Superheavy element
Superheavy elements, also known as transactinide elements, transactinides, or super-heavy elements, are the chemical elements with atomic number greater than 103. The superheavy elements are those beyond the actinides in the periodic table; the last actinide is lawrencium (atomic number 103). By definition, superheavy elements are also transuranium elements, i.e., having atomic numbers greater than that of uranium (92). Depending on the definition of group 3 adopted by authors, lawrencium may also be included to complete the 6d series.[1][2][3][4]
Superheavy elements are radioactive and have only been obtained synthetically in laboratories. No macroscopic sample of any of these elements have ever been produced. Superheavy elements are all named after physicists and chemists or important locations involved in the synthesis of the elements.
IUPAC defines an element to exist if its lifetime is longer than 10−14 seconds, which is the time it takes for the atom to form an electron cloud.[7]
The known superheavy elements form part of the 6d and 7p series in the periodic table. Except for rutherfordium and dubnium (and lawrencium if it is included), even the longest-lasting isotopes of superheavy elements have half-lives of minutes or less. The element naming controversy involved elements 102–109. Some of these elements thus used systematic names for many years after their discovery was confirmed. (Usually the systematic names are replaced with permanent names proposed by the discoverers relatively shortly after a discovery has been confirmed.)
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.[13] 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.[14] 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.[14]
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.[14][15] This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed.[14] 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.[14]
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Visualization of unsuccessful nuclear fusion, based on calculations from the Australian National University[17] |
The resulting merger is an excited state[18]—termed a compound nucleus—and thus it is very unstable.[14] To reach a more stable state, the temporary merger may fission without formation of a more stable nucleus.[19] 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.[19] 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.[20][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.[22] 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.[22] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[25] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[22]
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.[26] 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.[27][28] Superheavy nuclei are thus theoretically predicted[29] and have so far been observed[30] 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,[32] and the lightest nuclide primarily undergoing spontaneous fission has 238.[33] In both decay modes, nuclei are inhibited from decaying by corresponding energy barriers for each mode, but they can be tunnelled through.[27][28]
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.)[22] 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
Early predictions
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The heaviest element known at the end of the 19th century was uranium, with an
In 1914, German physicist
Discoveries
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Work performed from 1961 to 2013 at four labs – the
List of elements
- 103 Lawrencium, Lr (for Ernest Lawrence); sometimes but not always included[1][2]
- 104 Rutherfordium, Rf (for Ernest Rutherford)
- 105 Dubnium, Db (for the town of Dubna, near Moscow)
- 106 Seaborgium, Sg (for Glenn T. Seaborg)
- 107 Bohrium, Bh (for Niels Bohr)
- 108 Hassium, Hs (for Hassia [Hesse], location of Darmstadt)
- 109 Meitnerium, Mt (for Lise Meitner)
- 110 Darmstadtium, Ds (for Darmstadt)
- 111 Roentgenium, Rg (for Wilhelm Röntgen)
- 112 Copernicium, Cn (for Nicolaus Copernicus)
- 113 Nihonium, Nh (for Nihon [Japan], location of the Riken institute)
- 114 Flerovium, Fl (for Russian physicist Georgy Flyorov)
- 115 Moscovium, Mc (for Moscow)
- 116 Livermorium, Lv (for Lawrence Livermore National Laboratory)
- 117 Tennessine, Ts (for Tennessee, location of Oak Ridge National Laboratory)
- 118 Oganesson, Og (for Russian physicist Yuri Oganessian)
Characteristics
Due to their short half-lives (for example, the most stable known isotope of seaborgium has a half-life of 14 minutes, and half-lives decrease gradually with increasing atomic number) and the low yield of the
Elements 103 to 112, lawrencium to copernicium, form the 6d series of transition elements. Experimental evidence shows that elements 103–108 behave as expected for their position in the periodic table, as heavier homologues of lutetium through osmium. They are expected to have ionic radii between those of their 5d transition metal homologs and their actinide pseudohomologs: for example, Rf4+ is calculated to have ionic radius 76 pm, between the values for Hf4+ (71 pm) and Th4+ (94 pm). Their ions should also be less polarizable than those of their 5d homologs. Relativistic effects are expected to reach a maximum at the end of this series, at roentgenium (element 111) and copernicium (element 112). Nevertheless, many important properties of the transactinides are still not yet known experimentally, though theoretical calculations have been performed.[6]
Elements 113 to 118, nihonium to oganesson, should form a 7p series, completing the
Element 118 is the last element that has been synthesized. The next two elements, element 119 and element 120, should form an 8s series and be an alkali and alkaline earth metal respectively. The 8s electrons are expected to be relativistically stabilized, so that the trend toward higher reactivity down these groups will reverse and the elements will behave more like their period 5 homologs, rubidium and strontium. The 7p3/2 orbital is still relativistically destabilized, potentially giving these elements larger ionic radii and perhaps even being able to participate chemically. In this region, the 8p electrons are also relativistically stabilized, resulting in a ground-state 8s28p1 valence electron configuration for element 121. Large changes are expected to occur in the subshell structure in going from element 120 to element 121: for example, the radius of the 5g orbitals should drop drastically, from 25 Bohr units in element 120 in the excited [Og] 5g1 8s1 configuration to 0.8 Bohr units in element 121 in the excited [Og] 5g1 7d1 8s1 configuration, in a phenomenon called "radial collapse". Element 122 should add either a further 7d or a further 8p electron to element 121's electron configuration. Elements 121 and 122 should be similar to actinium and thorium respectively.[6]
At element 121, the
Beyond superheavy elements
It has been suggested that elements beyond Z = 126 be called beyond superheavy elements.[55] Other sources refer to elements around Z = 164 as hyperheavy elements.[56]
See also
- Bose–Einstein condensate (also known as Superatom)
- Island of stability
Notes
- superactinide series).[10]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.[16] - ^ This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[21]
- ^ 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.[23] 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.[24]
- ^ Not all decay modes are caused by electrostatic repulsion. For example, beta decay is caused by the weak interaction.[31]
- ^ 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.[36]
- ^ 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.[41] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[42] 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).[43]
- ^ 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).[32] 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,[44] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[45] 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.[21] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[44]
- ^ For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics in Stockholm, Stockholm County, Sweden.[46] 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.[47] 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.[47] JINR insisted that they were the first to create the element and suggested a name of their own for the new element, joliotium;[48] the Soviet name was also not accepted (JINR later referred to the naming of the element 102 as "hasty").[49] This name was proposed to IUPAC in a written response to their ruling on priority of discovery claims of elements, signed 29 September 1992.[49] The name "nobelium" remained unchanged on account of its widespread usage.[50]
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