Chemistry:Hassium

Hassium is a chemical element; it has symbol Hs and atomic number 108. Hassium is highly radioactive: its most stable known isotopes have half-lives of approximately ten seconds. One of its isotopes, ²⁷⁰Hs, has magic numbers of both protons and neutrons for deformed nuclei, which gives it greater stability against spontaneous fission. Hassium is a superheavy element; it has been produced in a laboratory only in very small quantities by fusing heavy nuclei with lighter ones. Natural occurrences of the element have been hypothesised but never found.

In the periodic table of 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, 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 principal innovation that led to the discovery of hassium was the technique of cold fusion, in which the fused nuclei did 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 neutron ejections during synthesis, creating heavier, more stable resulting nuclei. The technique was first tested at the 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 and the International Union of Pure and Applied Physics, 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

This section is an excerpt from Superheavy element § Introduction[ edit ]

Discovery

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 the target and the projectile, the formed compound nuclei often broke apart and did not survive to form a new element. Moreover, fusion processes inevitably produce neutron-poor nuclei, as heavier elements require more neutrons per proton to maximize stability; therefore, the necessary ejection of neutrons results in final products with typically have shorter lifetimes. 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 the 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 value of 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 gives the nuclide additional stability. 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. This leaves less excitation energy for the newly created compound nucleus, which necessitates fewer neutron ejections to reach a stable state. Because of this energy difference, the former mechanism became known as "hot fusion" and the latter as "cold fusion".

Cold fusion was first declared successful in 1974 at JINR, when it was tested for synthesis of the yet-undiscovered element 106. These new nuclei were projected to decay via spontaneous fission. The physicists at JINR concluded element 106 was produced in the experiment because no fissioning nucleus known at the time showed parameters of fission similar to what was observed during the experiment and because changing either of the two nuclei in the reactions negated the observed effects. Physicists at the Lawrence Berkeley Laboratory (LBL; originally Radiation Laboratory, RL, and later Lawrence Berkeley National Laboratory, LBNL) of the University of California in Berkeley, California , United States, also expressed great interest in the new technique. When asked about how far this new method could go and if lead targets were a physics' Klondike, Oganessian responded, "Klondike may be an exaggeration [...] But soon, we will try to get elements 107 ... 108 in these reactions."

Reports

The synthesis of element 108 was first attempted in 1978 by a research team led by Oganessian at the JINR. The team used a reaction that would generate element 108, specifically, the isotope ²⁷⁰108, from fusion of radium (specifically, the isotope 22688Ra) and calcium (4820Ca). 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 (20882Pb) and iron (5826Fe); they were uncertain in interpreting the data, suggesting the possibility that element 108 had not been created.

GSI's linear particle accelerator UNILAC, where hassium was discovered and where its chemistry was first observed

In 1983, new experiments were performed at JINR. The experiments probably resulted in the synthesis of element 108; bismuth (20983Bi) was bombarded with manganese (5525Mn) to obtain ²⁶³108, lead (20782Pb, 20882Pb) was bombarded with iron (5826Fe) to obtain ²⁶⁴108, and californium (24998Cf) was bombarded with neon (2210Ne) 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; the researchers concluded they were caused by ²⁶⁴108.

Later in 1984, a research team led by Peter Armbruster and Gottfried Münzenberg at Gesellschaft für Schwerionenforschung (GSI; Institute for Heavy Ion Research) in Darmstadt, Hesse, West Germany, attempted to create element 108. The team bombarded a lead (20882Pb) target with accelerated iron (5826Fe) 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. 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 corroborated by an experiment aimed at synthesizing isotopes of element 106. GSI reported synthesis of three atoms of ²⁶⁵108. Two years later, they reported synthesis of one atom of the even–even ²⁶⁴108.

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 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. However, that was determined in retrospect given the work from Darmstadt; the JINR work focused on chemically identifying remote granddaughters of element 108 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 element 108 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. These matters were to be resolved by the Commission of Inorganic Nomenclature and the Commission of Atomic Weights. They would review the names in case of a conflict and select one; the decision would be based on a number of factors, such as usage, and would not be an indicator of priority of a claim. The two commissions would recommend a name to the IUPAC Council, which would be the final authority. The discoverers held the right to name an element, but their name would be subject to approval by IUPAC. The Commission of Atomic Weights distanced itself from element naming in most cases.

Under Mendeleev's nomenclature for unnamed and undiscovered elements, hassium would be known as "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" 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, the competing physicists in the field ignored them. They either called it "element 108", with the symbols E108, (108) or 108, or used the proposed name "hassium".

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. (In addition, 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.

Armbruster and his colleagues, the officially recognized German discoverers, held a naming ceremony for the elements 107 through 109, which had all been recognized as discovered by GSI, on 7 September 1992. For element 108, the scientists proposed the name "hassium". It is derived from the Latin name Hassia for the German state of Hesse where the institute is located. This name was proposed to IUPAC in a written response to their ruling on priority of discovery claims of elements, signed 29 September 1992.

The process of naming of element 108 was a part of a larger process of naming a number of elements starting with element 101; three teams—JINR, GSI, and LBL—claimed discoveries of several elements and the right to name those elements. Sometimes, these claims clashed; since a discoverer was considered entitled to naming of an element, conflicts over priority of discovery often resulted in conflicts over names of these new elements. These conflicts became known as the Transfermium Wars. 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 element 108 be named "hahnium" (Hn) after the German physicist Otto Hahn so elements named after Hahn and Lise Meitner (it was recommended element 109 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. The name "hahnium", albeit with the different symbol Ha, had already been proposed and used by the American scientists for element 105, for which they had a discovery dispute with JINR; they thus protested the confusing scrambling of names. 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. Element 108 was again named hahnium; this proposal was also retracted. The final compromise was reached in 1996 and published in 1997; element 108 was named hassium (Hs). Simultaneously, the name dubnium (Db; from Dubna, the JINR location) was assigned to element 105, and the name hahnium was not used for any element.

The official justification for this naming, alongside that of darmstadtium for element 110, 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." Later, when commenting on the naming of element 112, Armbruster said, "I did everything to ensure that we do not continue with German scientists and German towns."

Isotopes

List of hassium isotopes

Isotope	Half-life		Decay mode	Discovery year	Discovery reaction
	Value	Ref
263Hs	760 760 μs		α, SF	2009	208Pb(56Fe,n)
264Hs	540 540 μs		α, SF	1986	207Pb(58Fe,n)
265Hs	1960 1.96 ms		α, SF	1984	208Pb(58Fe,n)
265mHs	360 360 μs		α	1995	208Pb(58Fe,n)
266Hs	3020 3.02 ms		α, SF	2001	270Ds(—,α)
266mHs	280000 280 ms		α	2011	270mDs(—,α)
267Hs	55000 55 ms		α	1995	238U(34S,5n)
267mHs	990 990 μs		α	2004	238U(34S,5n)
268Hs	1420000 1.42 s		α	2010	238U(34S,4n)
269Hs	16000000 16 s		α	1996	277Cn(—,2α)
270Hs	9000000 9 s		α	2003	248Cm(26Mg,4n)
271Hs	10000000 10 s		α	2008	248Cm(26Mg,3n)
272Hs	160000 160 ms		α	2022	276Ds(—,α)
273Hs	510000 510 ms		α	2010	285Fl(—,3α)
275Hs	600000 600 ms		α	2004	287Fl(—,3α)
277Hs	12000 12 ms		α	2010	289Fl(—,3α)
277mHs	130000000 130 s		SF	2012	293mLv(—,4α)

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. As of 2019, the quantity of all hassium ever produced was on the order of hundreds of atoms. Thirteen isotopes with mass numbers ranging from 263 to 277 (with the exceptions of 274 and 276) have been reported, four of which—hassium-265, -266, -267, 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. Lighter isotopes were usually synthesized by direct fusion between two lighter nuclei, whereas heavier isotopes were typically observed as decay products of nuclei with larger atomic numbers.

Atomic nuclei have well-established nuclear shells, and the existence of these shells provides nuclei with additional stability. If a nucleus has certain numbers of protons or neutrons, called magic numbers, that complete certain nuclear shells, then the nucleus is even more stable against decay. The highest known magic numbers are 82 for protons and 126 for neutrons. This notion is sometimes expanded to include additional numbers between those magic numbers, which also provide some additional stability and indicate closure of "sub-shells". In contrast to the better-known lighter nuclei, superheavy nuclei are deformed. Until the 1960s, the liquid drop model was the dominant explanation for nuclear structure. It suggested that the fission barrier would disappear for nuclei with about 280 nucleons. It was thus thought that spontaneous fission would occur nearly instantly before nuclei could form a structure that could stabilize them; it appeared that nuclei with Z ≈ 103 were too heavy to exist for a considerable length of time.

The later nuclear shell model suggested that nuclei with about three hundred nucleons would form an island of stability in which nuclei will be more resistant to spontaneous fission and will primarily undergo alpha decay with longer half-lives, and the next doubly magic nucleus (having magic numbers of both protons and neutrons) is expected to lie in the center of the island of stability in the vicinity of Z = 110–114 and the predicted magic neutron number N = 184. Subsequent discoveries suggested that the predicted island might be further than originally anticipated; they also showed that nuclei intermediate between the long-lived actinides and the predicted island are deformed, and gain additional stability from shell effects. The addition to the stability against the spontaneous fission should be particularly great against spontaneous fission, although increase in stability against the alpha decay would also be pronounced. The center of the region on a chart of nuclides that would correspond to this stability for deformed nuclei was determined as ²⁷⁰Hs, with 108 expected to be a magic number for protons for deformed nuclei—nuclei that are far from spherical—and 162 a magic number for neutrons for such nuclei. Experiments on lighter superheavy nuclei, as well as those closer to the expected island, have shown greater than previously anticipated stability against spontaneous fission, showing the importance of shell effects on nuclei.

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 eleven-millisecond half-life of ²⁷⁷Hs and the five-millisecond half-life of the neighbouring isobar ²⁷⁷Mt 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 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.

In 1991, Polish physicists Zygmunt Patyk and Adam Sobiczewski predicted 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 life-times toward spontaneous fission. Computational prospects for shell stabilization for ²⁷⁰Hs 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 ²⁶⁹Hs, ²⁷⁰Hs, and ²⁷¹Hs. In 1997, Polish physicist Robert Smolańczuk calculated that the isotope ²⁹²Hs may be the most stable superheavy nucleus against alpha decay and spontaneous fission as a consequence of the predicted N = 184 shell closure.

Natural occurrence

Molybdenite

Hassium is not known to occur naturally on Earth; the half-lives of all 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 1914, 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 1963, Soviet geologist and physicist 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); 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. 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 a hundred 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 2006, Russian geologist Alexei Ivanov 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 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; this would explain excesses of plutonium-239 found on the ocean floors of the Pacific Ocean and the Gulf of Finland. However, minerals enriched with ²⁷¹Hs are predicted to have excesses of its daughters uranium-235 and lead-207; they would also 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.

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. In 2008–09, an experiment run in the laboratory resulted in detection of several registered events of neutron multiplicity (number of emitted free neutrons after a nucleus hit has been hit by a neutron and fissioned) above three in natural osmium, and in 2012–13, these findings were reaffirmed in another experiment run in the laboratory. These results hinted natural hassium could potentially exist in nature in amounts that allow its detection by the means of analytical chemistry, but this conclusion is based on an explicit assumption that there is a long-lived hassium isotope to which the registered events could be attributed.

Since ²⁹²Hs 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 ²⁸⁶Hs would be too unstable in the other decay channels to be observed in nature. A 2012 search for ²⁹²Hs 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 3×10⁻¹⁵ grams 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

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_3/2 and d_5/2 orbitals) in nonrelativistic calculations.

Relativistic effects on hassium should arise due to the high charge of its nuclei, which causes the electrons around the nucleus to move faster—so fast their velocity becomes 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_1/2 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_1/2 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_3/2 orbitals, which are the outermost orbitals for an Hs⁸⁺ ion (although in practice such highly charged ions would be too polarised in chemical environments to have much reality).

There are several kinds of electronic orbitals, denoted by the letters 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). Spin of an electron relativistically interacts with its orbit, and this interaction leads to a split of a subshell into two with different energies (the one with j = l − 1/2 is lower in energy and thus these electrons more difficult to extract): for instance, of the six 6p electrons, two become 6p_1/2 and four become 6p_3/2. This is the spin–orbit splitting (sometimes also referred to as 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).

These relativistic effects are responsible for the expected increase of the ionization energy, decrease of the electron affinity, and increase of stability of the +8 oxidation state compared to osmium; without them, the trends would be reversed. Relativistic effects decrease the atomization energies of the compounds of hassium because the spin–orbit splitting of the d orbital lowers binding energy between electrons and the nucleus and because relativistic effects decrease ionic character in bonding.

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, 442 GPa. Hassium is expected to be one of the densest of the 118 known elements, with a predicted density of 27–29 g/cm³ vs. the 22.59 g/cm³ measured for osmium.

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. In chemical compounds, hassium is calculated to display bonding characteristic for a d-block element, whose bonding will be primarily executed by 6d_3/2 and 6d_5/2 orbitals; compared to the elements from the previous periods, 7s, 6p_1/2, 6p_3/2, and 7p_1/2 orbitals should be more important.

Chemical

Stable oxidation states in group 8

Element	Stable oxidation states
iron			+6			+3	+2
ruthenium	+8		+6	+5	+4	+3	+2
osmium	+8		+6	+5	+4	+3	+2

Hassium is the sixth member of the 6d series of transition metals and is expected to be much like the platinum group metals. 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: 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, but like them it should show lower stable oxidation states such as +6, +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.

The group 8 elements show a distinctive oxide chemistry. All 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 oxyanion 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, 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⁻. 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 experimentally 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.

Experimental chemistry

In ferrocene, the cyclopentadienyl rings are in a staggered conformation.

In ruthenocene and osmocene, the cyclopentadienyl rings are in an eclipsed conformation. Hassocene is also predicted to have this structure.

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, 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 was synthesized. Unfortunately, this hassium isotope, ²⁶⁹Hs, was 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. 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, research focused on ruthenium and osmium rather than iron because hassium was expected to be similar to ruthenium and osmium, as the predicted data on hassium closely matched that of those two.

The first chemistry experiments were performed using gas thermochromatography in 2001, using the synthetic osmium radioisotopes 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 of hassium tetroxide was higher than that of osmium tetroxide, which indicated the former was the less volatile one, and this placed 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. (The calculations that yielded a closer match to the experimental data came after the experiment, in 2008.) It is possible hassium tetroxide interacts differently with silicon nitride than with silicon dioxide, the chemicals used for the detector; further research is required to establish whether there is a difference between such interactions and whether it has influenced the measurements. Such research would include 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):

HsO₄ + 2 NaOH → Na₂[HsO₄(OH)₂]

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 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. The highly symmetrical structure of hassocene and its low number of atoms make relativistic calculations easier. (As of 2021), there are no experimental reports of hassocene.

Bibliography

• Audi, G.; Kondev, F. G.; Wang, M. et al. (2017). "The NUBASE2016 evaluation of nuclear properties". Chinese Physics C 41 (3): 030001. doi:10.1088/1674-1137/41/3/030001. Bibcode: 2017ChPhC..41c0001A. http://pdfs.semanticscholar.org/7c02/89532fe80c913edd952e8caffd555013f596.pdf. 
• Barber, R. C.; Greenwood, N. N.; Hrynkiewicz, A. Z. et al. (1993). "Discovery of the Transfermium elements". Pure and Applied Chemistry 65 (8): 1757–1814. doi:10.1351/pac199365081757. http://s3.documentcloud.org/documents/562229/iupac1.pdf. Retrieved 7 September 2016. 
• Beiser, A. (2003). Concepts of modern physics (6th ed.). McGraw-Hill. ISBN 978-0-07-244848-1. OCLC 48965418. 
• Greenwood, N. N.; Earnshaw, A. (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8. https://archive.org/details/chemistryelement00earn_087. 
• Hoffman, D. C.; Ghiorso, A.; Seaborg, G. T. (2000). The Transuranium People: The Inside Story. World Scientific. ISBN 978-1-78-326244-1. 
• Hoffman, D. C.; Lee, D. M.; Pershina, V. (2006). "Transactinides and the future elements". in Morss, L. R.; Edelstein, N. M.; Fuger, J.. The Chemistry of the Actinide and Transactinide Elements (3rd ed.). Springer Science+Business Media. pp. 1652–1752. ISBN 978-1-4020-3555-5. https://archive.org/details/chemistryactinid00katz. 
• Kragh, H. (2018). From Transuranic to Superheavy Elements: A Story of Dispute and Creation. Springer. ISBN 978-3-319-75813-8. 
• Lide, D. R. (2004). Handbook of chemistry and physics (84th ed.). CRC Press. ISBN 978-0-8493-0566-5. https://archive.org/details/crchandbookofche81lide. 
• Zagrebaev, V.; Karpov, A.; Greiner, W. (2013). "Future of superheavy element research: Which nuclei could be synthesized within the next few years?". Journal of Physics 420 (1): 012001. doi:10.1088/1742-6596/420/1/012001. ISSN 1742-6588. Bibcode: 2013JPhCS.420a2001Z.