Superheavy elements, also known astransactinide elements,transactinides, orsuper-heavy elements, orsuperheavies for short, are thechemical elements with anatomic number of at least 104.[1] The superheavy elements are those beyond theactinides in the periodic table; the last actinide islawrencium (atomic number 103). By definition, superheavy elements are alsotransuranium elements, i.e., having atomic numbers greater than that ofuranium (92). Depending on the definition ofgroup 3 adopted by authors, lawrencium may also be included to complete the 6d series.[2][3][4][5]
Glenn T. Seaborg first proposed theactinide concept, which led to the acceptance of theactinide series. He also proposed a transactinide series ranging from element 104 to121 and asuperactinide series approximately spanning elements122 to 153 (though more recent work suggests the end of the superactinide series to occur at element 157 instead). The transactinideseaborgium was named in his honor.[6][7]
Superheavies areradioactive and have only been obtained synthetically in laboratories. No macroscopic sample of any of these elements has ever been produced. Superheavies 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 10−14second, which is the time it takes for the atom to form an electron cloud.[8]
The known superheavies form part of the 6d and 7p series in the periodic table. Except forrutherfordium anddubnium (and lawrencium if it is included), all known isotopes of superheavies havehalf-lives of minutes or less. Theelement naming controversy involved elements102–109. Some of these elements thus usedsystematic names for many years after their discovery was confirmed. (Usually the systematic names are replaced with permanent names proposed by the discoverers relatively soon after a discovery has been confirmed.)
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 ofmass, 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 thebeam of lighter nuclei. Two nuclei can onlyfuse into one if they approach each other closely enough; normally, nuclei (all positively charged) repel each other due toelectrostatic repulsion. Thestrong interaction can overcome this repulsion but only within a very short distance from a nucleus; beam nuclei are thus greatlyaccelerated 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 thespeed 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 about 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 itscross 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 cantunnel 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.[15]
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 Visualization of unsuccessful nuclear fusion, based on calculations from theAustralian National University[18] |
The resulting merger is anexcited state[19]—termed acompound nucleus—and thus it is very unstable.[15] To reach a more stable state, the temporary merger mayfission without formation of a more stable nucleus.[20] Alternatively, the compound nucleus may eject a fewneutrons, which would carry away the excitation energy; if the latter is not sufficient for a neutron expulsion, the merger would produce agamma ray. This happens in about 10−16 seconds after the initial nuclear collision and results in creation of a more stable nucleus.[20] The definition by theIUPAC/IUPAP Joint Working Party (JWP) states that achemical element can only be recognized as discovered if a nucleus of it has notdecayed within 10−14 seconds. This value was chosen as an estimate of how long it takes a nucleus to acquireelectrons and thus display its chemical properties.[21][d]
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 asurface-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, theenergy, 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 outermostnucleons (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] Totalbinding 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 andspontaneous 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 correspondingenergy barriers for each mode, but they can be tunneled through.[28][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.[36] Spontaneous fission is caused by electrostatic repulsion tearing the nucleus apart and produces various nuclei in different instances of identical nuclei fissioning.[29] As the atomic number increases, spontaneous fission rapidly becomes more important: spontaneous fission partial half-lives decrease by 23 orders of magnitude fromuranium (element 92) tonobelium (element 102),[37] and by 30 orders of magnitude fromthorium (element 90) tofermium (element 100).[38] The earlierliquid drop model thus suggested that spontaneous fission would occur nearly instantly due to disappearance of thefission barrier for nuclei with about 280 nucleons.[29][39] The laternuclear shell model suggested that nuclei with about 300 nucleons would form anisland of stability in which nuclei will be more resistant to spontaneous fission and will primarily undergo alpha decay with longer half-lives.[29][39] 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.[40] Experiments on lighter superheavy nuclei,[41] as well as those closer to the expected island,[37] 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.)[23] The known nucleus can be recognized by the specific characteristics of decay it undergoes such as decay energy (or more specifically, thekinetic 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]
The heaviest element known at the end of the 19th century was uranium, with anatomic mass of about 240 (now known to be 238)amu. Accordingly, it was placed in the last row of the periodic table; this fueled speculation about the possible existence ofelements heavier than uranium and whyA = 240 seemed to be the limit. Following the discovery of thenoble gases, beginning withargon in 1895, the possibility of heavier members of the group was considered. Danish chemistJulius Thomsen proposed in 1895 the existence of a sixth noble gas withZ = 86,A = 212 and a seventh withZ = 118,A = 292, the last closing a 32-elementperiod containingthorium and uranium.[52] In 1913, Swedish physicistJohannes Rydberg extended Thomsen's extrapolation of the periodic table to include even heavier elements with atomic numbers up to 460, but he did not believe that these superheavy elements existed or occurred in nature.[53]
In 1914, German physicistRichard Swinne proposed that elements heavier than uranium, such as those aroundZ = 108, could be found incosmic rays. He suggested that these elements may not necessarily have decreasing half-lives with increasing atomic number, leading to speculation about the possibility of some longer-lived elements at Z = 98–102 and Z = 108–110 (though separated by short-lived elements). Swinne published these predictions in 1926, believing that such elements might exist inEarth's core,iron meteorites, or theice caps of Greenland where they had been locked up from their supposed cosmic origin.[54]
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Work performed from 1961 to 2013 at four labs –Lawrence Berkeley National Laboratory in the US, theGSI Helmholtz Centre for Heavy Ion Research in Germany,Riken in Japan, and theJoint Institute for Nuclear Research (JINR) in the USSR (later Russia) – identified and confirmed the elementslawrencium tooganesson according to the criteria of theIUPAC–IUPAP Transfermium Working Groups and subsequent Joint Working Parties.
The creation ofLawrencium was first claimed in 1961 by theLawrence Berkeley National Laboratory, although doubts from scientists at theJINR, who reported their own synthesis of Lr-256 later in the 1960s, would delay general acceptance until 1992 byIUPAC-IUPAP.[55]
These discoveries complete the seventh row of the periodic table. The next two elements,ununennium (Z = 119) andunbinilium (Z = 120), have not yet been synthesized. They would begin an eighth period.
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 with increasing atomic number) and the low yield of thenuclear reactions that produce them, new methods have had to be created to determine their gas-phase and solution chemistry based on very small samples of a few atoms each.Relativistic effects become very important in this region of the periodic table, causing the filled 7s orbitals, empty 7p orbitals, and filling 6d orbitals to all contract inward toward the atomic nucleus. This causes a relativistic stabilization of the 7s electrons and makes the 7p orbitals accessible in low excitation states.[7]
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 homologs of lutetium through osmium. They are expected to haveionic radii between those of their 5d transition metal homologs and theiractinide pseudohomologs: for example, Rf4+ is calculated to have ionic radius 76 pm, between the values forHf4+ (71 pm) andTh4+ (94 pm). Their ions should also be lesspolarizable 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.[7]
Elements 113 to 118, nihonium to oganesson, should form a 7p series, completing theseventh period in the periodic table. Their chemistry will be greatly influenced by the very strong relativistic stabilization of the 7s electrons and a strongspin–orbit coupling effect "tearing" the 7p subshell apart into two sections, one more stabilized (7p1/2, holding two electrons) and one more destabilized (7p3/2, holding four electrons). Lower oxidation states should be stabilized here, continuing group trends, as both the 7s and 7p1/2 electrons exhibit theinert-pair effect. These elements are expected to largely continue to follow group trends, though with relativistic effects playing an increasingly larger role. In particular, the large 7p splitting results in an effective shell closure at flerovium (element 114) and a hence much higher than expected chemical activity for oganesson (element 118).[7]
Oganesson is the last known element. The next two elements,119 and120, should form an 8s series and be analkali andalkaline 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 andstrontium. 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 forelement 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 toactinium andthorium respectively.[7]
At element 121, thesuperactinide series is expected to begin, when the 8s electrons and the filling 8p1/2, 7d3/2, 6f5/2, and 5g7/2 subshells determine the chemistry of these elements. Complete and accurate calculations are not available for elements beyond 123 because of the extreme complexity of the situation:[56] the 5g, 6f, and 7d orbitals should have about the same energy level, and in the region of element 160 the 9s, 8p3/2, and 9p1/2 orbitals should also be about equal in energy. This will cause the electron shells to mix so that theblock concept no longer applies very well, and will also result in novel chemical properties that will make positioning these elements in a periodic table very difficult.[7]
It has been suggested that elements beyondZ = 126 be calledbeyond superheavy elements.[57] Other sources refer to elements around Z = 164 ashyperheavy elements.[58]
Superheavy elements (Z > 102) are teetering at the limits of mass and charge.
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