Seaborgium is a synthetic chemical element; it has symbol Sg and atomic number 106. It is named after the American nuclear chemist Glenn T. Seaborg. As a synthetic element, it can be created in a laboratory but is not found in nature. It is also radioactive; the most stable known isotopes have half-lives on the order of several minutes.
In the periodic table of the elements, it is a d-block transactinide element. It is a member of the 7th period and belongs to the group 6 elements as the fourth member of the 6d series of transition metals. Very few chemical reactions of seaborgium have been studied: only a few atoms at a time can be produced and these decay quickly. A few inorganic compounds and a carbonyl believed to be have been synthesized. These experiments have confirmed that seaborgium behaves as the heavier homologue to tungsten in group 6.
In 1974, a few atoms of seaborgium were produced in laboratories in the Soviet Union and in the United States. 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 the International Union of Pure and Applied Chemistry (IUPAC) established seaborgium as the official name for the element. It is one of only two elements named after a living person at the time of naming, the other being oganesson, element 118.
Introduction
History Following claims of the observation of elements 104 and 105 in 1970 by Albert Ghiorso et al. at the Lawrence Livermore National Laboratory, a search for element 106 using oxygen-18 projectiles and the previously used californium-249 target was conducted. Several 9.1 MeV alpha decays were reported and are now thought to originate from element 106, though this was not confirmed at the time. In 1972, the HILAC accelerator received equipment upgrades, preventing the team from repeating the experiment, and data analysis was not done during the shutdown. This reaction was tried again several years later, in 1974, and the Berkeley team realized that their new data agreed with their 1971 data, to the astonishment of Ghiorso. Hence, element 106 could have actually been discovered in 1971 if the original data was analyzed more carefully.
Two groups claimed discovery of the element. Evidence of element 106 was first reported in 1974 by a Russian research team in Dubna led by Yuri Oganessian, in which targets of lead-208 and lead-207 were bombarded with accelerated ions of chromium-54. In total, fifty-one spontaneous fission events were observed with a half-life between four and ten milliseconds. After having ruled out nucleon transfer reactions as a cause for these activities, the team concluded that the most likely cause of the activities was the spontaneous fission of isotopes of element 106. The isotope in question was first suggested to be seaborgium-259, but was later corrected to seaborgium-260.
+ → + 2 + → +
A few months later in 1974, researchers including Glenn T. Seaborg, Carol Alonso and Albert Ghiorso at the University of California, Berkeley, and E. Kenneth Hulet from the Lawrence Livermore National Laboratory, also synthesized the element by bombarding a californium-249 target with oxygen-18 ions, using equipment similar to that which had been used for the synthesis of element 104 five years earlier, observing at least seventy alpha decays, seemingly from the isotope seaborgium-263m with a half-life of seconds. The alpha daughter rutherfordium-259 and granddaughter nobelium-255 had previously been synthesised and the properties observed here matched with those previously known, as did the intensity of their production. The cross-section of the reaction observed, 0.3 nanobarns, also agreed well with theoretical predictions. These bolstered the assignment of the alpha decay events to seaborgium-263m.
+ → + 4 → + → +
A dispute thus arose from the initial competing claims of discovery, though unlike the case of the synthetic elements up to element 105, neither team of discoverers chose to announce proposed names for the new elements, thus averting an element naming controversy temporarily. The dispute on discovery, however, dragged on until 1992, when the IUPAC/IUPAP Transfermium Working Group (TWG), formed to put an end to the controversy by making conclusions regarding discovery claims for elements 101 to 112, concluded that the Soviet synthesis of seaborgium-260 was not convincing enough, "lacking as it is in yield curves and angular selection results", whereas the American synthesis of seaborgium-263 was convincing due to its being firmly anchored to known daughter nuclei. As such, the TWG recognised the Berkeley team as official discoverers in their 1993 report.
Seaborg had previously suggested to the TWG that if Berkeley was recognised as the official discoverer of elements 104 and 105, they might propose the name kurchatovium (symbol Kt) for element 106 to honour the Dubna team, which had proposed this name for element 104 after Igor Kurchatov, the former head of the Soviet nuclear research programme. However, due to the worsening relations between the competing teams after the publication of the TWG report (because the Berkeley team vehemently disagreed with the TWG's conclusions, especially regarding element 104), this proposal was dropped from consideration by the Berkeley team.Hoffman, D.C., Ghiorso, A., Seaborg, G. T. The Transuranium People: The Inside Story, (2000), 369–399 After being recognized as official discoverers, the Berkeley team started deciding on a name in earnest:
Seaborg's son Eric remembered the naming process as follows:
The name seaborgium and symbol Sg were announced at the 207th national meeting of the American Chemical Society in March 1994 by Kenneth Hulet, one of the co-discovers. However, IUPAC resolved in August 1994 that an element could not be named after a living person, and Seaborg was still alive at the time. Thus, in September 1994, IUPAC recommended a set of names in which the names proposed by the three laboratories (the third being the GSI Helmholtz Centre for Heavy Ion Research in Darmstadt, Germany) with competing claims to the discovery for elements 104 to 109 were shifted to various other elements, in which rutherfordium (Rf), the Berkeley proposal for element 104, was shifted to element 106, with seaborgium being dropped entirely as a name.
+ Summary of element naming proposals and final decisions for elements 101–112 (those covered in the TWG report) Atomic number Systematic American Russian German Compromise 92 IUPAC 94 ACS 94 IUPAC 95 IUPAC 97 Present 101 unnilunium mendelevium mendelevium mendelevium mendelevium mendelevium mendelevium mendelevium 102 unnilbium nobelium joliotium joliotium nobelium nobelium flerovium nobelium nobelium 103 unniltrium lawrencium rutherfordium lawrencium lawrencium lawrencium lawrencium lawrencium lawrencium 104 unnilquadium rutherfordium kurchatovium meitnerium dubnium rutherfordium dubnium rutherfordium rutherfordium 105 unnilpentium hahnium nielsbohrium kurchatovium joliotium hahnium joliotium dubnium dubnium 106 unnilhexium seaborgium rutherfordium rutherfordium seaborgium seaborgium seaborgium seaborgium 107 unnilseptium nielsbohrium nielsbohrium bohrium nielsbohrium nielsbohrium bohrium bohrium 108 unniloctium hassium hassium hahnium hassium hahnium hassium hassium 109 unnilennium meitnerium hahnium meitnerium meitnerium meitnerium meitnerium meitnerium 110 ununnilium hahnium becquerelium darmstadtium darmstadtium 111 unununium roentgenium roentgenium 112 ununbium copernicium copernicium
This decision ignited a firestorm of worldwide protest for disregarding the historic discoverer's right to name new elements, and against the new retroactive rule against naming elements after living persons; the American Chemical Society stood firmly behind the name seaborgium for element 106, together with all the other American and German naming proposals for elements 104 to 109, approving these names for its journals in defiance of IUPAC. At first, IUPAC defended itself, with an American member of its committee writing: "Discoverers don't have a right to name an element. They have a right to suggest a name. And, of course, we didn't infringe on that at all." However, Seaborg responded:
Bowing to public pressure, IUPAC proposed a different compromise in August 1995, in which the name seaborgium was reinstated for element 106 in exchange for the removal of all but one of the other American proposals, which met an even worse response. Finally, IUPAC rescinded these previous compromises and made a final, new recommendation in August 1997, in which the American and German proposals for elements 104 to 109 were all adopted, including seaborgium for element 106, with the single exception of element 105, named dubnium to recognise the contributions of the Dubna team to the experimental procedures of transactinide synthesis. This list was finally accepted by the American Chemical Society, which wrote:
Seaborg commented regarding the naming:
Seaborg died a year and a half later, on 25 February 1999, at the age of 86.
Isotopes
Superheavy elements such as seaborgium are produced by bombarding lighter elements in particle accelerators that induces fusion reactions. Whereas most of the isotopes of seaborgium can be synthesized directly this way, some heavier ones have only been observed as decay products of elements with higher atomic numbers.
Depending on the energies involved, fusion reactions that generate superheavy elements are separated into "hot" and "cold". In hot fusion reactions, very light, high-energy projectiles are accelerated toward very heavy targets (actinides), giving rise to compound nuclei at high excitation energy (~40–50 MeV) that may either fission or evaporate several (3 to 5) neutrons. In cold fusion reactions, the produced fused nuclei have a relatively low excitation energy (~10–20 MeV), which decreases the probability that these products will undergo fission reactions. As the fused nuclei cool to the ground state, they require emission of only one or two neutrons, and thus, allows for the generation of more neutron-rich products. The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (see cold fusion).
Seaborgium has no stable or naturally occurring isotopes. Several radioactive isotopes have been synthesized in the laboratory, either by fusing two atoms or by observing the decay of heavier elements. Fourteen different isotopes of seaborgium have been reported with mass numbers 257–269 and 271, four of which, seaborgium-261, −263, −265, and −267, have known metastable states. All of these decay only through alpha decay and spontaneous fission, with the single exception of seaborgium-261 that can also undergo electron capture to dubnium-261.
There is a trend toward increasing half-lives for the heavier isotopes, though even–odd isotopes are generally more stable than their neighboring even–even isotopes, because the odd neutron leads to increased hindrance of spontaneous fission; among known seaborgium isotopes, alpha decay is the predominant decay mode in even–odd nuclei whereas fission dominates in even–even nuclei. Three of the heaviest known isotopes, 267Sg, 269Sg, and 271Sg, are also the longest-lived, having half-lives on the order of several minutes. Some other isotopes in this region are predicted to have comparable or even longer half-lives. Additionally, 263Sg, 265Sg, 265mSg, and 268Sg have half-lives measured in seconds. All the remaining isotopes have half-lives measured in milliseconds, with the exception of the shortest-lived isotope, 261mSg, with a half-life of only 9.3 microseconds.