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Hassium (pronounced /ˈhæsiəm/ ( listen) HASS-ee-əm or /ˈhɑːsiəm/ HAH-see-əm) is a synthetic element with the symbol Hs and atomic number 108 and is the heaviest member of the group 8 (VIII) elements. The element was first observed in 1984.
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:
20882Pb + 5826Fe → 265108Hs + n
The IUPAC/IUPAP Transfermium Working Group (TWG) recognised the GSI collaboration as official discoverers in their 1992 report. 
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 (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), 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.
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 then been proposed. Once the actinide concept became widely accepted, the name "eka-osmium" was used for element 108.
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.
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.
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.
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).
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.
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. 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.
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.
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.  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 265108.
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. 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. 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.
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.
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. 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. 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.
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. A few months later, the RIKEN team also published their results on the same reaction.
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.
First attempts to synthesise nuclei of element 108 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 263108 decay chain. Identical results were measured in a repeat run in 1984. In a subsequent experiment in 1983, they applied the method of chemical identification of a descendant to provide support to the synthesis of element 108. They were able to detect alpha decays from fermium isotopes, assigned as descendants of the decay of 262108. This reaction has not been tried since and 262Hs is currently unconfirmed.
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.
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. 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. In Jan 2009, the team repeated the experiment and a further 2 atoms of 270Hs were detected.
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.
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.
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.  The decay properties was confirmed by the team at GSI in their simultaneous study of element 110.
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.
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. The synthesis of the important isotope 270Hs was published in December 2006 by the team of scientists from the Technical University of Munich. 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.
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.
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 element 108. 
An isotope assigned to 277Hs has been observed on two occasions decaying by SF with a long half-life of ~12 minutes. 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.
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.
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.
Evidence for the magicity of the Z=108 proton shell can be deemed from two sources:
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.
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.
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.
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.
The tables below provides cross-sections and excitation energies for reactions producing hassium isotopes directly. Data in bold represent maxima derived from excitation function measurements. + represents an observed exit channel.
The below table contains various targets-projectile combinations for which calculations have provided estimates for cross section yields from various neutron evaporation channels. The channel with the highest expected yield is given.
DNS = Di-nuclear system ; σ = cross section
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 +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.
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 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−.
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.
269108Hs + 2 O2 → 269108HsO4
The measured deposition temperature indicated that hassium(VIII) oxide is less volatile than osmium tetroxide, OsO4, and places hassium firmly in group 8.
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:
HsO4 + 2 NaOH → Na2[HsO4(OH)2]
Summary of compounds and complex ions
1. ^ hassium at Dictionary.com
* WebElements.com: Hassium