Hassium

Hassium (pronEng|ˈhæsiəm or IPA|/ˈhɑːsiəm/ [ [http://dictionary.reference.com/browse/hassium hassium - Definitions from Dictionary.com ] ] ) is a synthetic element in the periodic table that has the symbol Hs and atomic number 108. Hassium oxidizes similarly to osmium above it, to a hassium tetroxide with a lower volatility than osmium tetroxide. [cite web|url=http://www.gsi.de/documents/DOC-2003-Jun-29-2.pdf|title="Chemistry of Hassium"|accessdate=2007-01-31|author=|date=2002|work= Gesellschaft für Schwerionenforschung mbH]

Official discovery

Hassium was first synthesized in 1984 by a German research team led by Peter Armbruster and Gottfried Münzenberg at the Institute for Heavy Ion Research (Gesellschaft für Schwerionenforschung) in Darmstadt. The team bombarded a lead target with ⁵⁸Fe nuclei to produce 3 atoms of ²⁶⁵Hs in the reaction:$,\; ^\{208\}\_\{82\}mathrm\{Pb\}\; +\; ,\; ^\{58\}\_\{26\}mathrm\{Fe\}\; ,\; o\; ,\; ^\{265\}\_\{108\}mathrm\{Hs\}\; +\; ,\; ^1\_0mathrm\{n\}.$

The IUPAC/IUPAP Transfermium Working Group (TWG) recognised the GSI collaboration as official discoverers in their 1992 report. [http://iupac.org/publications/pac/1993/pdf/6508x1757.pdf "Discovery of the transfermium elements"] , IUPAC Technical report, "Pure & Appl. Chem.", Vol. 65, No. 8, pp. 1757-1814,1993. Retrieved on 2008-03-07]

Naming

Element 108 has historically been known as "eka-osmium". During the period of controversy over the names of the elements (see element naming controversy) IUPAC adopted "unniloctium" (IPA|/ˌjuːnɨˈlɒktiəm/ [ [http://dictionary.reference.com/browse/unniloctium unniloctium - Definitions from Dictionary.com ] ] or IPA|/ˌʌnɨˈlɒktiəm/, symbol "Uno") as a temporary element name for this element.

The name hassium was proposed by the officially recognised German discoverers in 1992, derived from the Latin name for the German state of Hesse where the institute is located (L. "hassia" German "Hessen").

In 1994 a committee of IUPAC recommended that element 108 be named "hahnium" (Hn). [http://iupac.org/publications/pac/1994/pdf/6612x2419.pdf (IUPAC 1994 recomm)]

The name "hassium" (Hs) was adopted internationally in 1997. [http://iupac.org/publications/pac/1997/pdf/6912x2471.pdf (IUPAC 1997 recomm)]

Electronic structure

Hassium has 6 full shells, 7s+5p+3d+2f=17 full subshells, and 108 orbitals:

Bohr model: 2, 8, 18, 32, 32, 14, 2

Quantum mechanical model: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁶6s²4f¹⁴5d¹⁰6p⁶7s²5f¹⁴6d⁶

Extrapolated chemical properties of eka-osmium/dvi-ruthenium

Oxidation states

Element 108 is projected to be the fifth member of the 6d series of transition metals and the heaviest member of group VIII in the Periodic Table, below iron, ruthenium and osmium.The latter two members of the group readily portray their group oxidation state of +VIII and the state becomes more stable as the group is descended. Thus hassium is expected to form a stable +VIII state. Osmium also shows stable +V, +IV and +III states with the +IV state the most stable. For ruthenium, the +VI, +V and +III states are stable with the +III state being the most stable. Hassium is therefore expected to also show other stable lower oxidation states.

Chemistry

The group VIII elements show a vary distinctive oxide chemistry which allows facile extrapolations to be made for hassium. All the lighter members have known or hypothetical tetroxides, MO₄. The oxidising power decreases as one descends the group such that FeO₄ [ [http://prola.aps.org/abstract/PRA/v59/i5/p3681_1 "FeO₄: A unique example of a closed-shell cluster mimicking a superhalogen"] , Gutsev et al., "Phys. Rev. A" , 59, 3681- 3684 (1999). Retrieved on 2008-03-01] is not known due to an extraordinary electron affinity which results in the formation of the well-known oxo-ion ferrate(VI), FeO₄²⁻. Ruthenium tetroxide, RuO₄, formed by oxidation of ruthenium(VI) in acid, readily undergoes reduction to ruthenate(VI), RuO₄²⁻. Oxidation of ruthenium metal in air forms the dioxide, RuO₂. In contrast, osmium burns to form the stable tetroxide, OsO₄, which complexes with hydroxide ion to form an osmium(VIII) -"ate" complex, [OsO₄(OH)₂] ²⁻. Therefore, eka-osmium properties for hassium should be demonstrated by the formation of a volatile tetroxide HsO₄, which undergoes complexation with hydroxide to form a hassate(VIII), [HsO₄(OH)₂] ²⁻.

Experimental chemistry

Gas phase chemistry

Hassium is expected to have the electron configuration [Rn] 5f¹⁴ 6d⁶ 7s² and thus behave as the heavier homolog of osmium (Os). As such, it should form a volatile tetroxide, HsO₄, due to the tetrahedral shape of the molecule.

The first chemistry experiments were performed using gas thermochromatography in 2001, using ¹⁷²Os as a reference. During the experiment, 5 hassium atoms were detected using the reaction ²⁴⁸Cm(²⁶Mg,5n)²⁶⁹Hs. The resulting atoms were thermalized and oxidized in a He/O₂ mixture to form the oxide. The measured deposition temperature indicated that hassium(VIII) oxide is less volatile than osmium tetroxide, OsO₄, and places hassium firmly in group 8. [http://lch.web.psi.ch/pdf/anrep01/B-03heavies.pdf (Investigation of Hassium)] :$,^\{269\}\_\{108\}mathrm\{Hs\}\; +\; 2\; ,mathrm\{O\}\_2\; ,\; o\; ,^\{269\}\_\{108\}mathrm\{HsO\}\_4$

In order to further probe the chemistry of hassium, scientists decided to assess the reaction between hassium tetroxide and sodium hydroxide to form the sodium hassate(VIII), a reaction well-known with osmium. In 2004, scientists announced that they had succeeded in carrying out the first acid-base reaction with a hassium compound. [http://www.gsi.de/informationen/wti/library/scientificreport2003/files/172.pdf (CALLISTO result)] :$mathrm\{HsO\}\_4\; +\; 2\; ,mathrm\{NaOH\}\; ,\; o\; mathrm\{Na\}\_2\; [mathrm\{HsO\}\_4(mathrm\{OH\})\_2]$

ummary of compounds and complex ions

Isomerism in hassium isotopes

²⁶⁹Hs

The direct synthesis of ²⁶⁹Hs has resulted in three alpha lines at 9.21, 9.10, and 8.94 MeV. In the decay of ²⁷⁷112, only 9.21 MeV ²⁶⁹Hs alpha decays have been observed indicating that this decay occurs from an isomeric level. Further research is required to confirm this.

²⁶⁷Hs

The decay of ²⁶⁷Hs is known to occur by alpha decay with three alpha lines at 9.88, 9.83, and 9.75 MeV and a half-life of 52 ms. In the recent syntheses of ^271m,gDs additional activities have been observed. A .94ms activity decaying by 9.83 MeV alpha emission has been observed in addition to longer lived ~.8 s and ~6.0 s activities. Each of these is currently not assigned and confirmed and further research is required to positively identify them.

²⁶⁵Hs

The synthesis of ²⁶⁵Hs has also provided evidence for two levels. The ground state decays by 10.30 MeV alpha emission with a halflife of 2.0 ms. The isomeric state is placed at 300 keV above the ground state and decays by 10.57 MeV alpha emission with a halflife of .75 ms.

Chronology of isotope discovery

²⁷⁰Hs: prospects for a deformed doubly-magic nucleus

According to macroscopic-microscopic (MM) theory, Z=108 is a deformed proton magic number. This means that such nuclei are permanently deformed in their ground state but have high, narrow fission barriers to further deformation and hence relatively-long SF partial halflives. The SF halflives in this region are typically reduced by a factor of 10⁹ in comparison with those in the vicinity of the spherical doubly-magic nucleus ²⁹⁸114, caused by an increase in the probability of barrier penetration by quantum tunnelling, due to the narrower fission barrier. In addition, N=162 has been calculated as a deformed neutron magic number and hence the nucleus ²⁷⁰Hs has promise as a deformed doubly-magic nucleus. Experimental data from the decay of Z=110 isotopes ²⁷¹Ds and ²⁷³Ds, provides strong evidence for the magic nature of the N=162 subshell. The recent synthesis of ²⁶⁹Hs, ²⁷⁰Hs, and ²⁷¹Hs also fully support the assignment of N=162 as a magic closed shell.

= Evidence for the Z=108 deformed proton shell=

Evidence for the effect of the Z=108 closed shell in the vicinity of the N=162 shell is limited at this moment in time. This is caused by the low production yields of the isotopes in question and thus poor statistics regarding SF partial halflives resulting from branching of the decay mode. In the case of the isotonic pair ²⁶⁴Hs and ²⁶²Sg (N=156 isotones), the lifetimes and decay modes do not support the stabilising effect of Z=108 but this is most likely due to a retreat from the N=162 shell. More conclusive evidence would come from the measurement of SF partial halflives for ²⁶⁶Hs (vs. ²⁶⁴Sg), ²⁶⁸Hs (vs. ²⁶⁶Sg), and especially ²⁷⁰Hs itself (vs ²⁶⁸Sg and ²⁶⁶Rf), although ²⁶⁸Sg and ²⁶⁸Hs are currently unknown and ²⁶⁶Rf has not been produced via alpha decay (which would provide T_SF for this N=162 isotone). Analysis of partial SF halflives of nuclei with Z>108 (e.g ²⁷²Ds) would also help to confirm the Z=108 closed shell. It should be noted that whilst ²⁷⁰Hs is expected to be a doubly-magic nucleus, it is not expected to have the longest halflife in this region of the periodic table. The reason is that whilst the N=162 shell staves off fission, alpha decay will predominate. As an example, the nucleus ²⁶⁸Sg (Z=106,N=162) is calculated to have a halflife of the order of two hours. However, recent data from the decay of ²⁶⁴Sg (T_SF = 70 ms) and ²⁶⁶Sg (T_SF = 360 ms) indicate that the influence of the N=162 shell for seaborgium isotopes against fission is some 1–2 orders of magnitude overestimated, so ²⁶⁸Sg may in fact decay by SF will a short half life of ~5 s. The recently-synthesized nucleus ²⁶⁸Db (T_SF = 29 h) has such a long halflife because the presence of both the odd proton and odd neutron hinder SF, relative to neighbouring even-even nuclei.

Unconfirmed isotopes

²⁷⁷Hs

An isotope assigned to ²⁷⁷Hs has been observed on two occasions decaying by SF with a long halflife of ~12 minutes. The isotope is not observed in the decay of ground state ²⁸¹Ds but is observed in the decay from an nonconfirmed isomeric level. The halflife is very long for the ground state and it is possible that it belongs to an isomeric level in ²⁷⁷Hs. Further research is required to confirm this result.

Retracted isotopes

²⁷³Hs

The claimed synthesis of element 118 by LBNL in 1999 involved the intermediate ²⁷³Hs. This isotope was claimed to decay by 9.78 and 9.47 MeV alpha emission with a half-life of 1.2 s. The claim to discovery of ²⁹³118 was retracted and this hassium isotope is currently unknown.

Future experiments

The team at GSI has plans to further study the synthesis of hassium isotopes by hot fusion. During 2007–2008, they hope to study the reaction [http://www.gsi.de/documents/DOC-2007-Mar-178-1.pdf (238U + 34S proposal)] :$,\; ^\{238\}\_\{92\}mathrm\{U\}\; +\; ,\; ^\{34\}\_\{16\}mathrm\{S\}\; ,\; o\; ,\; ^\{269,268,267\}mathrm\{Hs\}$

This will give them experience at producing intense ³⁴S beams before continuing with a measurement of the excitation function for the 3n, 4n, and 5n channels in the reaction using the expensive ³⁶S isotope [http://www.gsi.de/documents/DOC-2007-Mar-186-1.pdf (238U + 36S proposal)] :$,\; ^\{238\}\_\{92\}mathrm\{U\}\; +\; ,\; ^\{36\}\_\{16\}mathrm\{S\}\; ,\; o\; ,\; ^\{271,270,269\}mathrm\{Hs\}$

In a continuation of studies towards the synthesis of ²⁷⁰Hs, two further reactions are to be studied in 2008 by the Technische Universitaet Muenchen (TUM) and the FLNR.

The team at the Universitaet Mainz are planning to study the electrodeposition of hassium atoms using the new reaction: [Kratz,J.V., "Liquid-phase studies of the transactinides, TAN07, Davos, Switzerland, September 2007] :$,\; ^\{244\}\_\{94\}mathrm\{Pu\}\; +\; ,\; ^\{30\}\_\{14\}mathrm\{Si\}\; ,\; o\; ,\; ^\{271,270\}mathrm\{Hs\}$

In addition, scientists at the FLNR will reattempt the reaction: [ [http://flerovlab.jinr.ru/flnr/experiments.html "Experiments 2008"] ] :$,\; ^\{226\}\_\{88\}mathrm\{Ra\}\; +\; ,\; ^\{48\}\_\{20\}mathrm\{Ca\}\; ,\; o\; ,\; ^\{271,270\}mathrm\{Hs\}$

The FLNR is currently the only facility which is able to utilise the intensively radioactive Ra-226 target and will help to complete a survey of production methods for the ²⁷⁴Hs compound nucleus, which will provide useful information on the mechanism of hot fusion.

As part of their continued program on the study of the effect of isospin on the yield of evaporation residues in cold fusion reactions, the team at LBNL are planning to study the following reaction in 2008 with the search for a new isotope ²⁶³Hs:$,\; ^\{208\}\_\{82\}mathrm\{Pb\}\; +\; ,\; ^\{56\}\_\{26\}mathrm\{Fe\}\; ,\; o\; ,\; ^\{263\}\_\{108\}mathrm\{Hs\}\; +\; ,\; ^1\_0mathrm\{n\}$

Eka-osmium

Eka-osmium was a temporary name used to refer to the element that goes under osmium in the periodic table. The name "eka" was used in the same way as in Mendeleev's predicted elements. During the first half of the 20th century, eka-osmium referred to plutonium, because the actinide concept, which postulates the actinides form an inner transition series similar to the lanthanides, had not been proposed yet. Once the actinide concept became widely accepted, eka-osmium started to refer to element 108, now called Hassium, which was discovered in 1984.

References

External links

* [http://www.webelements.com/webelements/elements/text/Hs/index.html WebElements.com: Hassium]
* [http://www.apsidium.com/elements/108.htm Apsidium: Hassium 108]