Seaborgium

Seaborgium (pronEng|siːˈbɔrgiəm) is a chemical element in the periodic table that has the symbol Sg and atomic number 106, Image of Seaborgium . Seaborgium is a synthetic element whose most stable isotope ²⁷¹Sg has a half-life of 1.9 minutes. Chemistry experiments with seaborgium have firmly placed it in group 6 as a heavier homologue to tungsten.

Official discovery

Element 106 was officially discovered in June 1974 by an American research team led by Albert Ghiorso at the Lawrence Radiation Laboratory at the University of California, Berkeley. They reported creating ²⁶³106, with a half-life of 1.0 s, by the hot fusion reaction:

:$,\; ^\{249\}\_\{98\}mathrm\{Cf\}\; +\; ,\; ^\{18\}\_\{8\}mathrm\{O\}\; ,\; o\; ,\; ^\{263\}\_\{106\}mathrm\{Sg\}\; +\; 4,\; ^\{1\}\_\{0\}mathrm\{n\}$

A team led by Ken Gregorich at LBNL confirmed the synthesis in 1994.

The IUPAC/IUPAP Transfermium Working Group (TWG) officially recognised the LBNL team as discoverers in their 1992 report.http://www.iupac.org/publications/pac/1993/pdf/6508x1757.pdf (TWG report)]

Proposed names

(main article: Element naming controversy)

Historically, element 106 has been referred to as eka-tungsten.

The Berkeley team suggested the name "seaborgium" (Sg) to honor the American chemist Glenn T. Seaborg credited as a member of the American group in recognition of his participation in the discovery of several other actinides. The name selected by the team became controversial. An element naming controversy erupted and as a result IUPAC adopted "unnilhexium" (pronEng|ˌjuːnɪlˈhɛksiəm or IPA|/ˌʌnɪlˈhɛksiəm/, symbol "Unh") as a temporary, systematic element name. In 1994 a committee of IUPAC recommended that element 106 be named "rutherfordium" and adopted a rule that no element can be named after a living person. [http://www.iupac.org/publications/pac/1994/pdf/6612x2419.pdf (IUPAC 1994 recomm)] This ruling was fiercely objected to by the American Chemical Society. Critics pointed out that a precedent had been set in the naming of einsteinium during Albert Einstein's life and a survey indicated that chemists were not concerned with the fact that Seaborg was still alive. In 1997, as part of a compromise involving elements 104 to 108, the name "seaborgium" for element 106 was recognized internationally. [http://www.iupac.org/publications/pac/1997/pdf/6912x2471.pdf (IUPAC 1997 recomm)]

Electronic structure

Seaborgium is element 106 in the Periodic Table. The two forms of the projected electronic structure are:

Bohr model: 2, 8, 18, 32, 32, 12, 2

Quantum mechanical model: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁶6s²4f¹⁴5d¹⁰6p⁶7s²5f¹⁴6d⁴

Extrapolated chemical properties of eka-tungsten/dvi-molybdenum

Oxidation states

Element 106 is projected to be the third member of the 6d series of transition metals and the heaviest member of group VI in the Periodic Table, below chromium, molybdenum and tungsten.All the members of the group readily portray their group oxidation state of +VI and the state becomes more stable as the group is descended. Thus seaborgium is expected to form a stable +VI state.For this group, stable +V and +IV states are well represented for the heavier members and the +III state is known but reducing, unlike Cr(III).

Chemistry

Molybdenum and tungsten readily form stable trioxides MO₃ Therefore, seaborgium should from SgO₃. The oxides MO₃ are soluble in alkali with the formation of oxyanions, so seaborgium should form a seaborgate ion, SgO₄^2-. In addition, WO₃ reacts with acid so again eka-tungsten reactivity will show a lack of amphotericity for SgO₃. Molybdenum oxide, MoO₃, also reacts with moisture to form a hydroxide MoO₂(OH)₂, so SgO₂(OH)₂ is also feasible.The heavier homologues readily form the volatile, reactive hexahalides MX₆ (X=Cl,F). Only tungsten forms the unstable hexabromide, WBr₆. Therefore, the compounds SgF₆ and SgCl₆ are predicted. An eka-tungsten character may show itself in the formation of a hexabromide, SgBr₆. These halides are unstable to oxygen and moisture and readily form volatile oxyhalides, MOX₄ and MO₂X₂. Therefore SgOX₄ (X=F,Cl) and SgO₂X₂ (X=F,Cl) should be possible. In aqueous solution, a variety of anionic oxyfluoro-complexes are formed with fluoride ion, examples being MOF₅^- and MO₃F₃^3-. Similar seaborgium complexes are expected.

Experimental chemistry

Gas phase chemistry

Initial experiments aiming at probing the chemistry of seaborgium focused on the study of the gas thermochromatography of a volatile oxychloride.Seaborgium atoms were produced in the reaction ²⁴⁸Cm(²²Ne,4n)²⁶⁶Sg, thermalised and reacted with an O₂/HCl mixture. The resulting adsorption properties of the oxychloride was measured and compared with those of molybdenum and tungsten. The result indicated that seaborgium formed a volatile oxychloride in a manner akin to the group 6 elements:

:$mathrm\{Sg\}\; +\; mathrm\{O\}\_\{2\}/,mathrm\{HCl\}\; o\; ,mathrm\{SgO\}\_\{2\}mathrm\{Cl\}\_\{2\}$

In 2001, a team continued the study of the gas phase chemsitry 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 with lighter group 6 homologues. [ [http://www-w2k.gsi.de/kernchemie/images/pdf_Artikel/Radiochim_Acta_89_737_2001.pdf "Physico-chemical characterization of seaborgium as oxide hydroxide"] , Huebener et al., "Radiochim. Acta", 89, 737–741 (2001).Retrieved on 2008-02-29]

:$2,mathrm\{Sg\}\; +\; 3,mathrm\{O\}\_\{2\}\; o\; 2,mathrm\{SgO\}\_\{3\}$:$mathrm\{SgO\}\_\{3\}\; +\; mathrm\{H\}\_\{2\}mathrm\{O\}\; o\; mathrm\{SgO\}\_\{2\}(mathrm\{OH\})\_\{2\}$

Aqueous phase chemistry

In its aqueous chemistry, seaborgium has been shown to resemble its lighter homologues molybdenum and tungsten in group 6, forming a stable +6 oxidation state. Seaborgium was eluted from cation exchange resin using a HNO₃/HF solution, most likely as neutral SgO₂F₂ or the anionic complex ion [SgO₂F₃] ^-. In contrast, in 0.1 M HNO₃, seaborgium does not elute, unlike Mo and W, indicating that the hydrolysis of [Sg(H₂O)₆] ⁶⁺ only proceeds as far as the cationic complex [Sg(OH)₅(H₂O] ⁺.

ummary of investigated compounds and complex ions

Isotopes

There are 11 known isotopes of seaborgium (exclusing meta-stable and K-spin isomers). The longest-lived is ²⁷¹Sg which decays through alpha decay and spontaneous fission. It has a half-life of 1.9 minutes. The shortest-lived isotope is ²⁵⁸Sg which also decays through alpha decay and spontaneous fission. It has a half-life of 2.9 ms.

Isomerism in seaborgium nuclides

²⁶⁶Sg

Initial work identified an 8.63 MeV alpha-decaying activity with a half-life of ~21s and assigned to the ground state of ²⁶⁶Sg. Later work identified a nuclide decaying by 8.52 and 8.77 MeV alpha emission with a half-life of ~21s, which is unusual for an even-even nuclide. Recent work on the synthesis of ²⁷⁰Hs identified ²⁶⁶Sg decaying by SF with a short 360 ms half-life. The recent work on ²⁷⁷112 and ²⁶⁹Hs has provided new information on the decay of ²⁶⁵Sg and ²⁶¹Rf. This work suggested that the initial 8.77 MeV activity should be reassigned to ²⁶⁵Sg. Therefore the current information suggests that the SF activity is the ground state and the 8.52 MeV activity is a high spin K-isomer. Further work is required to confirm these assignments. A recent re-evaluation of the data has suggested that the 8.52 MeV activity should be associated with ²⁶⁵Sg and that ²⁶⁶Sg only undergoes fission.

²⁶⁵Sg

The recent direct synthesis of ²⁶⁵Sg resulted in four alpha-lines at 8.94,8.84,8.76 and 8.69 MeV with a half-life of 7.4 seconds. The observation of the decay of ²⁶⁵Sg from the decay of ²⁷⁷112 and ²⁶⁹Hs indicated that the 8.69 MeV line may be associated with an isomeric level with an associated half-life of ~ 20 s. It is plausible that this level is causing confusion between assignments of ²⁶⁶Sg and ²⁶⁵Sg since both can decay to fissioning rutherfordium isotopes.

A recent re-evaluation of the data has indicated that there are indeed two isomers, one with a principal decay energy of 8.85 MeV with a half-life of 8.9 s, and a second isomer which decays with energy 8.70 MeV with a half-life of 16.2 s.

²⁶³Sg

The discovery synthesis of ²⁶³Sg resulted in an alpha-line at 9.06 MeV. Observation of this nuclide by decay of ^271gDs, ^271mDs and ²⁶⁷Hs has confirmed an isomer decaying by 9.25 MeV alpha emission. The 9.06 MeV decay was also confirmed. The 9.06 MeV activity has been assigned to the ground state isomer with an associated half-life of 0.3 s. The 9.25 MeV activity has been assigned to an isomeric level decaying with a half-life of 0.9 s.

Recent work on the synthesis of ^271g,mDs was resulted in some confusing data regarding the decay of ²⁶⁷Hs. In one such decay, ²⁶⁷Hs decayed to ²⁶³Sg which decayed by alpha emission with a half-life of ~ 6 s. This activity has not yet been positively assigned to an isomer and further research is required.

pectroscopic decay schemes for seaborgium isotopes

²⁶¹Sg

Retracted isotopes

²⁶⁹Sg

In the claimed synthesis of ²⁹³118 in 1999 the isotope ²⁶⁹Sg was identified as a daughter product. It decayed by 8.74 MeV alpha emission with a half-life of 22 s. The claim was retracted in 2001 and thus this seaborgium isotope is currently unknown or unconfirmed. [see ununoctium]

Chemical yields of isotopes

Cold fusion

The table below provides cross-sections and excitation energies for cold fusion reactions producing seaborgium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel.

Hot fusion

The table below provides cross-sections and excitation energies for hot fusion reactions producing seaborgium isotopes directly. Data in bold represents maxima derived from excitation function measurements. + represents an observed exit channel.

References

External links

* [http://www.webelements.com/webelements/elements/text/Sg/index.html WebElements.com - Seaborgium]