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Hassium, 108Hs
Hassium
Pronunciation/ˈhæsiəm/ ⓘ[1] ​(HASS-ee-əm)
Mass number[271] (data not decisive)[a]
Hassium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Os
↑
Hs
↓
—
bohrium ← hassium → meitnerium
Atomic number (Z)108
Groupgroup 8
Periodperiod 7
Block  d-block
Electron configuration[Rn] 5f14 6d6 7s2[4]
Electrons per shell2, 8, 18, 32, 32, 14, 2
Physical properties
Phase at STPsolid (predicted)[5]
Density (near r.t.)27–29 g/cm3 (predicted)[6][7]
Atomic properties
Oxidation states(+2), (+3), (+4), (+6), +8[8][9][10] (parenthesized: prediction)
Ionization energies
  • 1st: 730 kJ/mol
  • 2nd: 1760 kJ/mol
  • 3rd: 2830 kJ/mol
  • (more) (predicted)[11]
Atomic radiusempirical: 126 pm (estimated)[12]
Covalent radius134 pm (estimated)[13]
Other properties
Natural occurrencesynthetic
Crystal structure ​hexagonal close-packed (hcp)
Hexagonal close-packed crystal structure for hassium

(predicted)[5]
CAS Number54037-57-9
History
Namingafter Hassia, Latin for Hesse, Germany, where it was discovered[14]
DiscoveryGesellschaft für Schwerionenforschung (1984)
Isotopes of hassium
Main isotopes[15] Decay
abun­dance half-life (t1/2) mode pro­duct
269Hs synth 12 s α 265Sg
270Hs synth 7.6 s α 266Sg
271Hs synth 46 s α 267Sg
277mHs synth 130 s SF –
 Category: Hassium
| references

Hassium (/ˈhæsiəm/ ⓘ HASS-ee-əm or /ˈhɑːsiəm/ HAH-see-əm)[16] is a synthetic element with the symbol Hs and atomic number 108. It is the heaviest member of the group 8 (VIII) elements. The element was first observed in 1984. Experiments have confirmed that hassium is a typical member of group 8 showing a stable +8 oxidation state, analogous to osmium. Hassium is the heavier homologue of osmium.

Several isotopes are known, with 269Hs being the longest-lived with a half-life of ~10 s. More than 100 atoms of hassium have been synthesized to date in various cold and hot fusion reactions, both as a parent nucleus and decay product.

History

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
82Pb
 + 58
26Fe
 → 265
108Hs
 +
n

The IUPAC/IUPAP Transfermium Working Group (TWG) recognised the GSI collaboration as official discoverers in their 1992 report. [17]

Naming

Hassium 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 (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 (Latin: Hassia German: Hessen).

In 1994 a committee of IUPAC recommended that element 108 be named hahnium (Hn),[18] in spite of the long-standing convention to give the discoverer the right to suggest a name. After protests from the German discoverers, the name hassium (Hs) was adopted internationally in 1997.[19]

Future experiments

Spectroscopy

Scientists at the GSI are planning to search for K-isomers in 270Hs using the reaction 226Ra(48Ca,4n) in 2010. They will use the new TASISpec method developed alongside the introduction of the new TASCA facility at the GSI.[20]

In addition, they also hope to study the spectroscopy of 269Hs, 265Sg and 261Rf, using the reaction 248Cm(26Mg,5n) or 226Ra(48Ca,5n). This will allow them to determine the level structure in 265Sg and 261Rf and attempt to give spin and parity assignments to the various proposed isomers.[21]

Chemistry

The team from the universität Mainz are planning to study the electrodeposition of hassium atoms using TASCA at the GSI. The current aim is to use the reaction 226Ra(48Ca,4n)270Hs.[22]

In addition, scientists at the GSI are hoping to utilize the new TASCA facility to study the synthesis and properties of the hassium(II) compound, hassocene, Hs(Cp)2 using the reaction 226Ra(48Ca,xn).[23]

Nucleosynthesis

Cold fusion

This section deals with the synthesis of nuclei of hassium by so-called "cold" fusion reactions. These are processes which create compound nuclei at low excitation energy (~10-20 MeV, hence "cold"), leading to a higher probability of survival from fission. The excited nucleus then decays to the ground state via the emission of one or two neutrons only.

136Xe(136Xe,xn)272−xHs

Important future experiments will involve the attempted synthesis of hassium isotopes in this symmetric reaction using the fission fragments. This reaction was carried out at Dubna in 2007 but no atoms were detected, leading to a cross section limit of 1 pb.[24] If confirmed, this would indicate that such symmetric fusion reactions should be modelled as 'hot fusion' reactions rather than 'cold fusion' ones, as first suggested. This would indicate that such reactions will unfortunately have limited use in the synthesis of superheavy elements.

198Pt(70Zn,xn)268−xHs

This reaction was performed in May 2002 at the GSI. Unfortunately, the experiment was cut short due to a failure of the zinc-70 beam.

208Pb(58Fe,xn)266−xHs (x=1,2)

This reaction was first reported in 1978 by the team at Dubna. In a later experiment in 1984, using the rotating drum technique, they were able to detect a spontaneous fission activity assigned to 260Sg, daughter of 264Hs. [25] In a repeat experiment in the same year, they applied the method of chemical identification of a descendant to provide support to the synthesis of element 108. They were able to detect several alpha decays of 253Es and 253Fm, descendants of 265Hs.

In the official discovery of the element in 1984, the team at GSI studied the reaction using the alpha decay genetic correlation method. They were able to positively identify 3 atoms of 265Hs.[26] After an upgrade of their facilities in 1993, the team repeated the experiment in 1994 and detected 75 atoms of 265Hs and 2 atoms of 264Hs, during the measurement of a partial excitation function for the 1n neutron evaporation channel.[27] The maximum of the 1n channel was measured as 69 pb in a further run in late 1997 in which a further 20 atoms were detected.[28]

The discovery experiment was successfully repeated in 2002 at RIKEN (10 atoms) and in 2003 at GANIL (7 atoms).

The team at RIKEN further studied the reaction in 2008 in order to conduct first spectroscopic studies of the even-even nucleus 264Hs. They were also able to detect a further 29 atoms of 265Hs.

207Pb(58Fe,xn)265−xHs (x=1)

The use of a Pb-207 target was first used in 1984 at Dubna. They were able to detect the same SF activity as observed in the Pb-208 run and once again assigned it to 260Sg, daughter of 264Hs.[17] The team at GSI first studied the reaction in 1986 using the method of correlation of genetic alpha decays and identified a single atom of 264Hs with a cross section of 3.2 pb.[29] The reaction was repeated in 1994 and the team were able to measure both alpha decay and spontaneous fission for 264Hs.

This reaction was studied in 2008 at RIKEN in order to conduct first spectrscopic studies of the even-even nucleus 264Hs. The team detected 11 atoms of the isotope.

208Pb(56Fe,xn)264−xHs (x=1)

This reaction was studied for the first time in 2008 by the team at LBNL. They were able to produce and identify 6 atoms of the new isotope 263Hs.[30] A few months later, the RIKEN team also published their results on the same reaction.[31]

206Pb(58Fe,xn)264−xHs (x=1)

This reaction was studied for the first time in 2008 by the team at RIKEN. They were able to identify 8 atoms of the new isotope 263Hs.[32]

209Bi(55Mn,xn)264−xHs

First attempts to synthesise nuclei of hassium were performed using this reaction by the team at Dubna in 1983. Using the rotating drum technique, they were able to detect a spontaneous fission activity assigned to 255Rf, descendant of the 263Hs decay chain. Identical results were measured in a repeat run in 1984.[17] In a subsequent experiment in 1983, they applied the method of chemical identification of a descendant to provide support to the synthesis of hassium. They were able to detect alpha decays from fermium isotopes, assigned as descendants of the decay of 262Hs. This reaction has not been tried since and 262Hs is currently unconfirmed.[17]

Hot fusion

This section deals with the synthesis of nuclei of hassium by so-called "hot" fusion reactions. These are processes which create compound nuclei at high excitation energy (~40-50 MeV, hence "hot"), leading to a reduced probability of survival from fission and quasi-fission. The excited nucleus then decays to the ground state via the emission of 3-5 neutrons.

226Ra(48Ca,xn)274−xHs (x=4)

This reaction was reportedly first studied in 1978 by the team at the Flerov Laboratory of Nuclear Reactions (FLNR) under the leadership of Yuri Oganessian. However, results are not available in the literature.[17] The reaction was repeated at the FLNR in June 2008 and results show that the 4 atoms of the isotope 270Hs were detected with a yield of 9 pb. The decay data for the recently discovered isotope was confirmed, although the alpha energy was slightly higher.[33] In Jan 2009, the team repeated the experiment and a further 2 atoms of 270Hs were detected.[34]

232Th(40Ar,xn)272−xHs

This reaction was first studied at Dubna in 1987. Detection was by spontaneous fission and no activities were found leading to a calculated cross section limit of 2 pb.[17]

238U(36S,xn)274−xHs (x=4)

This reaction with the rare and expensive 36S isotope was conducted at the GSI in April–May 2008. Preliminary results show that a single atom of 270Hs was detected with a yield of 0.8 pb. The data confirms the decay properties of 270Hs and 266Sg.[35]

238U(34S,xn)272−xHs (x=4,5)

In March 1994, the team at Dubna led by the late Yuri Lazerev announced the detection of 3 atoms of 267Hs from the 5n neutron evaporation channel. [36] The decay properties were confirmed by the team at GSI in their simultaneous study of darmstadtium.

The reaction was repeated at the GSI in Jan-Feb 2009 in order to search for the new isotope 268Hs. The team, led by Prof. Nishio, detected a single atom of both 268Hs and 267Hs. The new isotope underwent alpha-decay to the previously known isotope 264Sg.

248Cm(26Mg,xn)274−xHs (x=3,4,5)

Most recently, a GSI-PSI collaboration has studied the nuclear reaction of curium-248 with magnesium-26 ions. Between May 2001 and August 2005, the team has studied the excitation function of the 3n, 4n, and 5n evaporation channels leading to 269Hs, 270Hs, and 271Hs.[37][dead link][38][dead link] The synthesis of the important isotope 270Hs was published in December 2006 by the team of scientists from the Technical University of Munich.[39] It was reported that this isotope decayed by emission of an alpha-particle with an energy of 8.83 MeV and a projected half-life of ~22 s, assuming a 0+ to 0+ ground state decay to 266Sg using the Viola-Seaborg equation.

248Cm(25Mg,xn)273−xHs

This new reaction was studied at the GSI in July–August 2006 in a search for the new isotope 268Hs. They were unable to detect any atoms from neutron evaporation and calculated a cross section limit of 1 pb.

249Cf(22Ne,xn)271−xHs

The team at Dubna studied this reaction in 1983 using detection by spontaneous fission (SF). Several short SF activities were found indicating the formation of nuclei of hassium. [17]

Isotopes

Chronology of isotope discovery
Isotope Year discovered Discovery reaction
263Hs 2008 208Pb(56Fe,n)
264Hs 1986 207Pb(58Fe,n)
265Hs 1984 208Pb(58Fe,n)
266Hs 2000 207Pb(64Ni,n)[40]
267Hs 1995 238U(34S,5n)
268Hs 2009 238U(34S,4n)
269Hs 1996 208Pb(70Zn,n)[41]
270Hs 2004 248Cm(26Mg,4n)
271Hs 2004 248Cm(26Mg,3n)[citation needed]
272Hs unknown
273Hs 2010 242Pu(48Ca,5n)
274Hs unknown
275Hs 2003 242Pu(48Ca,3n) [42]
276Hs unknown
277aHs 2009 244Pu(48Ca,3n)
277bHs? 1999 244Pu(48Ca,3n) [42]

Unconfirmed isotopes

277bHs

An isotope assigned to 277Hs has been observed on one occasion decaying by SF with a long half-life of ~11 minutes.[43] The isotope is not observed in the decay of the most common isotope of 281Ds but is observed in the decay from a rare, as yet unconfirmed isomeric level, namely 281bDs . The half-life is very long for the ground state and it is possible that it belongs to an isomeric level in 277Hs. Furthermore, in 2009, the team at the GSI observed a small alpha decay branch for 281aDs producing an isotope of 277Hs decaying by SF in a short lifetime. The measured half-life is close to the expected value for ground state isomer, 277aHs. Further research is required to confirm the production of the isomer.

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 in 2001. This isotope was finally created in 2010 and the data confirmed the fabrication of previous data.

270Hs
prospects for a deformed doubly magic nucleus

According to macroscopic-microscopic (MM) theory, Z=108 is a deformed proton magic number, in combination with the neutron shell at N=162. 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 half-lives. The SF half-lives 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 sub-shell. The recent synthesis of 269Hs, 270Hs, and 271Hs also fully support the assignment of N=162 as a magic closed shell. In particular, the low decay energy for 270Hs is in complete agreement with calculations.[44]

Evidence for the Z=108 deformed proton shell

Evidence for the magicity of the Z=108 proton shell can be deemed from two sources:

  1. the variation in the partial spontaneous fission half-lives for isotones
  2. the large gap in Qα for isotonic pairs between Z=108 and Z=110.

For SF, it is necessary to measure the half-lives for the isotonic nuclei 268Sg, 270Hs and 272Ds. Since the seaborgium and darmstadtium isotopes are not known at this time, and fission of 270Hs has not been measured, this method can be used to date to confirm the stabilizing nature of the Z=108 shell. However, good evidence for the magicity of the Z=108 can be deemed from the large differences in the alpha decay energies measured for 270Hs, 271Ds and 273Ds. More conclusive evidence would come from the determination of the decay energy for the nucleus 272Ds.

Nuclear isomerism

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 half-life 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 half-life of .75 ms.

Chemical properties

Extrapolated chemical properties

Oxidation states

Hassium 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 +8 and this state becomes more stable as the group is descended. Thus hassium is expected to form a stable +8 state. Osmium also shows stable +5, +4 and +3 states with the +4 state the most stable. For ruthenium, the +6, +5 and +3 states are stable with the +3 state being the most stable. Hassium is therefore expected to also show other stable lower oxidation states.

Chemistry

The group VIII elements show a very 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[45] 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−.

Density

Hassium is predicted to have a bulk density of 41 g/cm3, the highest of any of the 118 known elements and nearly twice the density of osmium, the most dense measured element, at 22.6 g/cm3. This results from hassium's high atomic weight, the lanthanide and actinide contractions, and relativistic effects, although production of enough hassium to measure this quantity would be impractical, and the sample would quickly decay.[46]

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.

269
108Hs
 + 2 O
2 → 269
108Hs
O
4

The measured deposition temperature indicated that hassium(VIII) oxide is less volatile than osmium tetroxide, OsO4, and places hassium firmly in group 8.[47][48]

In order to further probe the chemistry of hassium, scientists decided to assess the reaction between hassium tetroxide and sodium hydroxide to form 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:[49]

HsO
4 + 2 NaOH → Na
2[HsO
4(OH)
2]

Summary of compounds and complex ions

Formula Names(s)
HsO4 hassium tetroxide; hassium(VIII) oxide
Na
2[HsO
4(OH)
2]		 sodium hassate(VIII); disodium dihydroxytetraoxohassate(VIII)

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  41. ^ see copernicium
  42. ^ a b see flerovium
  43. ^ Cite error: The named reference Ogan was invoked but never defined (see the help page).
  44. ^ Robert Smolanczuk (1997). "Properties of the hypothetical spherical superheavy nuclei". Physical Review C. 56 (2): 812–824. Bibcode:1997PhRvC..56..812S. doi:10.1103/PhysRevC.56.812.
  45. ^ Gutsev, Gennady L.; Khanna, S.; Rao, B.; Jena, P. (1999). "FeO4: A unique example of a closed-shell cluster mimicking a superhalogen". Physical Review A. 59 (5): 3681. Bibcode:1999PhRvA..59.3681G. doi:10.1103/PhysRevA.59.3681.
  46. ^ Darleane C. Hoffman, Diana M. Lee, and Valeria Pershina Transactinide Elements and Future Elements, Ch. 14 in Lester R. Morss, Norman M. Edelstein, Jean Fuger (Eds.) The Chemistry of the Actinide and Transactinide Elements, Springer-Verlag, Dordrecht 2006, ISBN 1-4020-3555-1 p. 1691.
  47. ^ Investigation of Hassium
  48. ^ "Chemistry of Hassium" (PDF). Gesellschaft für Schwerionenforschung mbH. 2002. Retrieved 2007-01-31.
  49. ^ CALLISTO result

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