stockholm County, Sweden.[41] There were no earlier definitive claims of creation of this element, and the element was assigned a name by its Swedish, American, and British discoverers, nobelium. It was later shown that the identification was incorrect.[42] The following year, RL was unable to reproduce the Swedish results and announced instead their synthesis of the element; that claim was also disproved later.[42] JINR insisted that they were the first to create the element and suggested a name of their own for the new element, joliotium;[43] the Soviet name was also not accepted (JINR later referred to the naming of the element 102 as "hasty").[44] This name was proposed to IUPAC in a written response to their ruling on priority of discovery claims of elements, signed 29 September 1992.[44] The name "nobelium" remained unchanged on account of its widespread usage.[45]
- ^ Atomic numbers 114, 120, 122, 124 have also been suggested as closed proton shells in different models.
- parentnucleus, as a superheavy atom that decays before reaching the detector will not be registered at all.
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