Seaborgium

2008/9 Schools Wikipedia Selection. Related subjects: Chemical elements

106 dubnium ← seaborgium → bohrium W ↑ Sg ↓ (Uph) Periodic Table - Extended Periodic Table
General
Name, Symbol, Number	seaborgium, Sg, 106
Element category	transition metals
Group, Period, Block	6, 7, d
Appearance	unknown, probably silvery white or metallic gray
Standard atomic weight	[271]  g·mol−1
Electron configuration	[Rn] 7s2 5f14 6d4
Electrons per shell	2, 8, 18, 32, 32, 12, 2
Physical properties
Phase	presumably a solid
Density (near r.t.)	unknown  g·cm−3
Atomic properties
Crystal structure	unknown
Oxidation states	6
Atomic radius (calc.)	unknown  pm
Covalent radius	unknown pm
Miscellaneous
CAS registry number	54038-81-2
Selected isotopes
Main article: Isotopes of seaborgium iso NA half-life DM DE ( MeV) DP 271Sg syn 1.9 min 67% α 8.54 267Rf 33% SF 267Sg syn 1.4 min 17% α 8.20 263Rf 83% SF 266mSg syn 21 s α 8.57 262Rf 266gSg syn 0.36 s SF 265mSg syn 15 s α 8.69 261Rf 265gSg syn 7.4 s α 8.90,8.84,8.76 261Rf 264Sg syn 68 ms SF 263mSg syn 0.9 s 87% α 9.25 259Rf 13% SF 263gSg syn 0.3 s α 9.06 259Rf 262Sg syn 15 ms SF 261Sg syn 0.18 s 98.1% α 9.62,9.55,9.47,9.42,9.37 257gRf 1.3% ε 261Db 0.6% SF 260Sg syn 3.6 ms 26% α 9.81,9.77,9.72 256Rf 74% SF 259Sg syn 0.48 s α 9.62,9.36,9.03 255Rf 258Sg syn 2.9 ms SF
References

Seaborgium (pronounced /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:

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.

Proposed names

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

The Berkeley team suggested the name seaborgium (Sg) to honour 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 (pronounced /ˌjuːnɪlˈhɛksiəm/ or /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. 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.

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: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰ 4p⁶5s²4d¹⁰5p⁶6s²4f¹⁴5d¹⁰ 6p⁶7s²5f¹⁴6d⁴

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:

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.

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]⁺.

Summary of investigated compounds and complex ions

Formula	Names(s)
SgO2Cl2	seaborgium oxychloride ; seaborgium(VI) dioxide dichloride ; seaborgyl dichloride
SgO2F2	seaborgium oxyfluoride ; seaborgium(VI) dioxide difluoride ; seaborgyl difluoride
SgO3	seaborgium oxide ; seaborgium(VI) oxide ; seaborgium trioxide
SgO2(OH)2	seaborgium oxide hydroxide ; seaborgium(VI) dioxide dihydroxide
[SgO2F3]-	trifluorodioxoseaborgate(VI)
[Sg(OH)5(H2O)]+	aquapentahydroxyseaborgium(VI)

History of synthesis of isotopes by cold fusion

²⁰⁸Pb(⁵⁴Cr,xn)^262-xSg (x=1,2,3)

The first attempt to synthesise element 106 in cold fusion reactions was performed in September 1974 by a Soviet team led by G. N. Flerov at the Joint Institute for Nuclear Research at Dubna. They reported producing a 0.48 s spontaneous fission (SF) activity which they assigned to the isotope ²⁵⁹106. Based on later evidence it was suggested that the team most likely measured the decay of ²⁶⁰Sg and its daughter ²⁵⁶Rf. The TWG concluded that, at the time, the results were insufficiently convincing.

The Dubna team revisited this problem in 1983-1984 and were able to detect a 5 ms SF activity assigned directly to ²⁶⁰Sg.

The team at GSI studied this reaction for the first time in 1985 using the improved method of correlation of genetic parent-daughter decays. They were able to detect ²⁶¹Sg (x=1) and ²⁶⁰Sg and measured a partial 1n neutron evaporation excitation function.

In December 2000, the reaction was studied by a team at GANIL, France and were able to detect 10 atoms of ²⁶¹Sg and 2 atoms of ²⁶⁰Sg to add to previous data on the reaction.

After a facility upgrade, the GSI team measured the 1n excitation function in 2003 using a metallic lead target. Of significance, in May 2003, the team successfully replaced the lead-208 target with more resistant lead(II) sulfide targets (PbS) which will allow more intense beams to be used in the future. They were able to measure the 1n,2n and 3n excitation functions and performed the first detailed alpha-gamma spectroscopy on the isotope ²⁶¹Sg. They detected ~1600 atoms of the isotope and identified new alpha lines as well as measuring a more accurate half-life and new EC and SF branchings. Furthermore, they were able to detect the K X-rays from the daughter rutherfordium element for the first time. They were also able to provide improved data for ²⁶⁰Sg, including the tentative observation of an isomeric level. The study was continued in September 2005 and March 2006. The accumulated work on ²⁶¹Sg was published in 2007. Work in September 2005 also aimed to begin spectroscopic studies on ²⁶⁰Sg.

²⁰⁷Pb(⁵⁴Cr,xn)^261-xSg (x=1,2)

The team at Dubna also studied this reaction in 1974 with identical results as for their first experiments with a Pb-208 target. The SF activities were first assigned to ²⁵⁹Sg and later to ²⁶⁰Sg and/or ²⁵⁶Rf. Further work in 1983-1984 also detected a 5 ms SF activity assigned to the parent ²⁶⁰Sg.

The GSI team studied this reaction for the first time in 1985 using the method of correlation of genetic parent-daughter decays. They were able to positively identify ²⁵⁹Sg as a product from the 2n neutron evaporation channel.

The reaction was further used in March 2005 using PbS targets to begin a spectroscopic study of the even-even isotope ²⁶⁰Sg.

²⁰⁶Pb(⁵⁴Cr,xn)^260-xSg

This reaction was studied in 1974 by the team at Dubna. It was used to assist them in their assignment of the observed SF activities in reactions using Pb-207 and Pb-208 targets. They were unable to detect any SF, indicating the formation of isotopes decaying primarily by alpha decay.

²⁰⁸Pb(⁵²Cr,xn)^260-xSg (x=1,2)

The team at Dubna also studied this reaction in their series of cold fusion reactions performed in 1974. Once again they were unable to detect any SF activities. The reaction was revisited in 2006 by the team at LBNL as part of their studies on the effect of the isospin of the projectile and hence the mass number of the compound nucleus on the yield of evaporation residues. They were able to identify ²⁵⁹Sg and ²⁵⁸Sg in their measurement of the 1n excitation function.

²⁰⁹Bi(⁵¹V,xn)^260-xSg (x=2)

The team at Dubna also studied this reaction in their series of cold fusion reactions performed in 1974. Once again they were unable to detect any SF activities. In 1994, the synthesis of seaborgium was revisited using this reaction by the GSI team, in order to study the new even-even isotope ²⁵⁸Sg. Ten atoms of ²⁵⁸Sg were detected and decayed by spontaneous fission.

History of synthesis of isotopes by hot fusion

²³⁸U(³⁰Si,xn)^268-xSg (x=3,4,5,6)

This reaction was first studied by Japanese scientists at the Japan Atomic Energy Research Institute (JAERI) in 1998. They detected a spontaneous fission activity which they tentatively assigned to the new isotope ²⁶⁴Sg or ²⁶³Db, formed by EC of ²⁶³Sg. In 2006, the teams at GSI and LBNL both studied this reaction using the method of correlation of genetic parent-daughter decays. The LBNL team measured an excitation function for the 4n,5n and 6n channels, whilst the GSI team were able to observe an additional 3n activity. Both teams were able to identify the new isotope ²⁶⁴Sg which decayed with a short lifetime by spontaneous fission.

²⁴⁸Cm(²²Ne,xn)^270-xSg (x=4,5)

In 1993, at Dubna, Yuri Lazarev and his team announced the discovery of long-lived ²⁶⁶Sg and ²⁶⁵Sg produced in the 4n and 5n channels of this nuclear reaction following the search for seaborgium isotopes suitable for a first chemical study. It was announced that ²⁶⁶Sg decayed by 8.57 MeV alpha-particle emission with a projected half-life of ~20 s, lending strong support to the stabilising effect of the Z=108,N=162 closed shells. This reaction was studied further in 1997 by a team at GSI and the yield, decay mode and half-lives for ²⁶⁶Sg and ²⁶⁵Sg have been confirmed, although there are still some discrepancies. In the recent synthesis of ²⁷⁰Hs (see hassium), ²⁶⁶Sg was found to undergo exclusively SF with a short half-life (T_SF = 360 ms). It is probable that this is the ground state, (^266gSg) and that the other activity, produced directly, belongs to a high spin K-isomer, ^266mSg, but further results are required to confirm this.

However, this reaction has been successfully used in the recent attempts to study the chemistry of seaborgium (see below).

²⁴⁹Cf(¹⁸O,xn)^267-xSg (x=4)

The synthesis of element 106 was first attempted in 1974 by the team at LBNL. In their discovery experiment, they were able to apply the new method of correlation of genetic parent-daughter decays to identify the new isotope ²⁶³Sg. In 1975, the team at Oak Ridge were able to confirm the decay data but were unable to identify coincident X-rays in order to prove that seaborgium as produced. In 1979, the team at Dubna studied the reaction by detection of SF activities. In comparison with data from Berkeley, they calculated a 70% SF branching for ²⁶³Sg. The synthesis and discovery reaction was confirmed in 1994 by a different team at LBNL.

Synthesis of isotopes as decay products

Isotopes of seaborgium have also been observed in the decay of heavier elements. Observations to date are summarised in the table below:

Evaporation Residue	Observed Sg isotope
291116 , 287114 , 283112	271Sg
271Hs	267Sg
270Hs	266Sg
277Uub , 273Ds , 269Hs	265Sg
271Ds , 267Ds	263Sg
270Ds	262Sg
269Ds , 265Hs	261Sg
264Hs	260Sg

Chronology of isotope discovery

Isotope	Year discovered	discovery reaction
258Sg	1994	209Bi(51V,2n)
259Sg	1985	207Pb(54Cr,2n)
260Sg	1985	208Pb(54Cr,2n)
261Sg	1985	208Pb(54Cr,n)
262Sg	2001	207Pb(64Ni,n)
263Sgm	1974	249Cf(18O,4n)
263Sgg	1994	208Pb(64Ni,n)
264Sg	2006	238U(30Si,4n)
265Sg	1993	248Cm(22Ne,5n)
266Sg	1993	248Cm(22Ne,4n)
267Sg	2004	248Cm(26Mg,3n)
268Sg	unknown
269Sg	unknown
270Sg	unknown
271Sg	2003	242Pu(48Ca,3n)

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.

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. Further research is required to confirm these two isomers of ²⁶⁵Sg.

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.

Spectroscopic 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.

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.

Projectile	Target	CN	1n	2n	3n
54Cr	207Pb	261Sg
54Cr	208Pb	262Sg	4.23 nb , 13.0 MeV	500 pb	10 pb
51V	209Bi	260Sg		38 pb , 21.5 MeV
52Cr	208Pb	260Sg	281 pb , 11.0 MeV

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.

Projectile	Target	CN	3n	4n	5n	6n
30Si	238U	268Sg	+	9 pb, 40.0	~ 80 pb , 51.0 MeV	~30 pb , 58.0 MeV
22Ne	248Cm	270Sg		~25 pb	~250 pb
18O	249Cf	267Sg		+