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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 is a chemical element with symbol Hs and atomic number 108, named in honor of the German state of Hesse. It is a synthetic element (an element that can be created in a laboratory but is not found in nature) and radioactive; the most stable known isotope, 269Hs, has a half-life of approximately 9.7 seconds, although an unconfirmed metastable state, 277mHs, may have a longer half-life of about 11 minutes. More than 100 atoms of hassium have been synthesized to date.[16]

In the periodic table of the elements, it is a d-block transactinide element. It is a member of the 7th period and belongs to the group 8 elements. Chemistry experiments have confirmed that hassium behaves as the heavier homologue to osmium in group 8. The chemical properties of hassium are characterized only partly, but they compare well with the chemistry of the other group 8 elements. In bulk quantities, hassium is expected to be a silvery metal that reacts readily with oxygen in the air, forming a volatile tetroxide.

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.[17] The team bombarded a target of lead-208 with accelerated nuclei of iron-58 to produce 3 atoms of the isotope hassium-265 in the reaction:

The element Link does not exist. + The element Link does not exist. → The element Link does not exist. +
n

The IUPAC/IUPAP Transfermium Working Group (TWG) recognised the GSI collaboration as official discoverers in their 1992 report.[18]

Naming

The name hassium was proposed by the officially recognised German discoverers in 1992, derived from the Latin name (Hassia) for the German state of Hesse where the institute is located.[19]

In 1994 a committee of IUPAC recommended that element 108 be named hahnium (Hn) after the German physicist Otto Hahn, in spite of the long-standing convention to give the discoverer the right to suggest a name, so that elements named after Hahn and Lise Meitner (meitnerium) would be next to each other, honoring their joint discovery of nuclear fission.[20] This was because they felt that Hesse did not merit an element being named after it.[16] After protests from the German discoverers and the American Chemical Society, IUPAC relented and the name hassium (Hs) was adopted internationally in 1997.[16][21]

Nucleosynthesis

Main article: Isotopes of hassium

Super-heavy elements such as hassium are produced by bombarding lighter elements in particle accelerators that induce fusion reactions. Whereas most of the isotopes of hassium can be synthesized directly this way, some heavier ones have only been observed as decay products of elements with higher atomic numbers.[22]

Depending on the energies involved, the former are separated into "hot" and "cold". In hot fusion reactions, very light, high-energy projectiles are accelerated toward very heavy targets (actinides), giving rise to compound nuclei at high excitation energy (~40–50 MeV) that may either fission or evaporate several (3 to 5) neutrons.[23] In cold fusion reactions, the produced fused nuclei have a relatively low excitation energy (~10–20 MeV), which decreases the probability that these products will undergo fission reactions. As the fused nuclei cool to the ground state, they require emission of only one or two neutrons, and thus, allows for the generation of more neutron-rich products.[22] The latter is a distinct concept from that of where nuclear fusion claimed to be achieved at room temperature conditions (see cold fusion).[24]

Cold fusion

Before the first successful synthesis of hassium in 1984 by the GSI team, scientists at the Joint Institute for Nuclear Research (JINR) in Dubna, Russia also tried to synthesize hassium by bombarding lead-208 with iron-58 in 1978. No hassium atoms were identified. They repeated the experiment in 1984 and were able to detect a spontaneous fission activity assigned to 260Sg, the daughter of 264Hs.[25] Later that year, they tried the experiment again, and tried to chemically identify the decay products of hassium to provide support to their synthesis of element 108. They were able to detect several alpha decays of 253Es and 253Fm, decay products of 265Hs.[18]

In the official discovery of the element in 1984, the team at GSI studied the same reaction using the alpha decay genetic correlation method and were able to positively identify 3 atoms of 265Hs.[17] 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.[26] A further run of the reaction was conducted in late 1997 in which a further 20 atoms were detected.[27] This 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 the first spectroscopic studies of the even-even nucleus 264Hs. They were also able to detect a further 29 atoms of 265Hs.

The team at Dubna also conducted the analogous reaction with a lead-207 target instead of a lead-208 target in 1984:

The element Link does not exist. + The element Link does not exist. → The element Link does not exist. +
n

They were able to detect the same spontaneous fission activity as observed in the reaction with a lead-208 target and once again assigned it to 260Sg, daughter of 264Hs.[18] The team at GSI first studied the reaction in 1986 using the method of genetic correlation of alpha decays and identified a single atom of 264Hs with a cross section of 3.2 pb.[28] The reaction was repeated in 1994 and the team were able to measure both alpha decay and spontaneous fission for 264Hs. This reaction was also studied in 2008 at RIKEN in order to conduct the first spectroscopic studies of the even-even nucleus 264Hs. The team detected 11 atoms of 264Hs.

In 2008, the team at RIKEN conducted the analogous reaction with a lead-206 target for the first time:

The element Link does not exist. + The element Link does not exist. → The element Link does not exist. +
n

They were able to identify 8 atoms of the new isotope 263Hs.[29]

In 2008, the team at the Lawrence Berkeley National Laboratory (LBNL) studied the analogous reaction with iron-56 projectiles for the first time:

The element Link does not exist. + The element Link does not exist. → The element Link does not exist. +
n

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]

Further attempts to synthesise nuclei of hassium were performed the team at Dubna in 1983 using the cold fusion reaction between a bismuth-209 target and manganese-55 projectiles:

The element Link does not exist. + The element Link does not exist. → 264−x
108Hs
 + x
n
 (x = 1 or 2)

They were able to detect a spontaneous fission activity assigned to 255Rf, a product of the 263Hs decay chain. Identical results were measured in a repeat run in 1984.[18] 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.[18]

Hot fusion

Under the leadership of Yuri Oganessian, the team at the Joint Institute for Nuclear Research studied the hot fusion reaction between calcium-48 projectiles and radium-226 targets in 1978:

The element Link does not exist. + The element Link does not exist. → The element Link does not exist. + 4
n

However, results are not available in the literature.[18] The reaction was repeated at the JINR in June 2008 and 4 atoms of the isotope 270Hs were detected.[32] In January 2009, the team repeated the experiment and a further 2 atoms of 270Hs were detected.[33]

The team at Dubna studied the reaction between californium-249 targets and neon-22 projectiles in 1983 by detecting spontaneous fission activities:

The element Link does not exist. + The element Link does not exist. → 271−x
108Hs
 + x
n

Several short spontaneous fission activities were found, indicating the formation of nuclei of hassium.[18]

The hot fusion reaction between uranium-238 targets and projectiles of the rare and expensive isotope sulfur-36 was conducted at the GSI in April–May 2008:

The element Link does not exist. + The element Link does not exist. → The element Link does not exist. + 4
n

Preliminary results show that a single atom of 270Hs was detected. This experiment confirmed the decay properties of the isotopes 270Hs and 266Sg.[34]

In March 1994, the team at Dubna led by the late Yuri Lazarev attempted the analogous reaction with sulfur-34 projectiles:

The element Link does not exist. + The element Link does not exist. → 272−x
108Hs
 + x
n
 (x = 4 or 5)

They announced the detection of 3 atoms of 267Hs from the 5n neutron evaporation channel.[35] The decay properties were confirmed by the team at GSI in their simultaneous study of darmstadtium. The reaction was repeated at the GSI in January-February 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.

Between May 2001 and August 2005, a GSI-PSI (Paul Scherrer Institute) collaboration studied the nuclear reaction between curium-248 targets and magnesium-26 projectiles:

The element Link does not exist. + The element Link does not exist. → 274−x
108Hs
 + x
n
 (x = 3, 4, or 5)

The team studied the excitation function of the 3n, 4n, and 5n evaporation channels leading to the isotopes 269Hs, 270Hs, and 271Hs.[36][37] The synthesis of the important doubly magic isotope 270Hs was published in December 2006 by the team of scientists from the Technical University of Munich.[38] It was reported that this isotope decayed by emission of an alpha particle with an energy of 8.83 MeV and a half-life of ~22 s. This figure has since been revised to 3.6 s.[39]

As decay product

List of hassium isotopes observed by decay
Evaporation residue Observed hassium isotope
267Ds 263Hs[40]
269Ds 265Hs[41]
270Ds 266Hs[42]
271Ds 267Hs[43]
277Cn, 273Ds 269Hs[44]
285Fl, 281Cn, 277Ds 273Hs[45]
291Lv, 287Fl, 283Cn, 279Ds 275Hs[46]
293Lv, 289Fl, 285Cn, 281Ds 277Hs[47][48][49]

Hassium has been observed as decay products of darmstadtium. Darmstadtium currently has eight known isotopes, all of which have been shown to undergo alpha decays to become hassium nuclei, with mass numbers between 263 and 277. Hassium isotopes with mass numbers 266, 273, 275, and 277 to date have only been produced by darmstadtium nuclei decay. Parent darmstadtium nuclei can be themselves decay products of copernicium, flerovium, or livermorium. To date, no other elements have been known to decay to hassium.[39] For example, in 2004, the Dubna team identified hassium-277 as a final product in the decay of livermorium via an alpha decay sequence:[49]

293
116Lv
 → 289
114Fl
 + 4
2He
289
114Fl
 → 285
112Cn
 + 4
2He
285
112Cn
 → 281
110Ds
 + 4
2He
281
110Ds
 → 277
108Hs
 + 4
2He

Natural occurrence

Molybdenite

In the 1960s, it was predicted that long-lived deformed isomers of hassium might occur naturally on Earth in trace quantities. This was theorized in order to explain the extreme radiation damage in some minerals that could not have been caused by any known natural radioisotopes, but could have been caused by superheavy elements. In 1963, Victor Cherdyntsev claimed to have discovered element 108 in natural molybdenite and suggested the name sergenium for it, after a region in Kazakhstan where his molybdenite samples came from.[16] More recently, it was hypothesized that an isomer of 271Hs might have a half-life of around (2.5±0.5)×108 y, which would explain the observation of alpha particles with energies of around 4.4 MeV in some samples of molybdenite and osmiride.[50] This isomer of 271Hs could be produced from the beta decay of 271Bh and 271Sg, which, being homologous to rhenium and molybdenum respectively, should occur in molybdenite along with rhenium and molybdenum if they occurred in nature. Since hassium is homologous to osmium, it should also occur along with osmium in osmiride if it occurred in nature. However, the decay chains of these isotopes are very hypothetical and the predicted half-life of this hypothetical hassium isomer is not long enough for any sufficient quantity to remain on Earth.[50] It is possible that more 271Hs may be deposited on the Earth as the Solar System travels through the spiral arms of the Milky Way, which would also explain excesses of plutonium-239 found on the floors of the Pacific Ocean and the Gulf of Finland, but minerals enriched with 271Hs are predicted to also have to have excesses of uranium-235 and lead-207, and would have different proportions of elements that are formed during spontaneous fission, such as krypton, zirconium, and xenon. Thus, the occurrence of hassium in nature in minerals such as molybdenite and osmiride is theoretically possible, but highly unlikely.[50] In 2004, the Joint Institute for Nuclear Research conducted a search for natural hassium. This was done underground to avoid interference and false positives from cosmic rays, but no results have been released, strongly implying that no natural hassium was found. The possible extent of primordial hassium on Earth is uncertain; it might now only exist in traces, or could even have completely decayed by now after having caused the radiation damage long ago.[16]

Isotopes

Main article: Isotopes of hassium
List of hassium isotopes
Isotope
 Half-life
[39]		 Decay
mode[39]		 Discovery
year		 Reaction
263Hs 0.74 ms α, SF 2008 208Pb(56Fe,n)[30]
264Hs ~0.8 ms α, SF 1986 207Pb(58Fe,n)[28]
265Hs 1.9 ms α, SF 1984 208Pb(58Fe,n)[17]
265mHs 0.3 ms α 1984 208Pb(58Fe,n)[17]
266Hs 2.3 ms α, SF 2000 270Ds(—,α)[42]
267Hs 52 ms α, SF 1995 238U(34S,5n)[35]
267mHs 0.8 s α 1995 238U(34S,5n)[35]
268Hs 0.4 s α 2009 238U(34S,4n)
269Hs 3.6 s α 1996 277Cn(—,2α)[44]
269mHs 9.7 s α 2004 248Cm(26Mg,5n)[36]
270Hs 3.6 s α 2004 248Cm(26Mg,4n)[36]
271Hs ~4 s α 2004 248Cm(26Mg,3n)[37]
272Hs 40? s α, SF ? unknown —
273Hs 0.24 s α 2004 285Fl(—,3α)[45]
274Hs 1? min α, SF ? unknown —
275Hs 0.15 s α 2003 287Fl(—,3α)[46]
276Hs 1? h α, SF ? unknown —
277Hs 2 s α 2009 289Fl(—,3α)[47]
277mHs ? ~11 min ? α 1999 289Fl(—,3α)[48]

Hassium has no stable or naturally-occurring isotopes. Several radioactive isotopes have been synthesized in the laboratory, either by fusing two atoms or by observing the decay of heavier elements. Twelve different isotopes have been reported with atomic masses from 263 to 277 (with the exceptions of 272, 274, and 276), four of which, hassium-265, hassium-267, hassium-269, and hassium-277, have known metastable states (although that of hassium-277 is unconfirmed). Most of these decay predominantly through alpha decay, but some also undergo spontaneous fission.[39][45]

Half-lives

The lighter isotopes usually have shorter half-lives; half-lives of under 1 ms for 263Hs, 264Hs, and 265mHs were observed. 265Hs and 266Hs are more stable at around 2 ms, 267Hs has a half-life of about 50 ms, 267mHs, 268Hs, 273Hs, and 275Hs live between 0.1 and 1 second, and 269Hs, 269mHs, 270Hs, 271Hs, and 277Hs are more stable, at between 1 and 30 seconds. The heaviest isotopes are the most stable, with 277mHs having a measured half-life of about 11 minutes. The unknown isotopes 272Hs, 274Hs, and 276Hs are predicted to have even longer half-lives of around 40 seconds, 1 minute and 1 hour respectively. Before its discovery, 271Hs was also predicted to have a long half-life of 40 seconds, but it was found to have a shorter half-life of only about 4 seconds.[39]

The lightest isotopes were synthesized by direct fusion between two lighter nuclei and as decay products. The heaviest isotope produced by direct fusion is 271Hs; heavier isotopes have only been observed as decay products of elements with larger atomic numbers. In 1999, American scientists at the University of California, Berkeley, announced that they had succeeded in synthesizing three atoms of 293118.[51] These parent nuclei were reported to have successively emitted three alpha particles to form hassium-273 nuclei, which were claimed to have undergone an alpha decay, emitting alpha particles with decay energies of 9.78 and 9.47 MeV and half-life 1.2 s, but their claim was retracted in 2001.[52] The isotope, however, was produced in 2010 by the same team. The new data matched the previous (fabricated)[53] data.[45]

Nuclear isomerism

277Hs

An isotope assigned to 277Hs has been observed on one occasion decaying by spontaneous fission with a long half-life of ~11 minutes.[54] The isotope is not observed in the decay of the most common isomer of 281Ds but is observed in the decay from a rare, as yet unconfirmed isomeric level, namely 281mDs. 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 281Ds producing an isotope of 277Hs decaying by spontaneous fission with a short lifetime. The measured half-life is close to the expected value for ground state isomer, 277Hs. Further research is required to confirm the production of the isomer.[47]

269Hs

The direct synthesis of 269Hs has resulted in the observation of three alpha particles with energies 9.21, 9.10, and 8.94 MeV emitted from 269Hs atoms. However, when this isotope is indirectly synthesized from the decay of 277Cn, only alpha particles with energy 9.21 MeV have been observed, indicating that this decay occurs from an isomeric level. Further research is required to confirm this.[36][44]

267Hs

267Hs is known to decay by alpha decay, emitting alpha particles with energies of 9.88, 9.83, and 9.75 MeV. It has a half-life of 52 ms. In the recent syntheses of 271Ds and 271mDs, additional activities have been observed. A 0.94 ms activity emitting alpha particles with energy 9.83 MeV has been observed in addition to longer lived ~0.8 s and ~6.0 s activities. Currently, none of these are assigned and confirmed and further research is required to positively identify them.[35]

265Hs

The synthesis of 265Hs has also provided evidence for two isomeric levels. The ground state decays by emission of an alpha particle with energy 10.30 MeV and has a half-life of 2.0 ms. The isomeric state has 300 keV of excess energy and decays by the emission of an alpha particle with energy 10.57 MeV and has a half-life of 0.75 ms.[17]

Future experiments

Scientists at the GSI are planning to search for isomers of 270Hs using the reaction 226Ra(48Ca,4n) in 2010 using the new TASCA facility at the GSI.[55] 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.[56]

270Hs: prospects for a deformed doubly magic nucleus

According to calculations, 108 is a proton magic number for deformed nuclei (nuclei that are far from spherical), and 162 is a neutron magic number for deformed nuclei. 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 life-times to spontaneous fission.[57][58] The spontaneous fission 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 298Fl, caused by the narrower fission barrier for such deformed nuclei.[59] Hence, the nucleus 270Hs has promise as a deformed doubly magic nucleus. Experimental data from the decay of the darmstadtium (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 number. In particular, the low decay energy for 270Hs is in complete agreement with calculations.[57][58][59]

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

  1. the variation in the partial spontaneous fission half-lives for isotones
  2. the large gap in the alpha Q value for isotonic nuclei of hassium and darmstadtium.[59]

For spontaneous fission, it is necessary to measure the half-lives for the isotonic nuclei 268Sg, 270Hs and 272Ds.[59] Since the isotopes 268Sg and 272Ds are not currently known,[39] and fission of 270Hs has not been measured,[39][58] this method cannot be used to date to confirm the stabilizing nature of the Z=108 shell. However, good evidence for the magicity of the Z=108 shell can be deemed from the large differences in the alpha decay energies measured for 270Hs, 271Ds and 273Ds.[57][58][59] More conclusive evidence would come from the determination of the decay energy for the unknown nucleus 272Ds.[59]

Predicted properties

Chemical

Hassium is the sixth member of the 6d series of transition metals and is expected to be in the platinum group metals.[60] Calculations on its ionization potentials, atomic radius, as well as radii, orbital energies, and ground levels of its ionized states are similar to that of osmium. Therefore it was concluded that hassium's basic properties will resemble those of other group 8 elements, below iron, ruthenium, and osmium.[61][62] Some of its properties were determined by gas-phase chemistry experiments.[63][64][65] The group 8 elements portray a wide variety of oxidation states, but 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.[66][67][68] Ruthenium and osmium show the highest oxidation states of all the elements in their respective periods and are the only non-superheavy elements to show the +8 state,[68] except for xenon,[67][69][70] iridium,[71] and plutonium (unstable).[72] Thus hassium is expected to form a stable +8 state.[64] 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.[66] Hassium is therefore expected to also show other stable lower oxidation states, such as +6, +5, +4, +3, and +2.[16][61][10]

The group 8 elements show a very distinctive oxide chemistry which allows extrapolations to be made easily for hassium. All the lighter members have known or hypothetical tetroxides, MO4.[73] 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 oxoanion ferrate(VI), FeO2−
4.[74] Ruthenium tetroxide, RuO4, formed by oxidation of ruthenium(VI) in acid, readily undergoes reduction to ruthenate(VI), RuO2−
4.[75][76] Oxidation of ruthenium metal in air forms the dioxide, RuO2.[77] In contrast, osmium burns to form the stable tetroxide, OsO4,[78][79] which complexes with the hydroxide ion to form an osmium(VIII) -ate complex, [OsO4(OH)2]2−.[80] Therefore, eka-osmium properties for hassium should be demonstrated by the formation of a stable, very volatile tetroxide HsO4 (due to the tetrahedral nature of the molecule),[16][61][63][65][67] which undergoes complexation with hydroxide to form a hassate(VIII), [HsO4(OH)2]2−.[81] The trend of the volatilities of the group 8 tetroxides is expected to be RuO4 < OsO4 ≤ HsO4, based on the calculated enthalpies of adsorption for the three compounds;[61][82] this trend remains the same whether or not relativistic effects are taken into account.[82]

Physical and atomic

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,[61] the most dense measured element,[83] at 22.61 g/cm3.[84] 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.[61] The atomic radius of hassium is expected to be around 126 pm.[61] Due to the relativistic stabilization of the 7s orbital and destabilization of the 6d orbital, the Hs+ ion is predicted to have an electron configuration of [Rn] 5f14 6d5 7s2, giving up a 6d electron instead of a 7s electron, which is the opposite of the behavior of its lighter homologues. On the other hand, the Hs2+ ion is expected to have an electron configuration of [Rn] 5f14 6d5 7s1, analogous to that calculated for the Os2+ ion.[61]

Experimental atomic gas phase chemistry

Summary of compounds and complex ions
Formula Names
HsO4 hassium tetroxide; hassium(VIII) oxide
Na
2[HsO
4(OH)
2]		 sodium hassate(VIII); disodium dihydroxytetraoxohassate(VIII)

Despite the fact that the selection of a volatile hassium compound (hassium tetroxide) for gas-phase chemical studies was clear from the beginning,[61][67] the chemical characterization of hassium was considered a difficult task for a long time.[67] Although hassium isotopes were first synthesized in 1984, it was not until 1996 that a hassium isotope long-lived enough to allow chemical studies to be performed was synthesized. Unfortunately, this hassium isotope, 269Hs, was then synthesized indirectly from the decay of 277Cn;[67] not only are indirect synthesis methods not favourable for chemical studies,[61] but also the reaction that produced the isotope 277Cn had a low yield (its cross-section was only 1 pb),[67] and thus did not provide enough hassium atoms for a chemical investigation.[60] The direct synthesis of 269Hs and 270Hs in the reaction 248Cm(26Mg,xn)274−xHs (x = 4 or 5) appeared more promising, as the cross-section for this reaction was somewhat larger, at 7 pm.[61][67] However, this yield was still around an order of magnitude lower than that for the reaction used for the chemical characterization of bohrium.[67] New techniques for irradiation, separation, and detection had to be introduced before hassium could be successfully characterized chemically as a typical member of group 8 in early 2001.[61][67]

Ruthenium and osmium have very similar chemistry due to the lanthanide contraction, but iron shows some differences from them: for example, although ruthenium and osmium form stable tetroxides in which the metal is in the +8 oxidation state, iron does not.[67][73] Consequently, in preparation for the chemical characterization of hassium, researches focused on ruthenium and osmium rather than iron,[67] as hassium was expected to also be similar to ruthenium and osmium due to the actinide contraction.[61] However, in the planned experiment to study hassocene (Hs(C5H5)2), ferrocene may also be used for comparison along with ruthenocene and osmocene.[10]

File:Hassium chemistry experiment setup.png
Schematic diagram of the IVO-COLD system, which was used to study the gas-phase properties of hassium tetroxide (HsO4) and its lighter homologues ruthenium tetroxide (RuO4) and osmium tetroxide (OsO4).[61][85]
Ferrocene
In ferrocene, the cyclopentadienyl rings are in a staggered conformation.
Ruthenocene
In ruthenocene and osmocene, the cyclopentadienyl rings are in an eclipsed conformation. Hassocene is predicted to also have this structure.

The first chemistry experiments were performed using gas thermochromatography in 2001, using 172Os and 173Os as a reference. During the experiment, 5 hassium atoms were synthesized using the reaction 248Cm(26Mg,5n)269Hs. They were then thermalized and oxidized in a mixture of helium and oxygen gas to form the tetroxide.

269
Hs + 2 O
2 → 269
HsO
4

The measured deposition temperature indicated that hassium(VIII) oxide is less volatile than osmium tetroxide, OsO4, and places hassium firmly in group 8.[61][63][65] However, the enthalpy of adsorption for HsO4 measured, (−46 ± 2) kJ/mol, was significantly lower than what was predicted, (−36.7 ± 1.5) kJ/mol, indicating that OsO4 was more volatile than HsO4, contradicting earlier calculations, which implied that they should have very similar volatilities. For comparison, the value for OsO4 is (−39 ± 1) kJ/mol.[61] It is possible that hassium tetroxide interacts differently with the different chemicals (silicon nitride and silicon dioxide) used for the detector; further research is required, including more accurate measurements of the nuclear properties of 269Hs and comparisons with RuO4 in addition to OsO4.[61]

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:[81]

HsO
4 + 2 NaOH → Na
2[HsO
4(OH)
2]

The team from the University of Mainz are planning to study the electrodeposition of hassium atoms using the new TASCA facility at the GSI. The current aim is to use the reaction 226Ra(48Ca,4n)270Hs.[86] In addition, scientists at the GSI are hoping to utilize TASCA to study the synthesis and properties of the hassium(II) compound hassocene, Hs(C5H5)2, using the reaction 226Ra(48Ca,xn). This compound is analogous to the lighter ferrocene, ruthenocene, and osmocene, and is expected to have the two cyclopentadienyl rings in an eclipsed conformation like ruthenocene and osmocene and not in a staggered conformation like ferrocene.[10] Hassocene was chosen because it has hassium in the low formal oxidation state of +2 (although the bonding between the metal and the rings is mostly covalent in metallocenes) rather than the high +8 state which has been investigated, and relativistic effects were expected to be stronger in the lower oxidation state.[10] Many metals in the periodic table form metallocenes, so that trends could be more easily determined, and the highly symmetric structure of hassocene and its low number of atoms also make relativistic calculations easier. Hassocene should be a stable and highly volatile compound.[10]

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