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Nobelium is a synthetic chemical element with symbol No and atomic number 102. It is named in honor of Alfred Nobel, the inventor of dynamite and benefactor of science. A radioactive metal, it is the tenth transuranic element and is the penultimate member of the actinide series. Like all elements with atomic number over 100, nobelium can only be produced in particle accelerators by bombarding lighter elements with charged particles. A total of twelve nobelium isotopes are known to exist; the most stable is 259No with a half-life of 58 minutes, but the shorter-lived 255No (half-life 3.1 minutes) is most commonly used in chemistry because it can be produced on a larger scale. Chemistry experiments have confirmed that nobelium behaves as a heavier homolog to ytterbium in the periodic table. The chemical properties of nobelium are not completely known: they are mostly only known in aqueous solution. Before nobelium's discovery, it was predicted that it would show a stable +2 oxidation state as well as the +3 state characteristic of the other actinides: these predictions were later confirmed, as the +2 state is much more stable than the +3 state in aqueous solution and it is difficult to keep nobelium in the +3 state. In the 1950s and 1960s, many claims of the discovery of nobelium were made from laboratories in Sweden, the Soviet Union, and the United States. Although the Swedish scientists soon retracted their claims, the priority of the discovery and therefore the naming of the element was disputed between Soviet and American scientists, and it was not until 1997 that International Union of Pure and Applied Chemistry (IUPAC) established nobelium as the official name for the element and credited the Soviet team with the discovery. ==Discovery== The discovery of element 102 was a complicated process and was claimed by groups from Sweden, the United States, and the former Soviet Union. The first complete and incontrovertible report of its detection only came in 1966 from the Joint Institute of Nuclear Research at Dubna (then in the Soviet Union).〔 (Note: for Part I see Pure Appl. Chem., Vol. 63, No. 6, pp. 879–886, 1991)〕 The first announcement of the discovery of element 102 was announced by physicists at the Nobel Institute in Sweden in 1957. The team reported that they had bombarded a curium target with carbon-13 ions for twenty-five hours in half-hour intervals. Between bombardments, ion-exchange chemistry was performed on the target. Twelve out of the fifty bombardments contained samples emitting (8.5 ± 0.1) MeV alpha particles, which were in drops which eluted earlier than fermium (atomic number ''Z'' = 100) and californium (''Z'' = 98). The half-life reported was 10 minutes and was assigned to either 251102 or 253102, although the possibility that the alpha particles observed were from a presumably short-lived mendelevium (''Z'' = 101) isotope created from the electron capture of element 102 was not excluded.〔 The team proposed the name ''nobelium'' (No) for the new element,〔 which was immediately approved by IUPAC,〔 a decision which the Dubna group later characterized in 1968 as being hasty. The following year, scientists at the Lawrence Berkeley National Laboratory repeated the experiment but were unable to find any 8.5 MeV events which were not background effects.〔 In 1959, the Swedish team attempted to explain the Berkeley team's inability to detect element 102 in 1958, maintaining that they did discover it. However, later work has shown that no nobelium isotope lighter than 259No (no heavier isotopes could have been produced in the Swedish experiments) with a half-life over 3 minutes exist, and that the Swedish team's results are most likely from thorium-225, which has a half-life of 8 minutes and quickly undergoes triple alpha decay to polonium-213, which has a decay energy of 8.53612 MeV. This hypothesis is lent weight by the fact that thorium-225 can easily be produced in the reaction used and would not be separated out by the chemical methods used. Later work on nobelium also showed that the divalent state is more stable than the trivalent one and hence that the samples emitting the alpha particles could not have contained nobelium, as the divalent nobelium would not have eluted with the other trivalent actinides.〔 Thus, the Swedish team later retracted their claim and associated the activity to background effects. The Berkeley team, consisting of Albert Ghiorso, Glenn T. Seaborg, John R. Walton and Torbjørn Sikkeland, then claimed the synthesis of element 102 in 1958. The team used the new heavy-ion linear accelerator (HILAC) to bombard a curium target (95% 244Cm and 5% 246Cm) with 13C and 12C ions. They were unable to confirm the 8.5 MeV activity claimed by the Swedes but were instead able to detect decays from fermium-250, supposedly the daughter of 254102 (produced from the curium-246), which had an apparent half-life of ~3 s. Later 1963 Dubna work confirmed that 254102 could be produced in this reaction, but that its half-life was actually 50 ± 10 s. In 1967, the Berkeley team attempted to defend their work, stating that the isotope found was indeed 250Fm but the isotope that the half-life measurements actually related to was californium-244, granddaughter of 252102, produced from the more abundant curium-244. Energy differences were then attributed to "resolution and drift problems", although these had not been previously reported and should also have influenced other results. 1977 experiments showed that 252102 indeed had a 2.3-second half-life. However, 1973 work also showed that the 250Fm recoil could have also easily been produced from the isomeric transition of 250mFm (half-life 1.8 s) which could also have been formed in the reaction at the energy used.〔 Given this, it is probable that no nobelium was actually produced in this experiment.〔 In 1959 the team continued their studies and claimed that they were able to produce an isotope that decayed predominantly by emission of an 8.3 MeV alpha particle, with a half-life of 3 s with an associated 30% spontaneous fission branch. The activity was initially assigned to 254No but later changed to 252No. However, they also noted that it was not certain that nobelium had been produced due to difficult conditions.〔 The Berkeley team decided to adopt the proposed name of the Swedish team, "nobelium", for the element.〔 : + → → + 4 Meanwhile, in Dubna, experiments were carried out in 1958 and 1960 aiming to synthesize element 102 as well. The first 1958 experiment bombarded plutonium-239 and -241 with oxygen-16 ions. Some alpha decays with energies just over 8.5 MeV were observed, and they were assigned to 251,252,253102, although the team wrote that formation of isotopes from lead or bismuth impurities (which would not produce nobelium) could not be ruled out. While later 1958 experiments noted that new isotopes could be produced from mercury, thallium, lead, or bismuth impurities, the scientists still stood by their conclusion that element 102 could be produced from this reaction, mentioning a half-life of under 30 seconds and a decay energy of (8.8 ± 0.5) MeV. Later 1960 experiments proved that these were background effects. 1967 experiments also lowered the decay energy to (8.6 ± 0.4) MeV, but both values are too high to possibly match those of 253No or 254No.〔 The Dubna team later stated in 1970 and again in 1987 that these results were not conclusive.〔 In 1961, Berkeley scientists claimed the discovery of element 103 in the reaction of californium with boron and carbon ions. They claimed the production of the isotope 257103, and also claimed to have synthesized an alpha decaying isotope of element 102 that had a half-life of 15 s and alpha decay energy 8.2 MeV. They assigned this to 255102 without giving a reason for the assignment. The values do not agree with those now known for 255No, although they do agree with those now known for 257No, and while this isotope probably played a part in this experiment its discovery was inconclusive.〔 Work on element 102 also continued in Dubna, and in 1964, experiments were carried out there to detect alpha-decay daughters of element 102 isotopes by synthesizing element 102 from the reaction of a uranium-238 target with neon ions. The products were carried along a silver catcher foil and purified chemically, and the isotopes 250Fm and 252Fm were detected. The yield of 252Fm was interpreted as evidence that its parent 256102 was also synthesized: as it was noted that 252Fm could also be produced directly in this reaction by the simultaneous emission of an alpha particle with the excess neutrons, steps were taken to ensure that 252Fm could not go directly to the catcher foil. The half-life detected for 256102 was 8 s, which is much higher than the more modern 1967 value of (3.2 ± 0.2) s.〔 Further experiments were conducted in 1966 for 254102, using the reactions 243Am(15N,4n)254102 and 238U(22Ne,6n)254102, finding a half-life of (50 ± 10) s: at that time the discrepancy between this value and the earlier Berkeley value was not understood, although later work proved that the formation of the isomer 250mFm was less likely in the Dubna experiments than at the Berkeley ones. In hindsight, the Dubna results on 254102 were probably correct and can be now considered a conclusive detection of element 102.〔 One more very convincing experiment from Dubna came in 1966, again using the same two reactions, which concluded that 254102 indeed had a half-life much longer than the 3 seconds claimed by Berkeley.〔 Later work in 1967 at Berkeley and 1971 at the Oak Ridge National Laboratory fully confirmed the discovery of element 102 and clarified earlier observations.〔 In December 1966, the Berkeley group repeated the Dubna experiments and fully confirmed them, and used this data to finally assign correctly the isotopes they had previously synthesized but could not yet identify at the time, and thus claimed to have discovered nobelium in 1958 to 1961.〔 : + → → + 6 In 1969, the Dubna team carried out chemical experiments on element 102 and concluded that it behaved as the heavier homologue of ytterbium. The Russian scientists proposed the name ''joliotium'' (Jo) for the new element, creating an element naming controversy that would not be resolved for several decades, which each group using its own proposed names.〔 In 1992, the IUPAC-IUPAP Transfermium Working Group (TWG) reassessed the claims of discovery and concluded that only the Dubna work from 1966 correctly detected and assigned decays to nuclei with atomic number 102 at the time. The Dubna team are therefore officially recognised as the discoverers of nobelium although it is possible that it was detected at Berkeley in 1959.〔 This decision was criticized by Berkeley the following year, calling the reopening of the cases of elements 101 to 103 a "futile waste of time", while Dubna agreed with the IUPAC's decision.〔 In 1994, as part of an attempted resolution to the element naming controversy, the IUPAC ratified names for elements 101–109. For element 102, it ratified the name ''nobelium'' (No) on the basis that it had become entrenched in the literature over the course of 30 years and that Alfred Nobel should be commemorated in this fashion. Because of outcry over the 1994 names, which mostly did not respect the choices of the discoverers, a comment period ensued, and in 1995 IUPAC named element 102 ''flerovium'' (Fl) as part of a new proposal, after either Georgy Flyorov or his eponymous Flerov Laboratory of Nuclear Reactions.〔 〕 This proposal was also not accepted, and in 1997 the name "nobelium" was restored.〔 Today, the name "flerovium", with the same symbol, now refers to element 114. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nobelium」の詳細全文を読む スポンサード リンク
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