Nobelium

Nobelium is a synthetic chemical element; it has symbol No and atomic number 102. It is named after Alfred Nobel, the inventor of dynamite and benefactor of science. A radioactive metal, it is the tenth transuranium element, the second transfermium, and is the fourteenth 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 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. It was not until 1992 that the International Union of Pure and Applied Chemistry credited the Soviet team with the discovery. Even so, nobelium, the Swedish proposal, was retained as the name of the element due to its long-standing use in the literature.

Introduction

Discovery

The discovery of element 102 was a complicated process and was claimed by groups from Sweden, the United States, and the Soviet Union. The first complete and incontrovertible report of its detection only came in 1966 from the Joint Institute of Nuclear Research at Dubna.
The first announcement of the discovery of element 102 was announced by physicists at the Nobel Institute for Physics 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 MeV alpha particles, which were in drops which eluted earlier than fermium and californium. The half-life reported was 10 minutes and was assigned to either ²⁵¹No or ²⁵³No, although the possibility that the alpha particles observed were from a presumably short-lived mendelevium isotope created from the electron capture of element 102 was not excluded. The team proposed the name nobelium for the new element, which was immediately approved by IUPAC, a decision which the Dubna group characterized in 1968 as being hasty.
In 1958, scientists at the Lawrence Berkeley National Laboratory repeated the experiment. The Berkeley team, consisting of Albert Ghiorso, Glenn T. Seaborg, John R. Walton and Torbjørn Sikkeland, used the new heavy-ion linear accelerator to bombard a curium target 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 ²⁵⁰Fm, supposedly the daughter of ²⁵⁴No, which had an apparent half-life of ~3 s. Probably this assignment was also wrong, as later 1963 Dubna work showed that the half-life of ²⁵⁴No is significantly longer. It is more likely that the observed alpha decays did not come from element 102, but rather from ^250mFm.
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 isotopes lighter than ²⁵⁹No with a half-life over 3 minutes exist, and that the Swedish team's results are most likely from ²²⁵Th, which has a half-life of 8 minutes and quickly undergoes triple alpha decay to ²¹³Po, which has a decay energy of 8.53612 MeV. This hypothesis is lent weight by the fact that ²²⁵Th 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.
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 element 102 had been produced due to difficult conditions. The Berkeley team decided to adopt the proposed name of the Swedish team, "nobelium", for the element.
Meanwhile, in Dubna, experiments were carried out in 1958 and 1960 aiming to synthesize element 102 as well. The first 1958 experiment bombarded ²³⁹Pu and ²⁴¹Pu with ¹⁶O ions. Some alpha decays with energies just over 8.5 MeV were observed, and they were assigned to ^251,252,253No, although the team wrote that formation of isotopes from lead or bismuth impurities 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 MeV. Later 1960 experiments proved that these were background effects. 1967 experiments also lowered the decay energy to 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 lawrencium in the reaction of californium with boron and carbon ions. They claimed the production of the isotope ²⁵⁷Lr, 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 ²⁵⁵No 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 ²³⁸U 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 ²⁵⁶No 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 ²⁵⁶No was 8 s, which is much higher than the more modern 1967 value of s. Further experiments were conducted in 1966 for ²⁵⁴No, using the reactions ²⁴³Am²⁵⁴No and ²³⁸U²⁵⁴No, finding a half-life of 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 ²⁵⁴No were probably correct and can be now considered a conclusive detection of element 102.
One more very convincing experiment from Dubna was published in 1966, again using the same two reactions, which concluded that ²⁵⁴No 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. Thus they claimed to have discovered nobelium in 1958 to 1961.
File:Frederic and Irene Joliot-Curie.jpg|thumb|right|Frédéric Joliot and Irène Joliot-Curie
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 for the new element after Irène Joliot-Curie, who had recently died, creating an element naming controversy that would not be resolved for several decades, with each group using its own proposed names.
In 1992, the IUPAC-IUPAP Transfermium Working Group 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 recognized 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 IUPAC's decision.
In 1994, as part of an attempted resolution to the element naming controversy, IUPAC ratified names for elements 101–109. For element 102, it ratified the name nobelium 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 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, refers to element 114.