Unbinilium
Theoretical element | ||||||
---|---|---|---|---|---|---|
Unbinilium | ||||||
Pronunciation | /ˌuːnbaɪˈnɪliəm/ | |||||
Alternative names | element 120, eka-radium | |||||
Unbinilium in the periodic table | ||||||
| ||||||
kJ/mol (extrapolated)[2] | ||||||
Atomic properties | ||||||
Oxidation states | (+1),[4] (+2), (+4), (+6) (predicted)[1][5] | |||||
Electronegativity | Pauling scale: 0.91 (predicted)[6] | |||||
Ionization energies | ||||||
Atomic radius | empirical: 200 pm (predicted)[1] | |||||
Covalent radius | 206–210 pm (extrapolated)[2] | |||||
Other properties | ||||||
Crystal structure | body-centered cubic (bcc) (extrapolated)[8] | |||||
CAS Number | 54143-58-7 | |||||
History | ||||||
Naming | IUPAC systematic element name | |||||
Isotopes of unbinilium | ||||||
Experiments and theoretical calculations | ||||||
Unbinilium, also known as eka-radium or element 120, is a hypothetical
Unbinilium has not yet been synthesized, despite multiple attempts from German and Russian teams. Experimental evidence from these attempts shows that the period 8 elements would likely be far more difficult to synthesise than the previous known elements. New attempts by American, Russian, and Chinese teams to synthesize unbinilium are planned to begin in the mid-2020s.
Unbinilium's position as the seventh alkaline earth metal suggests that it would have similar properties to its lighter
Introduction
Synthesis of superheavy nuclei
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.[14] 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.[15] 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.[15]
Coming close enough alone is not enough for two nuclei to fuse: when two nuclei approach each other, they usually remain together for approximately 10−20 seconds and then part ways (not necessarily in the same composition as before the reaction) rather than form a single nucleus.[15][16] This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed.[15] 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.[15]
External videos | |
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Visualization of unsuccessful nuclear fusion, based on calculations from the Australian National University[18] |
The resulting merger is an excited state[19]—termed a compound nucleus—and thus it is very unstable.[15] To reach a more stable state, the temporary merger may fission without formation of a more stable nucleus.[20] 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 approximately 10−16 seconds after the initial nuclear collision and results in creation of a more stable nucleus.[20] 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.[21][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.[23] 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.[23] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[26] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[23]
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.[27] 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.[28][29] Superheavy nuclei are thus theoretically predicted[30] and have so far been observed[31] 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,[33] and the lightest nuclide primarily undergoing spontaneous fission has 238.[34] In both decay modes, nuclei are inhibited from decaying by corresponding energy barriers for each mode, but they can be tunnelled through.[28][29]
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.)[23] 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
Elements 114 to 118 (flerovium through oganesson) were discovered in "hot fusion" reactions bombarding the actinides plutonium through californium with calcium-48, a quasi-stable neutron-rich isotope which could be used as a projectile to produce more neutron-rich isotopes of superheavy elements.[52] This cannot easily be continued to elements 119 and 120, because it would require a target of the next actinides einsteinium and fermium. Tens of milligrams of these would be needed to create such targets, but only micrograms of einsteinium and picograms of fermium have so far been produced.[53] More practical production of further superheavy elements would require bombarding actinides with projectiles heavier than 48Ca,[52] but this is expected to be more difficult.[53] Attempts to synthesize elements 119 and 120 push the limits of current technology, due to the decreasing cross sections of the production reactions and their probably short half-lives,[54] expected to be on the order of microseconds.[1][55]
Synthesis attempts
Past
Following their success in obtaining
- 244
94Pu
+ 58
26Fe
→ 302
120Ubn
* → no atoms
In April 2007, the team at the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany attempted to create unbinilium using a 238U target and a 64Ni beam:[59]
- 238
92U
+ 64
28Ni
→ 302
120Ubn
* → no atoms
No atoms were detected. The GSI repeated the experiment with higher sensitivity in three separate runs in April–May 2007, January–March 2008, and September–October 2008, all with negative results, reaching a cross section limit of 90 fb.[59]
In 2011, after upgrading their equipment to allow the use of more radioactive targets, scientists at the GSI attempted the rather asymmetrical fusion reaction:[60]
- 248
96Cm
+ 54
24Cr
→ 302
120Ubn
* → no atoms
It was expected that the change in reaction would quintuple the probability of synthesizing unbinilium,[61] as the yield of such reactions is strongly dependent on their asymmetry.[54] Although this reaction is less asymmetric than the 249Cf+50Ti reaction, it also creates more neutron-rich unbinilium isotopes that should receive increased stability from their proximity to the shell closure at N = 184.[62] Three signals were observed in May 2011; a possible assignment to 299Ubn and its daughters was considered,[63] but could not be confirmed,[64][65][62] and a different analysis suggested that what was observed was simply a random sequence of events.[66]
In August–October 2011, a different team at the GSI using the TASCA facility tried a new, even more asymmetrical reaction:[60][67]
- 249
98Cf
+ 50
22Ti
→ 299
120Ubn
* → no atoms
Because of its asymmetry,[68] the reaction between 249Cf and 50Ti was predicted to be the most favorable practical reaction for synthesizing unbinilium, though it produces a less neutron-rich isotope of unbinilium than any other reaction studied. No unbinilium atoms were identified.[67]
This reaction was investigated again in April to September 2012 at the GSI. This experiment used a 249Bk target and a 50Ti beam to produce element 119, but since 249Bk decays to 249Cf with a half-life of about 327 days, both elements 119 and 120 could be searched for simultaneously:
- 249
97Bk
+ 50
22Ti
→ 299
119Uue
* → no atoms - 249
98Cf
+ 50
22Ti
→ 299
120Ubn
* → no atoms
Neither element 119 nor element 120 was observed.[69]
Planned
In May 2021, the JINR announced plans to investigate the 249Cf+50Ti reaction in their new facility.[70] The 249Cf target would be produced by the Oak Ridge National Laboratory in Oak Ridge, Tennessee, United States; the 50Ti beam would be produced by the Hubert Curien Pluridisciplinary Institute in Strasbourg, Alsace, France. If diplomatic relations between Russia and the United States made this impossible, then the plan was to investigate the 248Cm+54Cr reaction instead, with a Russian-produced 248Cm target and a French-produced 54Cr beam, though the cross section would likely be three to ten times lower.[71]
After the Russian invasion of Ukraine began in February 2022, collaboration between the JINR and other institutes completely ceased due to sanctions.[72] Starting from 2022,[53] plans began to be made to use the 88-inch cyclotron in the Lawrence Berkeley National Laboratory (LBNL) in Berkeley, California, United States to attempt to make new elements using 50Ti projectiles. The plan is to first test them on a plutonium target to create livermorium (element 116) in late 2023. If that is successful, an attempt to make element 120 in the 249Cf+50Ti reaction will begin, probably in 2024 at the earliest.[73][74]
In March 2022,
The team at the Heavy Ion Research Facility in
Naming
Mendeleev's nomenclature for unnamed and undiscovered elements would call unbinilium eka-radium. The 1979 IUPAC recommendations temporarily call it unbinilium (symbol Ubn) until it is discovered, the discovery is confirmed and a permanent name chosen.[79] Although the IUPAC systematic names are widely used in the chemical community on all levels, from chemistry classrooms to advanced textbooks, scientists who work theoretically or experimentally on superheavy elements typically call it "element 120", with the symbol E120, (120) or 120.[1]
Predicted properties
Nuclear stability and isotopes
The stability of nuclei decreases greatly with the increase in atomic number after
Isotopes of unbinilium are predicted to have alpha decay half-lives of the order of
Given that element 120 fills the 2f5/2 proton orbital, much attention has been given to the compound nucleus 302Ubn* and its properties. Several experiments have been performed between 2000 and 2008 at the Flerov Laboratory of Nuclear Reactions in Dubna studying the fission characteristics of the compound nucleus 302Ubn*. Two nuclear reactions have been used, namely 244Pu+58Fe and 238U+64Ni. The results have revealed how nuclei such as this 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.[88]
In 2008, the team at GANIL, France, described the results from a new technique which attempts to measure the fission half-life of a compound nucleus at high excitation energy, since the yields are significantly higher than from neutron evaporation channels. It is also a useful method for probing the effects of shell closures on the survivability of compound nuclei in the super-heavy region, which can indicate the exact position of the next proton shell (Z = 114, 120, 124, or 126). The team studied the nuclear fusion reaction between uranium ions and a target of natural nickel:[89][90]
- 238
92U
+ nat
28Ni
→ 296,298,299,300,302
120Ubn
* → fission
The results indicated that nuclei of unbinilium were produced at high (~70 MeV) excitation energy which underwent fission with measurable half-lives just over 10−18 s.
Atomic and physical
Being the second
The main reason for the predicted differences between unbinilium and the other alkaline earth metals is the spin–orbit (SO) interaction—the mutual interaction between the electrons' motion and spin. The SO interaction is especially strong for the superheavy elements because their electrons move faster—at velocities comparable to the speed of light—than those in lighter atoms.[4] In unbinilium atoms, it lowers the 7p and 8s electron energy levels, stabilizing the corresponding electrons, but two of the 7p electron energy levels are more stabilized than the other four.[93] The effect is called subshell splitting, as it splits the 7p subshell into more-stabilized and the less-stabilized parts. Computational chemists understand the split as a change of the second (azimuthal) quantum number l from 1 to 1/2 and 3/2 for the more-stabilized and less-stabilized parts of the 7p subshell, respectively.[4][l] Thus, the outer 8s electrons of unbinilium are stabilized and become harder to remove than expected, while the 7p3/2 electrons are correspondingly destabilized, perhaps allowing them to participate in chemical reactions.[1] This stabilization of the outermost s-orbital (already significant in radium) is the key factor affecting unbinilium's chemistry, and causes all the trends for atomic and molecular properties of alkaline earth metals to reverse direction after barium.[94]
Due to the stabilization of its outer 8s electrons, unbinilium's first
Unbinilium should be a solid at room temperature, with melting point 680 °C:[96] this continues the downward trend down the group, being lower than the value 700 °C for radium.[97] The boiling point of unbinilium is expected to be around 1700 °C, which is lower than that of all the previous elements in the group (in particular, radium boils at 1737 °C), following the downward periodic trend.[3] The density of unbinilium has been predicted to be 7 g/cm3, continuing the trend of increasing density down the group: the value for radium is 5.5 g/cm3.[3][2]
Chemical
Compound | Bond length (Å) |
Bond-dissociation energy (eV) |
---|---|---|
Ca2 | 4.277 | 0.14 |
Sr2 | 4.498 | 0.13 |
Ba2 | 4.831 | 0.23 |
Ra2 | 5.19 | 0.11 |
Ubn2 | 5.65 | 0.02 |
The chemistry of unbinilium is predicted to be similar to that of the alkaline earth metals,
Unbinilium may also show the +4
Compound | Bond length (Å) |
Bond-dissociation energy (kJ/mol) |
---|---|---|
CaAu | 2.67 | 2.55 |
SrAu | 2.808 | 2.63 |
BaAu | 2.869 | 3.01 |
RaAu | 2.995 | 2.56 |
UbnAu | 3.050 | 1.90 |
In the gas phase, the alkaline earth metals do not usually form covalently bonded diatomic molecules like the alkali metals do, since such molecules would have the same number of electrons in the bonding and antibonding orbitals and would have very low
Compound | Bond length (Å) |
Harmonic frequency, cm−1 |
Vibrational anharmonicity, cm−1 |
Bond-dissociation energy (eV) |
---|---|---|---|---|
UbnH | 2.38 | 1070 | 20.1 | 1.00 |
BaH | 2.23 | 1168 | 14.5 | 2.06 |
UbnAu | 3.03 | 100 | 0.13 | 1.80 |
BaAu | 2.91 | 129 | 0.18 | 2.84 |
The Ubn–Au bond should be the weakest of all bonds between gold and an alkaline earth metal, but should still be stable. This gives extrapolated medium-sized adsorption enthalpies (−ΔHads) of 172 kJ/mol on gold (the radium value should be 237 kJ/mol) and 50 kJ/mol on silver, the smallest of all the alkaline earth metals, that demonstrate that it would be feasible to study the chromatographic adsorption of unbinilium onto surfaces made of noble metals.[94] The ΔHsub and −ΔHads values are correlated for the alkaline earth metals.[94]
See also
- Island of stability: flerovium–unbinilium–unbihexium
Notes
- superactinide series).[11]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.[17] - ^ This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[22]
- ^ 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.[24] 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.[25]
- ^ Not all decay modes are caused by electrostatic repulsion. For example, beta decay is caused by the weak interaction.[32]
- ^ 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.[37]
- ^ 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.[42] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[43] 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).[44]
- ^ 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).[33] 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,[45] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[46] 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.[22] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[45]
- ^ For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics in Stockholm, Stockholm County, Sweden.[47] 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.[48] 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.[48] JINR insisted that they were the first to create the element and suggested a name of their own for the new element, joliotium;[49] the Soviet name was also not accepted (JINR later referred to the naming of the element 102 as "hasty").[50] This name was proposed to IUPAC in a written response to their ruling on priority of discovery claims of elements, signed 29 September 1992.[50] The name "nobelium" remained unchanged on account of its widespread usage.[51]
- ^ The quantum number corresponds to the letter in the electron orbital name: 0 to s, 1 to p, 2 to d, etc. See azimuthal quantum number for more information.
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Previously, we worked mainly with calcium. This is element 20 in the Periodic Table. It was used to bombard the target. And the heaviest element that can be used to make a target is californium, 98. Accordingly, 98 + 20 is 118. That is, to get element 120, we need to proceed to the next particle. This is most likely titanium: 22 + 98 = 120.
There is still much work to adjust the system. I don't want to get ahead of myself, but if we can successfully conduct all the model experiments, then the first experiments on the synthesis of element 120 will probably start this year. - ^ Riegert, Marion (19 July 2021). "In search of element 120 in the periodic table of elements". en.unistra.fr. University of Strasbourg. Retrieved 20 February 2022.
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В этом году мы фактически завершаем подготовительную серию экспериментов по отладке всех режимов ускорителя и масс-спектрометров для синтеза 120-го элемента. Научились получать высокие интенсивности ускоренного хрома и титана. Научились детектировать сверхтяжелые одиночные атомы в реакциях с минимальным сечением. Теперь ждем, когда закончится наработка материала для мишени на реакторах и сепараторах у наших партнеров в «Росатоме» и в США: кюрий, берклий, калифорний. Надеюсь, что в 2025 г. мы полноценно приступим к синтезу 120-го элемента.
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