|Pronunciation||// (listen) |
|Mass number||269 (most stable isotope)|
|Hassium in the periodic table|
|Atomic number (Z)||108|
|Element category||Transition metal|
|Electron configuration||[Rn] 5f14 6d6 7s2|
Electrons per shell
|2, 8, 18, 32, 32, 14, 2|
|Phase at STP||solid (predicted)|
|Density (near r.t.)||41 g/cm3 (predicted)|
|Oxidation states||(+2), (+3), (+4), (+5), (+6), +8 (brackets: prediction)|
|Atomic radius||empirical: 126 pm (estimated)|
|Covalent radius||134 pm (estimated)|
|Crystal structure|| hexagonal close-packed (hcp)|
|Naming||after Hassia, Latin for Hesse, Germany, where it was discovered|
|Discovery||Gesellschaft für Schwerionenforschung (1984)|
|Main isotopes of hassium|
Hassium is a chemical element with the symbol Hs and the atomic number 108. It is not known to occur in nature and has been made only in laboratories in minuscule quantities. Hassium is highly radioactive; the most stable known isotope, 269Hs, has a half-life of approximately 16 seconds.
The first attempt to synthesize element 108 was made at the Joint Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast, Soviet Union, in 1978. Another attempt was made at the same venue in 1983 and then in 1984; the latter resulted in a claim that element 108 had been produced. Later in 1984, an attempt was made at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Hesse, West Germany, which also claimed to have synthesized it. The 1993 report by the Joint Working Party between the International Union of Pure and Applied Chemistry and the International Union of Pure and Applied Physics concluded the report from Darmstadt was more conclusive on its own and the major credit was assigned to the German scientists, who then chose the name hassium after the German state of Hesse.
In the periodic table of the elements, hassium is a transactinide element, a member of the 7th period and group 8; it is thus the sixth member of the 6d series of transition metals. Chemistry experiments have confirmed that hassium behaves as the heavier homologue to osmium, also in group 8, reacting readily with oxygen to form a volatile tetroxide. The chemical properties of hassium have only been partly characterized but they compare well with the chemistry of the other group 8 elements.
The synthesis of element 108 was first attempted in 1978 by a research team led by Yuri Oganessian at the Joint Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast, Soviet Union. The team used a reaction that would generate 270108 from radium and calcium. The researchers were uncertain in interpreting their data, and their paper did not unambiguously claim to have discovered the element. The same year, another team at JINR investigated the possibility of synthesis of element 108 in reactions between lead and iron; they were also uncertain in interpreting the data, suggesting the possibility that element 108 had not been created.
In 1983, new experiments were performed at JINR. In each experiment, a thin layer of a target material was installed on a rotating wheel and bombarded at a shallow angle so fission fragments from spontaneously fissioning nuclides formed could escape the target and be detected in fission track detectors surrounding the wheel. The experiments probably resulted in the synthesis of element 108; bismuth was bombarded with manganese to obtain 263108, lead was bombarded with iron to obtain 264108, and californium was bombarded with neon to obtain 270108. These experiments were not claimed as a discovery and Oganessian announced them in a conference rather than in a written report.
In 1984, JINR researchers in Dubna performed experiments set up identically to the previous ones; they bombarded bismuth and lead targets with ions of lighter elements manganese and iron, respectively. Twenty-one spontaneous fission events were recorded; these were assigned to 264108.
Later in 1984, a research team led by Peter Armbruster and Gottfried Münzenberg at the Gesellschaft für Schwerionenforschung (GSI; Institute for Heavy Ion Research) in Darmstadt, Hesse, West Germany, attempted to create element 108. The team bombarded a lead target with accelerated iron nuclei. GSI's experiment to create element 108 was delayed until after their creation of element 109 in 1982, as prior calculations had suggested that even–even isotopes of element 108 would have spontaneous fission half-lives of less than one microsecond, making them difficult to detect and identify. (A nuclide decaying in less than a microsecond would decay before it reached the detectors, and a nuclide predominantly decaying by spontaneous fission rather than alpha emission would not produce a chain anchored to known daughters.) The element 108 experiment finally went ahead after 266109 had been synthesized and was found to decay by alpha emission, suggesting that isotopes of element 108 would do likewise, and this was also corroborated by an experiment aimed at synthesising isotopes of element 106. GSI reported synthesis of three atoms of 265108. Two years later, they also reported synthesis of one atom of the even–even 264108.
In 1985, the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP) formed a Joint Working Party (JWP) to assess discoveries and establish final names for elements with atomic numbers greater than 100. The party held meetings with delegates from the three competing institutes; in 1990, they established criteria for recognition of an element and in 1991, they finished the work of assessing discoveries and disbanded. These results were published in 1993.
According to the report, the 1984 works from JINR and GSI simultaneously and independently established synthesis of element 108. Of the two 1984 works, the one from GSI was said to be sufficient as a discovery on its own. The JINR work, which preceded the GSI one, "very probably" displayed synthesis of element 108 but that was determined in retrospect given the work from Darmstadt; the JINR work had focused on chemically identifying remote granddaughters of element 108 isotopes (which could not exclude the possibility that other ancestors were the source of these daughters), while the GSI work clearly identified the decay path of those element 108 isotopes. The report concluded that the major credit should be awarded to GSI.
According to Mendeleev's nomenclature for unnamed and undiscovered elements, hassium should be known as "eka-osmium". In 1979, IUPAC published recommendations according to which the element was to be called "unniloctium" and assigned the corresponding symbol of "Uno", a systematic element name as a placeholder until the element was discovered and the discovery then confirmed, and a permanent name was decided. Although these recommendations were widely followed in the chemical community, most scientists in the field ignored them. They either called it "element 108", with the symbols E108, (108) or 108, or used the proposed name "hassium".
Peter Armbruster and his colleagues, the officially recognized German discoverers, proposed the name "hassium" for element 108 in September 1992. It is derived from the Latin name Hassia for the German state of Hesse where the institute is located. A naming ceremony was held for elements 107, 108, and 109 on 7 September 1992 at GSI.
In 1994, IUPAC Commission on Nomenclature of Inorganic Chemistry recommended that element 108 be named "hahnium" (Hn) after the German physicist Otto Hahn so elements named after Hahn and Lise Meitner (meitnerium) would be next to each other, honouring their joint discovery of nuclear fission; it was reported that they felt the German suggestion was obscure. GSI protested, saying this proposal contradicted the long-standing convention of giving the discoverer the right to suggest a name; the American Chemical Society supported GSI. The name "hahnium", albeit with the different symbol Ha, had already been proposed and used by American scientists from the Lawrence Berkeley Laboratory for element 105, which they had a discovery dispute with JINR for; they thus also protested the confusing scrambling of names. IUPAC relented and the name hassium (Hs) was adopted internationally in 1997. Simultaneously, the name dubnium (Db; from Dubna, the JINR location) was assigned to element 105, and the name hahnium was abandoned.
Hassium is not known to occur naturally on Earth; the half-lives of all of its known isotopes are short enough that no primordial hassium would have survived to the present day. This does not rule out the possibility of the existence of unknown, longer-lived isotopes or nuclear isomers, some of which could still exist in trace quantities if they are long-lived enough. As early as 1926, German physicist Richard Swinne proposed element 108 as a source of X-rays in the Greenland ice sheet. Although Swinne was unable to verify this observation and thus did not claim discovery, he proposed in 1931 the existence of regions of long-lived transuranic elements, including one around Z = 108. In the early 1960s, it was predicted that long-lived, deformed isomers of hassium might occur naturally on Earth in trace quantities. This was theorized 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, Soviet scientist Viktor Cherdyntsev, who had previously claimed the existence of primordial curium-247, claimed to have discovered element 108—specifically the 267108 isotope, which supposedly had a half-life of 400 to 500 million years—in natural molybdenite and suggested the provisional name sergenium (symbol Sg);[a] this name takes its origin from the name for the Silk Road and was explained as "coming from Kazakhstan" for it. His rationale for claiming that sergenium was the heavier homologue to osmium was that minerals supposedly containing sergenium formed volatile oxides when boiled in nitric acid, similarly to osmium.
Cherdyntsev's findings were criticized by Soviet physicist Vladimir Kulakov on the grounds that some of the properties Cherdyntsev claimed sergenium had were inconsistent with the then-current nuclear physics. The chief questions raised by Kulakov were that the claimed alpha decay energy of sergenium was many orders of magnitude lower than expected and the half-life given was eight orders of magnitude shorter than what would be predicted for a nuclide alpha-decaying with the claimed decay energy but at the same time a corrected half-life in the region of 1016 years would be impossible because it would imply the samples contained about 100 milligrams of sergenium. In 2003, it was suggested that the observed alpha decay with energy 4.5 MeV could be due to a low-energy and strongly enhanced transition between different hyperdeformed states of a hassium isotope around 271Hs, thus suggesting that the existence of superheavy elements in nature was at least possible, although unlikely.
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. No results have been released, strongly implying no natural hassium was found. The possible extent of primordial hassium on Earth is uncertain; it might only exist in traces or could have completely decayed after long ago having caused the radiation damage.
In 2006, it was hypothesized that an isomer of 271Hs might have a half-life of around (2.5±0.5)×108 years, which would explain the observation of alpha particles with energies of around 4.4 MeV in some samples of molybdenite and osmiridium. 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. Because hassium is homologous to osmium, it should also occur along with osmium in osmiridium if it occurs in nature. The decay chains of 271Bh and 271Sg are hypothetical and the predicted half-life of this hypothetical hassium isomer is not long enough for any sufficient quantity to remain on Earth. 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 ocean floors of the Pacific Ocean and the Gulf of Finland; minerals enriched with 271Hs, however, are predicted to also 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. The natural occurrence of hassium in minerals such as molybdenite and osmiride is theoretically possible, but very unlikely.
A 2007 calculation on the decay properties of unknown, neutron-rich isotopes of superheavy elements suggested the isotope 292Hs may be the most stable, superheavy nucleus against alpha decay and spontaneous fission as a consequence of the shell closure at N = 184. As such, it was considered as a candidate to exist in nature. This nucleus, however, is predicted to be very unstable toward beta decay and any beta-stable isotopes of hassium such as 286Hs would be too unstable in the other decay channels to be observed in nature. Indeed, a subsequent search for 292Hs in nature along with its congener osmium was unsuccessful, setting an upper limit to its abundance at 3×10−15 grams of hassium per gram of osmium.
|263Hs||760 µs||||α, SF||2009||208Pb(56Fe,n)|
|264Hs||540 µs||||α, SF||1986||207Pb(58Fe,n)|
|265Hs||1.96 ms||||α, SF||1984||208Pb(56Fe,n)|
|266Hs||3.02 ms||||α, SF||2001||270Ds(—,α)|
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 isotopes with mass numbers ranging from 263 to 277 (with the exceptions of 272, 274, and 276) have been reported, four of which—hassium-265, -267, -269, and -277—have known metastable states, although that of hassium-277 is unconfirmed. Most of these isotopes decay predominantly through alpha decay; this is the most common for all isotopes for which comprehensive decay characteristics are available, the only exception being hassium-277, which undergoes spontaneous fission. The lightest isotopes, which usually have shorter half-lives, 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.
Theoretical models predict a region of instability for some hassium isotopes to lie around A = 275 and N = 168–170, which is between the predicted neutron shell closures at N = 162 for deformed nuclei and N = 184 for spherical nuclei. Nuclides within this region are predicted to have low fission barrier heights, resulting in short partial half-lives toward spontaneous fission. This prediction is supported by the observed 11 millisecond half-life of 277Hs and that of the neighbouring isobar 277Mt because the expected hindrance factors from the odd nucleon were shown to be much lower than expected. The measured half-lives are even lower than those predicted for the even–even 276Hs and 278Ds, which suggests a gap in stability away from the shell closures and perhaps a weakening of the shell closures in this region.
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 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. Computational prospects for shell stabilisation for 270Hs made it a promising candidate for 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 syntheses of 269Hs, 270Hs, and 271Hs also fully supports the assignment of N = 162 as a magic number. 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 obtained from two sources; the variation in the partial spontaneous fission half-lives for isotones and the large gap in the alpha Q value for isotonic nuclei of hassium and darmstadtium. For spontaneous fission, it is necessary to measure the half-lives for the isotonic nuclei 268Sg, 270Hs and 272Ds. Because the isotopes 268Sg and 272Ds are not currently known, and fission of 270Hs has not been measured, this method cannot yet be used to confirm the stabilizing nature of the Z = 108 shell. Good evidence for the magicity of the Z = 108 shell can nevertheless be found 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 unknown nucleus 272Ds.
Various calculations show hassium should be the heaviest known group 8 element, which is consistent with the periodic law. Its properties should generally match those expected for a heavier homologue of osmium with a few deviations arising from relativistic effects.
Physical and atomic
The previous members of group 8 have relatively high melting points; (Fe, 1538 °C; Ru, 2334 °C; Os, 3033 °C). Much like them, hassium is predicted to be a solid at room temperature although its melting point has not been precisely calculated. Hassium should crystallize in the hexagonal close-packed structure (c/a = 1.59), similarly to its lighter congener osmium. Pure metallic hassium is calculated to have a bulk modulus (resistance to uniform compression) of 450 GPa, comparable with that of diamond, which has bulk modulus 442 GPa. Hassium is expected to have a bulk density of 41 g/cm3 at the standard pressure and temperature, the highest of any of the 118 known elements and nearly twice the highest density of an element observed to this day at 22.6 g/cm3.[e] 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 decay quickly.
The atomic radius of hassium is expected to be around 126 pm. 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 behaviour of its lighter homologues. The Hs2+ ion is expected to have an electron configuration of [Rn] 5f14 6d5 7s1, analogous to that calculated for the Os2+ ion.
|Element||Stable oxidation states|
Hassium is the sixth member of the 6d series of transition metals and is expected to be much like the platinum group metals. Calculations on its ionisation potentials, atomic radii, orbital energies and ground levels of its ionized states are similar to those of osmium, implying hassium's properties would resemble those of the other group 8 elements: iron, ruthenium, and osmium. Some of these properties were confirmed by gas-phase chemistry experiments. The group 8 elements portray a wide variety of oxidation states but ruthenium and osmium readily portray their group oxidation state of +8; this state becomes more stable down the group. This oxidation state is extremely rare; it is the highest oxidation state that exists in a reasonably stable compound and only ruthenium, osmium, and xenon among stable elements show it in such compounds.[f] Hassium is expected to follow its congeners and also to form a stable +8 state but like them it should also show other stable, lower oxidation states such as +6, +5, +4, +3, and +2. Hassium(IV) is expected to be more stable than hassium(VIII) in aqueous solution. Hassium should be a rather noble metal; the standard reduction potential for the Hs4+/Hs couple is expected to be 0.4 V, which is more than that for the Cu2+/Cu couple of copper (0.3419 V), but less than that for the Ru2+/Ru couple for ruthenium (0.455 V).
The group 8 elements show a very distinctive oxide chemistry. All of the lighter members have known or hypothetical tetroxides, MO4. Their oxidizing power decreases as one descends the group. FeO4 is not known due to its extraordinarily large electron affinity—the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion— which results in the formation of the well-known oxoanion ferrate(VI), FeO2−
4. Ruthenium tetroxide, RuO4, which is formed by oxidation of ruthenium(VI) in acid, readily undergoes reduction to ruthenate(VI), RuO2−
4. Oxidation of ruthenium metal in air forms the dioxide, RuO2. In contrast, osmium burns to form the stable tetroxide, OsO4, which complexes with the 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 stable, very volatile tetroxide HsO4, which undergoes complexation with hydroxide to form a hassate(VIII), [HsO4(OH)2]2−. Ruthenium tetroxide and osmium tetroxide are both volatile due to their symmetrical tetrahedral molecular geometry and because they are charge-neutral; hassium tetroxide should similarly be a very volatile solid. The trend of the volatilities of the group 8 tetroxides is known to be RuO4 < OsO4 > HsO4, which confirms the calculated results. In particular, the calculated enthalpies of adsorption—the energy required for the adhesion of atoms, molecules, or ions from a gas, liquid or dissolved solid to a surface—of HsO4, −(45.4 ± 1) kJ/mol on quartz, agrees very well with the experimental value of −(46 ± 2) kJ/mol.
Despite the selection of a volatile hassium compound (hassium tetroxide) for gas-phase chemical studies being clear from the beginning, the chemical characterization of hassium was considered a difficult task for a long time. 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; not only are indirect synthesis methods not favourable for chemical studies but the reaction that produced the isotope 277Cn had a low yield—its cross-section was only 1 pb— and thus did not provide enough hassium atoms for a chemical investigation. The direct synthesis of 269Hs and 270Hs in the reaction 248Cm(26Mg,xn)274−xHs (x = 4 or 5) appeared more promising because the cross-section for this reaction was somewhat larger at 7 pb. This yield was still around ten times lower than that for the reaction used for the chemical characterization of bohrium. New techniques for irradiation, separation and detection had to be introduced before hassium could be successfully characterized chemically.
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. In preparation for the chemical characterization of hassium, researches focused on ruthenium and osmium rather than iron because hassium was expected to also be similar to ruthenium and osmium due to the actinide contraction.
The first chemistry experiments were performed using gas thermochromatography in 2001, using 172Os and 173Os as a reference. During the experiment, seven hassium atoms were synthesized using the reactions 248Cm(26Mg,5n)269Hs and 248Cm(26Mg,4n)270Hs. They were then thermalized and oxidized in a mixture of helium and oxygen gases to form hassium tetroxide molecules.
- Hs + 2 O2 → HsO4
The measured deposition temperature indicated hassium tetroxide is less volatile than osmium tetroxide, OsO4 and places hassium firmly in group 8. The enthalpy of adsorption for HsO4 measured, −46±2 kJ/mol, was significantly lower than the predicted value, −36.7±1.5 kJ/mol, indicating OsO4 is more volatile than HsO4, contradicting earlier calculations that implied they should have very similar volatilities. For comparison, the value for OsO4 is −39±1 kJ/mol. It is possible hassium tetroxide interacts differently with silicon nitride than with silicon dioxide, the chemicals 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.
In 2004, scientists reacted hassium tetroxide and sodium hydroxide to form sodium hassate(VIII), a reaction that is well known with osmium. This was the first acid-base reaction with a hassium compound, forming sodium hassate(VIII):
4 + 2 NaOH → Na
The team from the University of Mainz planned in 2008 to study the electrodeposition of hassium atoms using the new TASCA facility at GSI. Their aim was to use the reaction 226Ra(48Ca,4n)270Hs. Scientists at GSI were also hoping to use 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 compounds 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. Hassocene, which is expected to be a stable and highly volatile compound, 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 that had previously been investigated, and relativistic effects were expected to be stronger in the lower oxidation state. Many metals in the periodic table form metallocenes so trends could be more easily determined. The highly symmetrical structure of hassocene and its low number of atoms also make relativistic calculations easier. As of 2019[update] there are no experimental reports of hassocene.
- At the time, this symbol had not yet been taken by seaborgium.
- Different sources give different values for half-lives; the most recently published values are listed.
- Half-life not precisely measured
- This isotope is unconfirmed.
- The two densest elements whose densities have been measured so far are osmium and iridium, both from the sixth period. There have been different records on which is denser; different texts published different results. More precise measurements from the 1990s onward established that osmium was slightly denser at 22.589 ± 0.005 g/cm3 at the standard conditions (iridium may be the denser one at high pressures).
- While iridium is known to also show a +8 state in iridium tetroxide, as well as a unique +9 state in the iridium tetroxide cation IrO+
4, the former is only known in matrix isolation and the latter in the gas phase, and no iridium compounds in such high oxidation states have been synthesized in macroscopic amounts.
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