Hassium

Hassium
108Hs
Os ↑ Hs ↓ (Uhn) Periodic table bohrium ← hassium → meitnerium
Appearance
silvery (predicted)
General properties
Name, symbol, number	hassium, Hs, 108
Pronunciation	/ ˈ h æ s i ə m / HASS-ee-əm
Element category	transition metal
Group, period, block	8, 7, d
Standard atomic weight	[269]
Electron configuration	[Rn] 5f14 6d6 7s2 (predicted) 2, 8, 18, 32, 32, 14, 2 (predicted)
History
Discovery	Gesellschaft für Schwerionenforschung (1984)
Physical properties
Phase	solid (predicted)
Density (near r.t.)	41 (predicted) g·cm−3
Atomic properties
Oxidation states	8, 6, 5, 4, 3, 2 (predicted) (only bolded oxidation states are known experimentally)
Ionization energies ( more)	1st: 733.3 (estimated) kJ·mol−1
	2nd: 1756.0 (estimated) kJ·mol−1
	3rd: 2827.0 (estimated) kJ·mol−1
Atomic radius	126 (estimated) pm
Covalent radius	134 (estimated) pm
Miscellanea
CAS registry number	54037-57-9
Most stable isotopes
Main article: Isotopes of hassium
iso NA half-life DM DE ( MeV) DP 277Hs syn 2 s SF 277mHs ? syn ~11 min SF 271Hs syn ~4 s α 9.27,9.13 267Sg 270Hs syn 3.6 s α 9.02,8.88 266Sg 269Hs syn 9.7 s α 9.21,9.10,8.97 265Sg only isotopes with half-lives over 1 second are included here

Hassium is a chemical element with symbol Hs and atomic number 108, named in honour 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.

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. 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:

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.

Naming

Using Mendeleev's nomenclature for unnamed and undiscovered elements, hassium should be known as eka-osmium or dvi-ruthenium. In 1979, during the Transfermium Wars (but before the synthesis of hassium), IUPAC published recommendations according to which the element was to be called unniloctium (with 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 on. The recommendations were mostly ignored among scientists, who either called it "element 108", with the symbol of (108) or even simply 108, or used the proposed name "hassium".

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.

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. This was because they felt that Hesse did not merit an element being named after it. After protests from the German discoverers and the American Chemical Society, IUPAC relented and the name hassium (Hs) was adopted internationally in 1997.

Natural occurrence

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 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.

In 2006, 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. 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. 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.

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.

Isotopes

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.

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.

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. 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. The isotope, however, was produced in 2010 by the same team. The new data matched the previous (fabricated) data.

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. 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. 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.

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.

For spontaneous fission, it is necessary to measure the half-lives for the isotonic nuclei ²⁶⁸Sg, ²⁷⁰Hs and ²⁷²Ds. Since the isotopes ²⁶⁸Sg and ²⁷²Ds are not currently known, and fission of ²⁷⁰Hs has not been measured, 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. More conclusive evidence would come from the determination of the decay energy for the unknown nucleus ²⁷²Ds.

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. 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. Some of its properties were determined by gas-phase chemistry experiments. 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. 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, except for xenon, iridium, and plutonium (unstable). 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, such as +6, +5, +4, +3, and +2.

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. 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. Ruthenium tetroxide, RuO₄, 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 (due to the tetrahedral nature of the molecule), which undergoes complexation with hydroxide to form a hassate(VIII), [HsO₄(OH)₂]²⁻. 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; this trend remains the same whether or not relativistic effects are taken into account.

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, the most dense measured element, at 22.61 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. 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. 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.

Experimental atomic gas phase chemistry

Despite the fact that the selection of a volatile hassium compound (hassium tetroxide) for gas-phase chemical studies was 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 also 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, as the cross-section for this reaction was somewhat larger, at 7 pm. However, this yield was still around an order of magnitude 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 as a typical member of group 8 in early 2001.

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. Consequently, in preparation for the chemical characterization of hassium, researches focused on ruthenium and osmium rather than iron, as hassium was expected to also be similar to ruthenium and osmium due to the actinide contraction. However, in the planned experiment to study hassocene (Hs( C₅H₅)₂), ferrocene may also be used for comparison along with ruthenocene and osmocene.

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.

²⁶⁹Hs + 2 O₂ → ²⁶⁹HsO₄

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

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]

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. 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. 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. 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.