|Pronunciation||// (listen) |
|Seaborgium in the periodic table|
|Atomic number (Z)||106|
|Electron configuration||[Rn] 5f14 6d4 7s2|
|Electrons per shell||2, 8, 18, 32, 32, 12, 2|
|Phase at STP||solid (predicted)|
|Density (near r.t.)||35.0 g/cm3 (predicted)|
|Oxidation states||0, (+3), (+4), (+5), +6 (parenthesized: prediction)|
|Atomic radius||empirical: 132 pm (predicted)|
|Covalent radius||143 pm (estimated)|
|Crystal structure|| body-centered cubic (bcc)|
|Naming||after Glenn T. Seaborg|
|Discovery||Lawrence Berkeley National Laboratory (1974)|
|Main isotopes of seaborgium|
Seaborgium is a synthetic chemical element with the 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 isotope, 269Sg, has a half-life of approximately 14 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. Chemistry experiments have confirmed that seaborgium behaves as the heavier homologue to tungsten in group 6. The chemical properties of seaborgium are characterized only partly, but they compare well with the chemistry of the other group 6 elements.
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.
|Visualization of unsuccessful nuclear fusion, based on calculations by the Australian National University|
The heaviest[a] atomic nuclei are created in nuclear reactions that combine 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. 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. Coming close 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. If fusion does occur, the temporary merger—termed a compound nucleus—is an excited state. To lose its excitation energy and reach a more stable state, a compound nucleus either fissions or ejects one or several neutrons,[c] which carry away the energy. This occurs in approximately 10−16 seconds after the initial collision.[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. 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. The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long. The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.
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, as it has unlimited range. Nuclei of the heaviest elements are thus theoretically predicted and have so far been observed to primarily decay via decay modes that are caused by such repulsion: alpha decay and spontaneous fission;[f] these modes are predominant for nuclei of superheavy elements. 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 determined arithmetically.[g] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[h]
The information available to physicists aiming to synthesize one of the heaviest elements 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.[i]
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. Unambiguous 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.
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 0.9±0.2 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.
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. After being recognized as official discoverers, the Berkeley team started deciding on a name in earnest:
...we were given credit for the discovery and the accompanying right to name the new element. The eight members of the Ghiorso group suggested a wide range of names honoring Isaac Newton, Thomas Edison, Leonardo da Vinci, Ferdinand Magellan, the mythical Ulysses, George Washington, and Finland, the native land of a member of the team. There was no focus and no front-runner for a long period.
Then one day Al [Ghiorso] walked into my office and asked what I thought of naming element 106 "seaborgium." I was floored.— Glenn Seaborg
Seaborg's son Eric remembered the naming process as follows:
With eight scientists involved in the discovery suggesting so many good possibilities, Ghiorso despaired of reaching consensus, until he awoke one night with an idea. He approached the team members one by one, until seven of them had agreed. He then told his friend and colleague of 50 years: "We have seven votes in favor of naming element 106 seaborgium. Will you give your consent?" My father was flabbergasted, and, after consulting my mother, agreed.— Eric Seaborg
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.
|Atomic number||Systematic||American||Russian||German||Compromise 92||IUPAC 94||ACS 94||IUPAC 95||IUPAC 97||Present|
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:
This would be the first time in history that the acknowledged and uncontested discoverers of an element are denied the privilege of naming it.— Glenn Seaborg
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:
In the interest of international harmony, the Committee reluctantly accepted the name 'dubnium' for element 105 in place of 'hahnium' [the American proposal], which has had long-standing use in literature. We are pleased to note that 'seaborgium' is now the internationally approved name for element 106.— American Chemical Society
Seaborg commented regarding the naming:
I am, needless to say, proud that U.S. chemists recommended that element 106, which is placed under tungsten (74), be called 'seaborgium.' I was looking forward to the day when chemical investigators will refer to such compounds as seaborgous chloride, seaborgic nitrate, and perhaps, sodium seaborgate.
This is the greatest honor ever bestowed upon me—even better, I think, than winning the Nobel Prize.[j] Future students of chemistry, in learning about the periodic table, may have reason to ask why the element was named for me, and thereby learn more about my work.— Glenn Seaborg
Seaborg died a year and a half later, on 25 February 1999, at the age of 86.
|260Sg||4 ms||SF, α||1985||208Pb(54Cr,2n)|
|261Sg||200 ms||α, EC, SF||1985||208Pb(54Cr,n)|
|262Sg||7 ms||SF, α||2001||270Ds(—,2α)|
|263mSg||120 ms||α, SF||1974||249Cf(18O,4n)|
|267Sg||1.4 min||SF, α||2004||271Hs(—,α)|
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. Twelve different isotopes of seaborgium have been reported with atomic masses 258–267, 269, and 271, three of which, seaborgium-261, 263, and 265, 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; thus the heaviest three known isotopes, 267Sg, 269Sg, and 271Sg, are also the longest-lived, having half-lives in minutes. Some other isotopes in this region are predicted to have comparable or even longer half-lives. Additionally, 263Sg, 265Sg, and 265mSg 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 92 microseconds.
The proton-rich isotopes from 258Sg to 261Sg were directly produced by cold fusion; all heavier isotopes were produced from the repeated alpha decay of the heavier elements hassium, darmstadtium, and flerovium, with the exceptions of the isotopes 263mSg, 264Sg, 265Sg, and 265mSg, which were directly produced by hot fusion through irradiation of actinide targets. The twelve isotopes of seaborgium have half-lives ranging from 92 microseconds for 261mSg to 14 minutes for 269Sg.
Very few properties of seaborgium or its compounds have been measured; this is due to its extremely limited and expensive production and the fact that seaborgium (and its parents) decays very quickly. A few singular chemistry-related properties have been measured, but properties of seaborgium metal remain unknown and only predictions are available.
Seaborgium is expected to be a solid under normal conditions and assume a body-centered cubic crystal structure, similar to its lighter congener tungsten. It should be a very heavy metal with a density of around 35.0 g/cm3, which would be the fourth-highest of any of the 118 known elements, lower only than bohrium (37.1 g/cm3), meitnerium (37.4 g/cm3) and hassium (41 g/cm3), the three following elements in the periodic table. In comparison, the densest known element that has had its density measured, osmium, has a density of only 22.61 g/cm3. This results from seaborgium's high atomic weight, the lanthanide and actinide contractions, and relativistic effects, although production of enough seaborgium to measure this quantity would be impractical, and the sample would quickly decay.
Seaborgium is the fourth member of the 6d series of transition metals and the heaviest member of group 6 in the periodic table, below chromium, molybdenum, and tungsten. All the members of the group form a diversity of oxoanions. They readily portray their group oxidation state of +6, although this is highly oxidising in the case of chromium, and this state becomes more and more stable to reduction as the group is descended: indeed, tungsten is the last of the 5d transition metals where all four 5d electrons participate in metallic bonding. As such, seaborgium should have +6 as its most stable oxidation state, both in the gas phase and in aqueous solution, and this is the only oxidation state that is experimentally known for it; the +5 and +4 states should be less stable, and the +3 state, the most common for chromium, would be the least stable for seaborgium.
This stabilisation of the highest oxidation state occurs in the early 6d elements because of the similarity between the energies of the 6d and 7s orbitals, since the 7s orbitals are relativistically stabilised and the 6d orbitals are relativistically destabilised. This effect is so large in the seventh period that seaborgium is expected to lose its 6d electrons before its 7s electrons (Sg, [Rn]5f146d47s2; Sg+, [Rn]5f146d37s2; Sg2+, [Rn]5f146d37s1; Sg4+, [Rn]5f146d2; Sg6+, [Rn]5f14). Because of the great destabilisation of the 7s orbital, SgIV should be even more unstable than WIV and should be very readily oxidised to SgVI. The predicted ionic radius of the hexacoordinate Sg6+ ion is 65 pm, while the predicted atomic radius of seaborgium is 128 pm. Nevertheless, the stability of the highest oxidation state is still expected to decrease as LrIII > RfIV > DbV > SgVI. Some predicted standard reduction potentials for seaborgium ions in aqueous acidic solution are as follows:
2 SgO3 + 2 H+ + 2 e− ⇌ Sg2O5 + H2O E0 = −0.046 V Sg2O5 + 2 H+ + 2 e− ⇌ 2 SgO2 + H2O E0 = +0.11 V SgO2 + 4 H+ + e− ⇌ Sg3+ + 2 H2O E0 = −1.34 V Sg3+ + e− ⇌ Sg2+ E0 = −0.11 V Sg3+ + 3 e− ⇌ Sg E0 = +0.27 V
Seaborgium should form a very volatile hexafluoride (SgF6) as well as a moderately volatile hexachloride (SgCl6), pentachloride (SgCl5), and oxychlorides SgO2Cl2 and SgOCl4. SgO2Cl2 is expected to be the most stable of the seaborgium oxychlorides and to be the least volatile of the group 6 oxychlorides, with the sequence MoO2Cl2 > WO2Cl2 > SgO2Cl2. The volatile seaborgium(VI) compounds SgCl6 and SgOCl4 are expected to be unstable to decomposition to seaborgium(V) compounds at high temperatures, analogous to MoCl6 and MoOCl4; this should not happen for SgO2Cl2 due to the much higher energy gap between the highest occupied and lowest unoccupied molecular orbitals, despite the similar Sg–Cl bond strengths (similarly to molybdenum and tungsten).
Molybdenum and tungsten are very similar to each other and show important differences to the smaller chromium, and seaborgium is expected to follow the chemistry of tungsten and molybdenum quite closely, forming an even greater variety of oxoanions, the simplest among them being seaborgate, SgO2−
4, which would form from the rapid hydrolysis of Sg(H
6, although this would take place less readily than with molybdenum and tungsten as expected from seaborgium's greater size. Seaborgium should hydrolyse less readily than tungsten in hydrofluoric acid at low concentrations, but more readily at high concentrations, also forming complexes such as SgO3F− and SgOF−
5: complex formation competes with hydrolysis in hydrofluoric acid.
Experimental chemical investigation of seaborgium has been hampered due to the need to produce it one atom at a time, its short half-life, and the resulting necessary harshness of the experimental conditions. The isotope 265Sg and its isomer 265mSg are advantageous for radiochemistry: they are produced in the 248Cm(22Ne,5n) reaction.
In the first experimental chemical studies of seaborgium in 1995 and 1996, seaborgium atoms were produced in the reaction 248Cm(22Ne,4n)266Sg, thermalised, and reacted with an O2/HCl mixture. The adsorption properties of the resulting oxychloride were measured and compared with those of molybdenum and tungsten compounds. The results indicated that seaborgium formed a volatile oxychloride akin to those of the other group 6 elements, and confirmed the decreasing trend of oxychloride volatility down group 6:
- Sg + O
2 + 2 HCl → SgO
2 + H
In 2001, a team continued the study of the gas phase chemistry of seaborgium by reacting the element with O2 in a H2O environment. In a manner similar to the formation of the oxychloride, the results of the experiment indicated the formation of seaborgium oxide hydroxide, a reaction well known among the lighter group 6 homologues as well as the pseudohomologue uranium.
- 2 Sg + 3 O
2 → 2 SgO
3 + H
2O → SgO
Predictions on the aqueous chemistry of seaborgium have largely been confirmed. In experiments conducted in 1997 and 1998, seaborgium was eluted from cation-exchange resin using a HNO3/HF solution, most likely as neutral SgO2F2 or the anionic complex ion [SgO2F3]− rather than SgO2−
4. In contrast, in 0.1 M nitric acid, seaborgium does not elute, unlike molybdenum and tungsten, indicating that the hydrolysis of [Sg(H2O)6]6+ only proceeds as far as the cationic complex [Sg(OH)4(H2O)]2+ or [Sg(OH)3(H2O)2]+, while that of molybdenum and tungsten proceeds to neutral [MO2(OH)2)].
The only other oxidation state known for seaborgium other than the group oxidation state of +6 is the zero oxidation state. Similarly to its three lighter congeners, forming chromium hexacarbonyl, molybdenum hexacarbonyl, and tungsten hexacarbonyl, seaborgium has been shown in 2014 to also form seaborgium hexacarbonyl, Sg(CO)6. Like its molybdenum and tungsten homologues, seaborgium hexacarbonyl is a volatile compound that reacts readily with silicon dioxide.
- In nuclear physics, an element is called heavy if its atomic number is high; lead (element 82) is one example of such a heavy element. The term "superheavy elements" typically refers to elements with atomic number greater than 103 (although there are other definitions, such as atomic number greater than 100 or 112; sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypothetical superactinide series). 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.
- In 2009, a team at JINR led by Oganessian published results of their attempt to create hassium in a symmetric 136Xe + 136Xe reaction. They failed to observe a single atom in such a reaction, putting the upper limit on the cross section, the measure of probability of a nuclear reaction, as 2.5 pb. In comparison, the reaction that resulted in hassium discovery, 208Pb + 58Fe, had a cross section of ~20 pb (more specifically, 19+19
−11 pb), as estimated by the discoverers.
- The greater the excitation energy, the more neutrons are ejected. If the excitation energy is lower than energy binding each neutron to the rest of the nucleus, neutrons are not emitted; instead, the compound nucleus de-excites by emitting a gamma ray.
- The definition by the IUPAC/IUPAP Joint Working Party 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. This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.
- 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. 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.
- Not all decay modes are caused by electrostatic repulsion. For example, beta decay is caused by the weak interaction.
- 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 heaviest nuclei. The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL. 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).
- Spontaneous fission was discovered by Soviet physicist Georgy Flerov, a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility. 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. They thus preferred to link new isotopes to the already known ones by successive alpha decays.
- For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics in Stockholm, Stockholm County, Sweden. 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. The following year, LBNL was unable to reproduce the Swedish results and announced instead their synthesis of the element; that claim was also disproved later. JINR insisted that they were the first to create the element and suggested a name of their own for the new element, joliotium; the Soviet name was also not accepted (JINR later referred to the naming of element 102 as "hasty"). The name "nobelium" remained unchanged on account of its widespread usage.
- Seaborg had in fact previously won the 1951 Nobel Prize in Chemistry together with Edwin McMillan for "their discoveries in the chemistry of the first transuranium elements".
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