Hassium

Hassium is a synthetic chemical element; it has symbol Hs and atomic number 108. It is highly radioactive: its most stable known isotopes have half-lives of about ten seconds. One of its isotopes, Hs, has magic numbers of protons and neutrons for deformed nuclei, giving it greater stability against spontaneous fission. Hassium is a superheavy element; it has been produced in a laboratory in very small quantities by fusing heavy nuclei with lighter ones. Natural occurrences of hassium have been hypothesized but never found.

In the periodic table, hassium is a transactinide element, a member of period 7 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, reacting readily with oxygen to form a volatile tetroxide. The chemical properties of hassium have been only partly characterized, but they compare well with the chemistry of the other group 8 elements.

The main innovation that led to the discovery of hassium was cold fusion, where the fused nuclei do not differ by mass as much as in earlier techniques. It relied on greater stability of target nuclei, which in turn decreased excitation energy. This decreased the number of neutrons ejected during synthesis, creating heavier, more stable resulting nuclei. The technique was first tested at Joint Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast, Russian SFSR, Soviet Union, in 1974. JINR used this technique to attempt synthesis of element 108 in 1978, in 1983, and in 1984; the latter experiment resulted in a claim that element 108 had been produced. Later in 1984, a synthesis claim followed from the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Hesse, West Germany. The 1993 report by the Transfermium Working Group, formed by the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP), concluded that the report from Darmstadt was conclusive on its own whereas that from Dubna was not, and major credit was assigned to the German scientists. GSI formally announced they wished to name the element hassium after the German state of Hesse (Hassia in Latin), home to the facility in 1992; this name was accepted as final in 1997.

Introduction to the heaviest elements

Discovery

alt=Apparatus for creating superheavy elements|right|thumb|upright=2.00|Scheme of an apparatus for creating superheavy elements, based on the Dubna Gas-Filled Recoil Separator set up in the [[Flerov Laboratory of Nuclear Reactions in JINR. The trajectory within the detector and the beam focusing apparatus changes because of a dipole magnet in the former and quadrupole magnets in the latter.]]

Cold fusion

Nuclear reactions used in the 1960s resulted in high excitation energies that required expulsion of four or five neutrons; these reactions used targets made of elements with high atomic numbers to maximize the size difference between the two nuclei in a reaction. While this increased the chance of fusion due to the lower electrostatic repulsion between target and projectile, the formed compound nuclei often broke apart and did not survive to form a new element. Moreover, fusion inevitably produces neutron-poor nuclei, as heavier elements need more neutrons per proton for stability; therefore, the necessary ejection of neutrons results in final products that are typically shorter-lived. As such, light beams (six to ten protons) allowed synthesis of elements only up to 106.

To advance to heavier elements, Soviet physicist Yuri Oganessian at Joint Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast, Russian SFSR, Soviet Union, proposed a different mechanism, in which the bombarded nucleus would be lead-208, which has magic numbers of protons and neutrons, or another nucleus close to it. Each proton and neutron has a fixed rest energy; those of all protons are equal and so are those of all neutrons. In a nucleus, some of this energy is diverted to binding protons and neutrons; if a nucleus has a magic number of protons and/or neutrons, then even more of its rest energy is diverted, which makes the nuclide more stable. This additional stability requires more energy for an external nucleus to break the existing one and penetrate it. More energy diverted to binding nucleons means less rest energy, which in turn means less mass (mass is proportional to rest energy). More equal atomic numbers of the reacting nuclei result in greater electrostatic repulsion between them, but the lower mass excess of the target nucleus balances it.

Cold fusion was first declared successful in 1974 at JINR, when it was tested for synthesis of the yet-undiscovered element106. The same year, another team at JINR investigated the possibility of synthesis of element108 in reactions between lead and iron ; they were uncertain in interpreting the data, suggesting the possibility that element108 had not been created.

thumb|upright=1.15|alt=GSI's particle accelerator UNILAC|GSI's linear [[particle accelerator UNILAC, where hassium was discovered and where its chemistry was first observed|left]]

In 1983, new experiments were performed at JINR. The experiments probably resulted in the synthesis of element108; bismuth was bombarded with manganese to obtain 108, lead (Pb) was bombarded with iron (Fe) to obtain 108, and californium was bombarded with neon to obtain 108. GSI's experiment to create element108 was delayed until after their creation of element109 in 1982, as prior calculations had suggested that even–even isotopes of element108 would have spontaneous fission half-lives of less than one microsecond, making them difficult to detect and identify.

Arbitration

In 1985, the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP) formed the Transfermium Working Group (TWG) 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 element108. 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 element108. However, that was determined in retrospect given the work from Darmstadt; the JINR work focused on chemically identifying remote granddaughters of element108 isotopes (which could not exclude the possibility that these daughter isotopes had other progenitors), while the GSI work clearly identified the decay path of those element108 isotopes. The report concluded that the major credit should be awarded to GSI. In written responses to this ruling, both JINR and GSI agreed with its conclusions. In the same response, GSI confirmed that they and JINR were able to resolve all conflicts between them.

Naming

Historically, a newly discovered element was named by its discoverer. The first regulation came in 1947, when IUPAC decided naming required regulation in case there are conflicting names.

In Mendeleev's nomenclature for unnamed and undiscovered elements, hassium would be called "eka-osmium", as in "the first element below osmium in the periodic table" (from Sanskrit eka meaning "one"). In 1979, IUPAC published recommendations according to which the element was to be called "unniloctium" (symbol "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, the competing physicists in the field ignored them. They either called it "element108", with the symbols E108, (108) or 108, or used the proposed name "hassium".

thumb|upright=0.6|[[Coat of arms of the German state of Hesse, after which hassium is named.

]]

In 1990, in an attempt to break a deadlock in establishing priority of discovery and naming of several elements, IUPAC reaffirmed in its nomenclature of inorganic chemistry that after existence of an element was established, the discoverers could propose a name. (Also, the Commission of Atomic Weights was excluded from the naming process.) The first publication on criteria for an element discovery, released in 1991, specified the need for recognition by TWG. It is derived from the Latin name Hassia for the German state of Hesse where the institute is located. Different suggestions to name the whole set of elements from 101 onward and they occasionally assigned names suggested by one team to be used for elements discovered by another. However, not all suggestions were met with equal approval; the teams openly protested naming proposals on several occasions.

In 1994, IUPAC Commission on Nomenclature of Inorganic Chemistry recommended that element108 be named "hahnium" (Hn) after German physicist Otto Hahn so elements named after Hahn and Lise Meitner (it was recommended element109 should be named meitnerium, following GSI's suggestion) would be next to each other, honouring their joint discovery of nuclear fission; IUPAC commented 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. Following the uproar, IUPAC formed an ad hoc committee of representatives from the national adhering organizations of the three countries home to the competing institutions; they produced a new set of names in 1995. Element108 was again named hahnium; this proposal was also retracted. The final compromise was reached in 1996 and published in 1997; element108 was named hassium (Hs). Simultaneously, the name dubnium (Db; from Dubna, the JINR location) was assigned to element105, and the name hahnium was not used for any element.

The official justification for this naming, alongside that of darmstadtium for element110, was that it completed a set of geographic names for the location of the GSI; this set had been initiated by 19th-century names europium and germanium. This set would serve as a response to earlier naming of americium, californium, and berkelium for elements discovered in Berkeley. Armbruster commented on this, "this bad tradition was established by Berkeley. We wanted to do it for Europe."

Isotopes

Hassium has no stable or naturally occurring isotopes. Several radioisotopes have been synthesized in the lab, either by fusing two atoms or by observing the decay of heavier elements. As of 2019, the quantity of all hassium ever produced was on the order of hundreds of atoms. Thirteen isotopes with mass numbers 263 through 277 (except for 274 and 276) have been reported, six of which—Hs—have known metastable states, though that of Hs is unconfirmed. Most of these isotopes decay mainly through alpha decay; this is the most common for all isotopes for which comprehensive decay characteristics are available; the only exception is Hs, which undergoes spontaneous fission. Lighter isotopes were usually synthesized by direct fusion of two nuclei, whereas heavier isotopes were typically observed as decay products of nuclei with larger atomic numbers. It was thus thought that spontaneous fission would occur nearly instantly before nuclei could form a structure that could stabilize them;

The later nuclear shell model suggested that nuclei with ~300 nucleons would form an island of stability where nuclei will be more resistant to spontaneous fission and will mainly undergo alpha decay with longer half-lives, Experiments on lighter superheavy nuclei, as well as those closer to the expected island, Nuclides in 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 Hs and the 5-millisecond half-life of the neighbouring isobar Mt because the hindrance factors from the odd nucleon were shown to be much lower than otherwise expected. The measured half-lives are even lower than those originally predicted for the even–even Hs and Ds, which suggests a gap in stability away from the shell closures and perhaps a weakening of the shell closures in this region. that 108 is a proton magic number for deformed nuclei and 162 is a neutron magic number for such 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 spontaneous-fission half-lives. Computational prospects for shell stabilization for Hs made it a promising candidate for a deformed doubly magic nucleus. Experimental data is scarce, but the existing data is interpreted by the researchers to support the assignment of N=162 as a magic number. In particular, this conclusion was drawn from the decay data of Hs, Hs, and Hs. In 1997, Polish physicist Robert Smolańczuk calculated that the isotope Hs may be the most stable superheavy nucleus against alpha decay and spontaneous fission as a consequence of the predicted N=184 shell closure. claimed to have discovered element108—specifically the 108 isotope, which supposedly had a half-life of 400 to 500million years—in natural molybdenite and suggested the provisional name sergenium (symbol Sg); 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. In 2003, it was suggested that the observed alpha decay with energy 4.5MeV could be due to a low-energy and strongly enhanced transition between different hyperdeformed states of a hassium isotope around Hs, thus suggesting that the existence of superheavy elements in nature was at least possible, but unlikely.

In 2006, Russian geologist Alexei Ivanov hypothesized that an isomer of Hs might have a half-life of ~ years, which would explain the observation of alpha particles with energies of ~4.4MeV in some samples of molybdenite and osmiridium.

In 2004, JINR started a search for natural hassium in the Modane Underground Laboratory in Modane, Auvergne-Rhône-Alpes, France; this was done underground to avoid interference and false positives from cosmic rays.

Since Hs may be particularly stable against alpha decay and spontaneous fission, it was considered as a candidate to exist in nature. This nuclide, however, is predicted to be very unstable toward beta decay and any beta-stable isotopes of hassium such as Hs would be too unstable in the other decay channels to be observed in nature. A 2012 search for Hs in nature along with its homologue osmium at the Maier-Leibnitz Laboratory in Garching, Bavaria, Germany, was unsuccessful, setting an upper limit to its abundance at of hassium per gram of osmium.

Predicted properties

Various calculations suggest hassium should be the heaviest group 8 element so far, consistently with the periodic law. Its properties should generally match those expected for a heavier homologue of osmium; as is the case for all transactinides, a few deviations are expected to arise from relativistic effects.

Very few properties of hassium or its compounds have been measured; this is due to its extremely limited and expensive production and the fact that hassium (and its parents) decays very quickly. A few singular chemistry-related properties have been measured, such as enthalpy of adsorption of hassium tetroxide, but properties of hassium metal remain unknown and only predictions are available.

Relativistic effects

left|thumb|upright=1.50|alt=Energy levels of outermost orbitals of Hs and Os|Energy levels of outermost orbitals of hassium and osmium atoms in [[electronvolts, with and without taking relativistic effects into account. Note the lack of spin–orbit splitting (and thus the lack of distinction between d and d orbitals) in nonrelativistic calculations.]]

Relativistic effects in hassium should arise due to the high charge of its nuclei, which causes the electrons around the nucleus to move faster—so fast their speed is comparable to the speed of light. There are three main effects: the direct relativistic effect, the indirect relativistic effect, and spin–orbit splitting. (The existing calculations do not account for Breit interactions, but those are negligible, and their omission can only result in an uncertainty of the current calculations of no more than 2%.)

As atomic number increases, so does the electrostatic attraction between an electron and the nucleus. This causes the velocity of the electron to increase, which leads to an increase in its mass. This in turn leads to contraction of the atomic orbitals, most specifically the s and p orbitals. Their electrons become more closely attached to the atom and harder to pull from the nucleus. This is the direct relativistic effect. It was originally thought to be strong only for the innermost electrons, but was later established to significantly influence valence electrons as well.

Since the s and p orbitals are closer to the nucleus, they take a bigger portion of the electric charge of the nucleus on themselves ("shield" it). This leaves less charge for attraction of the remaining electrons, whose orbitals therefore expand, making them easier to pull from the nucleus. This is the indirect relativistic effect. As a result of the combination of the direct and indirect relativistic effects, the Hs ion, compared to the neutral atom, lacks a 6d electron, rather than a 7s electron. In comparison, Os lacks a 6s electron compared to the neutral atom. The ionic radius (in oxidation state +8) of hassium is greater than that of osmium because of the relativistic expansion of the 6p orbitals, which are the outermost orbitals for an Hs ion (although in practice such highly charged ions would be too polarized in chemical environments to have much reality).

There are several kinds of electron orbitals, denoted s, p, d, and f (g orbitals are expected to start being chemically active among elements after element 120). Each of these corresponds to an azimuthal quantum number l: s to 0, p to 1, d to 2, and f to 3. Every electron also corresponds to a spin quantum number s, which may equal either +1/2 or −1/2. Thus, the total angular momentum quantum number j = l + s is equal to j = l ± 1/2 (except for l = 0, for which for both electrons in each orbital j = 0 + 1/2 = 1/2). for instance, of the six 6p electrons, two become 6p and four become 6p. This is the spin–orbit splitting (also called subshell splitting or jj coupling). It is most visible with p electrons, which do not play an important role in the chemistry of hassium, but those for d and f electrons are within the same order of magnitude (quantitatively, spin–orbit splitting in expressed in energy units, such as electronvolts). to have a bulk modulus (resistance to uniform compression) of 450GPa, comparable with that of diamond, 442GPa. Hassium is expected to be one of the densest of the 118 known elements, with a predicted density of 27–29 g/cm vs. the 22.59 g/cm measured for osmium. Some of these properties were confirmed by gas-phase chemistry experiments. The group8 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: among stable elements, only ruthenium, osmium, and xenon are able to attain it in reasonably stable compounds. Hassium is expected to follow its congeners and have a stable +8 state, The standard reduction potential for the Hs⁴⁺/Hs couple is expected to be 0.4V.

The group 8 elements show a distinctive oxide chemistry. All the lighter members have known or hypothetical tetroxides, MO. Their oxidizing power decreases as one descends the group. FeO 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 oxyanion ferrate(VI), . Ruthenium tetroxide, RuO₄, which is formed by oxidation of ruthenium(VI) in acid, readily undergoes reduction to ruthenate(VI), . 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, hassium should behave as a heavier homologue of osmium by forming of a stable, very volatile tetroxide HsO₄, which undergoes complexation with hydroxide to form a hassate(VIII), [HsO₄(OH)₂]²⁻.

Experimental chemistry

The first goal for chemical investigation was the formation of the tetroxide; it was chosen because ruthenium and osmium form volatile tetroxides, being the only transition metals to display a stable compound in the +8 oxidation state. Despite this selection for gas-phase chemical studies being clear from the beginning,

The first chemistry experiments were performed using gas thermochromatography in 2001, using the synthetic osmium radioisotopes Os as a reference. During the experiment, seven hassium atoms were synthesized using the reactions Cm(Mg,5n)Hs and Cm(Mg,4n)Hs. They were then thermalized and oxidized in a mixture of helium and oxygen gases to form hassium tetroxide molecules.

: + 2 NaOH →

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 Ra(Ca,4n)Hs. Scientists at GSI were hoping to use TASCA to study the synthesis and properties of the hassium(II) compound hassocene, Hs(CH), using the reaction Ra(Ca,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. The highly symmetrical structure of hassocene and its low number of atoms make relativistic calculations easier.

Notes

References

Bibliography

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