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Hassium, 108Hs
Hassium
Pronunciation/ˈhæsiəm/ ⓘ[1] ​(HASS-ee-əm)
Mass number[271] (data not decisive)[a]
Hassium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
Os
↑
Hs
↓
—
bohrium ← hassium → meitnerium
Atomic number (Z)108
Groupgroup 8
Periodperiod 7
Block  d-block
Electron configuration[Rn] 5f14 6d6 7s2[4]
Electrons per shell2, 8, 18, 32, 32, 14, 2
Physical properties
Phase at STPsolid (predicted)[5]
Density (near r.t.)27–29 g/cm3 (predicted)[6][7]
Atomic properties
Oxidation statescommon: (none)
(+3), (+4), (+6), (+8)[8]
Ionization energies
  • 1st: 730 kJ/mol
  • 2nd: 1760 kJ/mol
  • 3rd: 2830 kJ/mol
  • (more) (predicted)[9]
Atomic radiusempirical: 126 pm (estimated)[10]
Covalent radius134 pm (estimated)[11]
Other properties
Natural occurrencesynthetic
Crystal structure ​hexagonal close-packed (hcp)
Hexagonal close-packed crystal structure for hassium

(predicted)[5]
CAS Number54037-57-9
History
Namingafter Hassia, Latin for Hesse, Germany, where it was discovered[12]
DiscoveryGesellschaft für Schwerionenforschung (1984)
Isotopes of hassium
Main isotopes[13] Decay
Isotope abun­dance half-life (t1/2) mode pro­duct
269Hs synth 13 s α 265Sg
270Hs synth 7.6 s α 266Sg
271Hs synth 46 s α 267Sg
277mHs synth 130 s? SF –
 Category: Hassium
| references

Hassium is a 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.[a] One of its isotopes, 270Hs, 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

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

Synthesis of superheavy nuclei

[edit]
A graphic depiction of a nuclear fusion reaction
A graphic depiction of a nuclear fusion reaction. Two nuclei fuse into one, emitting a neutron. Reactions that created new elements to this moment were similar, with the only possible difference that several singular neutrons sometimes were released, or none at all.

A superheavy[b] atomic nucleus is created in a nuclear reaction that combines two other nuclei of unequal size[c] into one; roughly, the more unequal the two nuclei in terms of mass, the greater the possibility that the two react.[19] The material made of the heavier nuclei is made into a target, which is then bombarded by the beam of lighter nuclei. Two nuclei can only fuse into one if they approach each other closely enough; normally, nuclei (all positively charged) repel each other due to electrostatic repulsion. The strong interaction can overcome this repulsion but only within a very short distance from a nucleus; beam nuclei are thus greatly accelerated in order to make such repulsion insignificant compared to the velocity of the beam nucleus.[20] The energy applied to the beam nuclei to accelerate them can cause them to reach speeds as high as one-tenth of the speed of light. However, if too much energy is applied, the beam nucleus can fall apart.[20]

Coming close enough alone is not enough for two nuclei to fuse: when two nuclei approach each other, they usually remain together for about 10−20 seconds and then part ways (not necessarily in the same composition as before the reaction) rather than form a single nucleus.[20][21] This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed.[20] Each pair of a target and a beam is characterized by its cross section—the probability that fusion will occur if two nuclei approach one another expressed in terms of the transverse area that the incident particle must hit in order for the fusion to occur.[d] This fusion may occur as a result of the quantum effect in which nuclei can tunnel through electrostatic repulsion. If the two nuclei can stay close past that phase, multiple nuclear interactions result in redistribution of energy and an energy equilibrium.[20]

External videos
video icon Visualization of unsuccessful nuclear fusion, based on calculations from the Australian National University[23]

The resulting merger is an excited state[24]—termed a compound nucleus—and thus it is very unstable.[20] To reach a more stable state, the temporary merger may fission without formation of a more stable nucleus.[25] Alternatively, the compound nucleus may eject a few neutrons, which would carry away the excitation energy; if the latter is not sufficient for a neutron expulsion, the merger would produce a gamma ray. This happens in about 10−16 seconds after the initial nuclear collision and results in creation of a more stable nucleus.[25] The definition by the IUPAC/IUPAP Joint Working Party (JWP) states that a chemical element can only be recognized as discovered if a nucleus of it has not decayed within 10−14 seconds. This value was chosen as an estimate of how long it takes a nucleus to acquire electrons and thus display its chemical properties.[26][e]

Decay and detection

[edit]

The beam passes through the target and reaches the next chamber, the separator; if a new nucleus is produced, it is carried with this beam.[28] In the separator, the newly produced nucleus is separated from other nuclides (that of the original beam and any other reaction products)[f] and transferred to a surface-barrier detector, which stops the nucleus. The exact location of the upcoming impact on the detector is marked; also marked are its energy and the time of the arrival.[28] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[31] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[28]

Stability of a nucleus is provided by the strong interaction. However, its range is very short; as nuclei become larger, its influence on the outermost nucleons (protons and neutrons) weakens. At the same time, the nucleus is torn apart by electrostatic repulsion between protons, and its range is not limited.[32] Total binding energy provided by the strong interaction increases linearly with the number of nucleons, whereas electrostatic repulsion increases with the square of the atomic number, i.e. the latter grows faster and becomes increasingly important for heavy and superheavy nuclei.[33][34] Superheavy nuclei are thus theoretically predicted[35] and have so far been observed[36] to predominantly decay via decay modes that are caused by such repulsion: alpha decay and spontaneous fission.[g] Almost all alpha emitters have over 210 nucleons,[38] and the lightest nuclide primarily undergoing spontaneous fission has 238.[39] In both decay modes, nuclei are inhibited from decaying by corresponding energy barriers for each mode, but they can be tunneled through.[33][34]

Apparatus for creation of superheavy elements
Scheme of an apparatus for creation of 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.[40]

Alpha particles are commonly produced in radioactive decays because the mass of an alpha particle per nucleon is small enough to leave some energy for the alpha particle to be used as kinetic energy to leave the nucleus.[41] Spontaneous fission is caused by electrostatic repulsion tearing the nucleus apart and produces various nuclei in different instances of identical nuclei fissioning.[34] As the atomic number increases, spontaneous fission rapidly becomes more important: spontaneous fission partial half-lives decrease by 23 orders of magnitude from uranium (element 92) to nobelium (element 102),[42] and by 30 orders of magnitude from thorium (element 90) to fermium (element 100).[43] The earlier liquid drop model thus suggested that spontaneous fission would occur nearly instantly due to disappearance of the fission barrier for nuclei with about 280 nucleons.[34][44] The later nuclear shell model suggested that nuclei with about 300 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.[34][44] 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.[45] Experiments on lighter superheavy nuclei,[46] as well as those closer to the expected island,[42] have shown greater than previously anticipated stability against spontaneous fission, showing the importance of shell effects on nuclei.[h]

Alpha decays are registered by the emitted alpha particles, and the decay products are easy to determine before the actual decay; if such a decay or a series of consecutive decays produces a known nucleus, the original product of a reaction can be easily determined.[i] (That all decays within a decay chain were indeed related to each other is established by the location of these decays, which must be in the same place.)[28] The known nucleus can be recognized by the specific characteristics of decay it undergoes such as decay energy (or more specifically, the kinetic energy of the emitted particle).[j] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[k]

The information available to physicists aiming to synthesize a superheavy element is thus the information collected at the detectors: location, energy, and time of arrival of a particle to the detector, and those of its decay. The physicists analyze this data and seek to conclude that it was indeed caused by a new element and could not have been caused by a different nuclide than the one claimed. Often, provided data is insufficient for a conclusion that a new element was definitely created and there is no other explanation for the observed effects; errors in interpreting data have been made.[l]

Discovery

[edit]
See also: Discovery of the chemical elements and Timeline of chemical element discoveries
Apparatus for creating superheavy elements
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.[57]

Cold fusion

[edit]

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;[m] 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.[60]

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.[61] 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.[62] 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.[61] This leaves less excitation energy for the new compound nucleus, which necessitates fewer neutron ejections to reach a stable state.[62] Due to this energy difference, the former mechanism became known as "hot fusion" and the latter as "cold fusion".[63]

Cold fusion was first declared successful in 1974 at JINR, when it was tested for synthesis of the yet-undiscovered element 106.[62] 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 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.[62] 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."[62]

Reports

[edit]

Synthesis of element 108 was first attempted in 1978 by a team led by Oganessian at JINR. The team used a reaction that would generate element 108, specifically, the isotope 270108,[n] from fusion of radium (specifically, the isotope 226
88Ra) and calcium (48
20Ca). The researchers were uncertain in interpreting their data, and their paper did not unambiguously claim to have discovered the element.[64] The same year, another team at JINR investigated the possibility of synthesis of element 108 in reactions between lead (208
82Pb) and iron (58
26Fe); they were uncertain in interpreting the data, suggesting the possibility that element 108 had not been created.[65]

GSI's particle accelerator UNILAC
GSI's linear particle accelerator UNILAC, where hassium was discovered[66] and where its chemistry was first observed[67]

In 1983, new experiments were performed at JINR.[68] The experiments probably resulted in the synthesis of element 108; bismuth (209
83Bi) was bombarded with manganese (55
25Mn) to obtain 263108, lead (207, 208Pb) was bombarded with iron (58Fe) to obtain 264108, and californium (249
98Cf) was bombarded with neon (22
10Ne) to obtain 270108.[12] These experiments were not claimed as a discovery and Oganessian announced them in a conference rather than in a written report.[68]

In 1984, JINR researchers in Dubna performed experiments set up identically to the previous ones; they bombarded bismuth and lead targets with ions of manganese and iron, respectively. Twenty-one spontaneous fission events were recorded; the researchers concluded they were caused by 264108.[69]

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, tried to create element 108. The team bombarded a lead (208Pb) target with accelerated iron (58Fe) nuclei.[70] 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.[71] 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.[71]

Arbitration

[edit]

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.[72]

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.[69] 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.[73]

Naming

[edit]
See also: Transfermium Wars

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.[74][o] 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.[74] The discoverers held the right to name an element, but their name would be subject to approval by IUPAC.[74] The Commission of Atomic Weights distanced itself from element naming in most cases.[74]

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"),[75] 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.[76][77] They either called it "element 108", with the symbols E108, (108) or 108, or used the proposed name "hassium".[78]

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.[74]

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".[79] It is derived from the Latin name Hassia for the German state of Hesse where the institute is located.[12][73] This name was proposed to IUPAC in a written response to their ruling on priority of discovery claims of elements, signed 29 September 1992.[73]

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 discovery 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.[80] 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.[p] However, not all suggestions were met with equal approval; the teams openly protested naming proposals on several occasions.[82]

In 1994, IUPAC Commission on Nomenclature of Inorganic Chemistry recommended that element 108 be named "hahnium" (Hn) after 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;[83] IUPAC commented that they felt the German suggestion was obscure.[84] GSI protested, saying this proposal contradicted the long-standing convention of giving the discoverer the right to suggest a name;[85] the American Chemical Society supported GSI.[12] 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.[86] 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.[87] The final compromise was reached in 1996 and published in 1997; element 108 was named hassium (Hs).[88] 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.[89][90][q]

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[r] was established by Berkeley. We wanted to do it for Europe."[92] 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."[92]

Isotopes

[edit]
Main article: Isotopes of hassium
List of hassium isotopes
Isotope Half-life[s][t] Decay
mode 		Discovery
year[94] 		Discovery
reaction[95][u]
Value ref
263Hs 760 μs [94] α, SF 2009 208Pb(56Fe,n)
264Hs 540 μs [94] α, SF 1986 207Pb(58Fe,n)
265Hs 1.96 ms [94] α, SF 1984 208Pb(58Fe,n)
265mHs 360 μs [94] α 1995 208Pb(58Fe,n)
266Hs 3.02 ms [94] α, SF 2001 270Ds(—,α)
266mHs 280 ms [94] α 2011 270mDs(—,α)
267Hs 55 ms [93] α 1995 238U(34S,5n)
267mHs 990 μs [93] α 2004 238U(34S,5n)
268Hs 1.42 s [93] α 2010 238U(34S,4n)
269Hs 13 s [96] α 1996 277Cn(—,2α)
269mHs 2.8 s [96] α, IT 2024 273Ds(—,α)
270Hs 7.6 s [93] α 2003 248Cm(26Mg,4n)
271Hs 46 s [96] α 2008 248Cm(26Mg,3n)
271mHs 7.1 s [96] α, IT 2024 275Ds(—,α)
272Hs 160 ms [97] α 2022 276Ds(—,α)
273Hs 510 ms [98] α 2010 285Fl(—,3α)
275Hs 600 ms [99] α 2004 287Fl(—,3α)
277Hs 12 ms [3] α 2010 289Fl(—,3α)
277mHs 130 s[v] [3][100] SF 2012 293mLv(—,4α)

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.[101][102] Thirteen isotopes with mass numbers 263 through 277 (except for 274 and 276) have been reported, six of which—265, 266, 267, 269, 271, 277Hs—have known metastable states,[103][w] though that of 277Hs is unconfirmed.[104] 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 277Hs, which undergoes spontaneous fission.[103] 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.[95]

Atomic nuclei have well-established nuclear shells, which make nuclei more stable. If a nucleus has certain numbers (magic numbers) of protons or neutrons, that complete a nuclear shell, 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". Unlike 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 ~280 nucleons.[105][106] It was thus thought that spontaneous fission would occur nearly instantly before nuclei could form a structure that could stabilize them;[60] it appeared that nuclei with Z ≈ 103[x] were too heavy to exist for a considerable length of time.[107]

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,[105][106] 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 near 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, against alpha decay and especially against spontaneous fission.[106] 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.[108] Experiments on lighter superheavy nuclei,[109] as well as those closer to the expected island,[60] 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[110] and N = 168–170, which is between the predicted neutron shell closures at N = 162 for deformed nuclei and N = 184 for spherical nuclei.[111] 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 277Hs and the 5-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.[111]

In 1991, Polish physicists Zygmunt Patyk and Adam Sobiczewski predicted[112] 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.[113][114] Computational prospects for shell stabilization for 270Hs made it a promising candidate for a deformed doubly magic nucleus.[115] 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.[y] 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.[118][119]

Natural occurrence

[edit]
Dark reflective crystal of molybdenite
Molybdenite

Hassium is not known to occur naturally on Earth; all its known isotopes are so short-lived that no primordial hassium would survive to today. This does not rule out the possibility 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. Though 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.[120]

In 1963, Soviet geologist and physicist Viktor Cherdyntsev, who had previously claimed the existence of primordial curium-247,[121] 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);[122][z] this name comes from the name for the Silk Road and was explained as "coming from Kazakhstan" for it.[122] 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.[123]

Soviet physicist Vladimir Kulakov criticized Cherdyntsev's findings on the grounds that some of the properties Cherdyntsev claimed sergenium had, were inconsistent with then-current nuclear physics. The chief questions Kulakov raised 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 ~100 milligrams of sergenium.[123] 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, but unlikely.[124]

In 2006, Russian geologist Alexei Ivanov hypothesized that an isomer of 271Hs might have a half-life of ~(2.5±0.5)×108 years, which would explain the observation of alpha particles with energies of ~4.4 MeV in some samples of molybdenite and osmiridium.[125] 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.[125] 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 by 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.[125]

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.[12] 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 is 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.[126]

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.[119] 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.[127]

Predicted properties

[edit]

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.[128]

Very few properties of hassium or its compounds have been measured; this is due to its extremely limited and expensive production[129] 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

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Main article: Relativistic quantum chemistry
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 d3/2 and d5/2 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.[130] 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%.)[131]

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 p1/2 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.[132]

Since the s and p1/2 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.[133] 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.[4] The ionic radius (in oxidation state +8) of hassium is greater than that of osmium because of the relativistic expansion of the 6p3/2 orbitals, which are the outermost orbitals for an Hs8+ ion (although in practice such highly charged ions would be too polarized in chemical environments to have much reality).[134]

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.[135] 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).[135] 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):[136] for instance, of the six 6p electrons, two become 6p1/2 and four become 6p3/2. This is the spin–orbit splitting (also called subshell splitting or jj coupling).[137][aa] It is most visible with p electrons,[131] which do not play an important role in the chemistry of hassium,[9] but those for d and f electrons are within the same order of magnitude[131] (quantitatively, spin–orbit splitting in expressed in energy units, such as electronvolts).[135]

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.[139] Relativistic effects decrease the atomization energies of hassium compounds 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.[139]

Physical and atomic

[edit]

The previous members of group 8 have high melting points: Fe, 1538°C; Ru, 2334°C; Os, 3033°C. Like them, hassium is predicted to be a solid at room temperature[5] though its melting point has not been precisely calculated. Hassium should crystallize in the hexagonal close-packed structure (c/a = 1.59),[5] similarly to its lighter congener osmium.[5] Pure metallic hassium is calculated[5][140] to have a bulk modulus (resistance to uniform compression) of 450 GPa, comparable with that of diamond, 442 GPa.[141] 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.[6][7]

Hassium's atomic radius is expected to be ≈126 pm.[10] Due to 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 electron configuration [Rn] 5f14 6d5 7s1, analogous to that calculated for the Os2+ ion.[4] In chemical compounds, hassium is calculated to display bonding characteristic for a d-block element, whose bonding will be primarily executed by 6d3/2 and 6d5/2 orbitals; compared to the elements from the previous periods, 7s, 6p1/2, 6p3/2, and 7p1/2 orbitals should be more important.[142]

Chemical

[edit]
Stable oxidation states in group 8[143]
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.[144] Some of these properties were confirmed by gas-phase chemistry experiments.[145][146][147] 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.[143][148][149] This oxidation state is extremely rare: among stable elements, only ruthenium, osmium, and xenon are able to attain it in reasonably stable compounds.[ab] Hassium is expected to follow its congeners and have a stable +8 state,[146] but like them it should show lower stable oxidation states such as +6, +4, +3, and +2.[10][152] Hassium(IV) is expected to be more stable than hassium(VIII) in aqueous solution.[153] Hassium should be a rather noble metal.[154] The standard reduction potential for the Hs4+/Hs couple is expected to be 0.4 V.[10]

The group 8 elements show a distinctive oxide chemistry. All the lighter members have known or hypothetical tetroxides, MO4.[155] 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[156]—which results in the formation of the well-known oxyanion ferrate(VI), FeO2−
4.[157] Ruthenium tetroxide, RuO4, which is formed by oxidation of ruthenium(VI) in acid, readily undergoes reduction to ruthenate(VI), RuO2−
4.[158][159] Oxidation of ruthenium metal in air forms the dioxide, RuO2.[160] In contrast, osmium burns to form the stable tetroxide, OsO4,[161][162] which complexes with the hydroxide ion to form an osmium(VIII) -ate complex, [OsO4(OH)2]2−.[163] Therefore, hassium should behave as a heavier homologue of osmium by forming of a stable, very volatile tetroxide HsO4,[12][145][147][148][164] which undergoes complexation with hydroxide to form a hassate(VIII), [HsO4(OH)2]2−.[165] 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.[166]

Experimental chemistry

[edit]
Ball-and-stick model of ferrocene molecule
In ferrocene, the cyclopentadienyl rings are in a staggered conformation.
Ball-and-stick model of ruthenocene molecule
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.[167] Despite this selection for gas-phase chemical studies being clear from the beginning,[148] chemical characterization of hassium was considered a difficult task for a long time.[148] Although hassium was first synthesized in 1984, it was not until 1996 that a hassium isotope long-lived enough to allow chemical studies was synthesized. Unfortunately, this isotope, 269Hs, was synthesized indirectly from the decay of 277Cn;[148] not only are indirect synthesis methods not favourable for chemical studies,[168] but the reaction that produced the isotope 277Cn had a low yield—its cross section was only 1 pb[148]—and thus did not provide enough hassium atoms for a chemical investigation.[144] 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.[148] This yield was still around ten times lower than that for the reaction used for the chemical characterization of bohrium.[148] New techniques for irradiation, separation, and detection had to be introduced before hassium could be successfully characterized chemically.[148]

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.[148][155] In preparation for the chemical characterization of hassium, research focused on ruthenium and osmium rather than iron[148] because hassium was expected to be similar to ruthenium and osmium, as the predicted data on hassium closely matched that of those two.[169][170]

The first chemistry experiments were performed using gas thermochromatography in 2001, using the synthetic osmium radioisotopes 172, 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.[145][147][171]

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.[145][147][172] 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.[173] (The calculations that yielded a closer match to the experimental data came after the experiment, in 2008.)[166] 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.[172]

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):[165]

HsO
4 + 2 NaOH → Na
2[HsO
4(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.[174] 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.[152] 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.[152] As of 2021, there are no experimental reports of hassocene.[175]

Notes

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References

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Bibliography

[edit]
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Hassium

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Hassium (Hs) is a synthetic with 108, positioned in group 8, period 7 of the periodic table as a member of the transactinide series and the third element in the 6d series. It is highly radioactive, with no isotopes, and exists only in trace amounts produced in particle accelerators through reactions. As one of the heaviest elements whose chemical properties have been experimentally investigated, hassium provides key insights into the behavior of superheavy elements, where relativistic effects on electrons influence bonding and reactivity. Recent studies (as of 2025) on even heavier elements like continue to build on these foundational investigations. The element was first synthesized on March 14, 1984, by a team led by Peter Armbruster and Gottfried Münzenberg at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, West Germany (now Germany), using the SHIP velocity filter to separate fusion products from the collision of iron-58 ions with a lead-208 target, producing the isotope hassium-265 via the reaction 208Pb+58Fe→265Hs+n^{208}\mathrm{Pb} + ^{58}\mathrm{Fe} \to ^{265}\mathrm{Hs} + n. This marked the first confirmed observation of element 108, identified through a chain of alpha decays leading to known isotopes of lighter elements. Subsequent experiments at GSI confirmed additional isotopes and refined production methods, including the use of curium-248 targets with magnesium-26 projectiles to generate hassium-269 and hassium-270. The name "hassium" derives from Hassia, the Latin name for the German state of , honoring the location of its discovery at GSI in . Initially, there was controversy over naming, with a 1994 IUPAC commission proposing "hahnium" to honor , but after review of discovery credits, the International Union of Pure and Applied Chemistry (IUPAC) officially approved "hassium" with symbol Hs in 1997 as part of its recommendations for transfermium elements. Hassium has 13 known isotopes (including isomers), with mass numbers ranging from 263 to 277, all highly unstable and decaying primarily by alpha emission or ; the longest-lived confirmed isotope is ^{270}Hs, with a of about 4–8 seconds, though ^{271}Hs may have a longer of around 46 seconds (tentative as of 2025). Experimental chemical studies, limited to just a handful of atoms, demonstrate that hassium exhibits group 8 behavior akin to and , forming a volatile tetroxide (likely HsO_4) under oxidative conditions, with adsorption enthalpies on surfaces matching those of , thus validating periodic table extrapolations despite relativistic influences. No macroscopic quantities or practical applications exist due to its extreme instability and production challenges, but ongoing research at facilities like GSI and others explores its nuclear structure to probe the "" for superheavy elements.

Introduction

General characteristics

Hassium is a synthetic with the 108 and Hs. Positioned in group 8, period 7, and the d-block of the periodic table, it is classified as a transactinide element, extending the series of transition metals beyond . The of hassium is predicted to be [Rn] 5f14 6d6 7s2[\mathrm{Rn}] \, 5f^{14} \, 6d^6 \, 7s^2, reflecting relativistic effects that influence the electronic structure of superheavy elements. Hassium has 13 known isotopes ranging from mass numbers 263 to 277, including isomers; the most stable known isotope is 271Hs^{271}\mathrm{Hs}, with a half-life of about 46 seconds through alpha decay. Other isotopes, such as 270Hs^{270}\mathrm{Hs} with a half-life of about 7.6 seconds and 269Hs^{269}\mathrm{Hs} with about 13 seconds, decay similarly rapidly by alpha emission to seaborgium isotopes. Hassium's extreme rarity underscores its laboratory origins; more than 100 atoms have been synthesized as of 2024, primarily through heavy-ion fusion reactions at facilities like GSI Helmholtz Centre. This scarcity limits direct study but informs models of nuclear stability in the regime.

Role in superheavy element research

Hassium, with 108, serves as a critical benchmark in the exploration of , conventionally defined as those beyond Z=103, where nuclear forces compete intensely with repulsion. Its synthesis marked an early success in extending the periodic table into this regime, enabling experimental probes of nuclear behavior under extreme proton excess. Studies of hassium have significantly advanced understanding of nuclear shell effects, which arise from quantized arrangements that enhance binding energies at specific ". Theoretical calculations indicate a proton subshell closure near =108, contributing to increased stability against in neutron-rich isotopes around N=162. This closure, predicted by macroscopic-microscopic models, helps validate relativistic mean-field theories that describe shell structures in heavy nuclei. Hassium's research also informs predictions of the "," a hypothetical of enhanced longevity for nuclei centered around Z=114–126 and N=184, where multiple shell closures could yield half-lives extending to seconds or longer. By examining hassium isotopes approaching these neutron numbers, such as the doubly ^{270}Hs (Z=108, N=162), experiments test the onset of these stabilizing effects and refine extrapolations toward the island. Despite these insights, hassium investigations are hampered by its isotopes' brief half-lives, often milliseconds to seconds, and production yields limited to a few atoms per experiment due to fusion cross-sections below 1 picobarn. These constraints demand specialized facilities, including high-intensity heavy-ion accelerators like GSI's UNILAC, which provides beams of up to 10^{12} particles per second for target bombardment.

History and Discovery

Initial synthesis attempts

The initial synthesis of hassium (element 108), temporarily named unniloctium, was claimed in 1984 by a team at the Gesellschaft für Schwerionenforschung (GSI) in , , led by Gottfried Münzenberg and Peter Armbruster. Using the approach, they bombarded a ^{208}Pb target with ^{58}Fe projectiles, producing three atoms of the isotope ^{265}Hs via the reaction ^{208}Pb(^{58}Fe, n)^{265}Hs at an excitation energy of 18 ± 2 MeV. Identification relied on observing chains with an alpha energy of 10.36 ± 0.03 MeV and a of 1.8^{+1}{-0.7} ms, genetically linked to the known decays of ^{261}{106}Sg and ^{257}_{104}Rf. Subsequent experiments at GSI through 1986 expanded production to other isotopes, including ^{264}Hs and further instances of ^{265}Hs. The even-even isotope ^{264}Hs was synthesized in one observed event using ^{207}Pb(^{58}Fe, n)^{264}Hs, decaying primarily by alpha emission at 10.59 ± 0.05 MeV with a of 0.39^{+0.34}_{-0.14} ms, alongside a single event. These runs confirmed the alpha decay mode for both isotopes and provided initial insights into their decay properties, though yields remained limited to single atoms or small numbers. These pioneering efforts faced significant challenges due to the extremely low production cross-sections, measured at approximately 19^{+19}_{-10} picobarns for ^{265}Hs, which required extended periods and high-intensity beams to detect . Additionally, the necessity of single-atom-at-a-time demanded advanced techniques, such as the velocity filter SHIP for separating fusion products and real-time correlation of sequential alpha decays and implantations, amid short isotopic half-lives on the millisecond scale that limited observation windows.

Confirmation and arbitration

The synthesis of hassium was verified through a series of experiments in the , with the GSI team in , , providing independent confirmation in 1994 by repeating the cold fusion reaction ^{208}Pb(^{58}Fe, n)^{265}Hs to produce additional decay chains and observing genetic links from element 110 decays (e.g., ^{269}110 → ^{265}Hs), which corroborated its nuclear properties. This work built on GSI's initial 1984 report and helped establish the of hassium production at the facility. Meanwhile, claims from the (JINR) in , —in 1978, 1983, and 1984, based on the reaction ^{136}Xe + ^{136}Xe—failed to be independently reproduced, as subsequent attempts did not yield consistent genetic links in decay chains. Priority disputes over the discovery arose due to overlapping efforts between GSI and JINR, prompting . In 1997, a of the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP), known as the Transfermium , reviewed the evidence and concluded that GSI's 1984 experiment unequivocally demonstrated the synthesis of element 108 through the detection of three decay chains from ^{265}Hs. The commission acknowledged JINR's parallel investigations but deemed their data insufficient for discovery credit owing to limited detail and lack of confirmation. This decision formally recognized GSI's team, led by Peter Armbruster and Gottfried Münzenberg, as the discoverers. Subsequent experiments in the 2000s at other facilities further solidified hassium's status. At in , researchers produced the new ^{263}Hs in 2008 using the reaction ^{206}Pb(^{58}Fe, n)^{263}Hs, observing its into known isotopes and confirming decay patterns consistent with prior GSI results. Similarly, at (LBNL) in the United States, the ^{263}Hs was synthesized in 2009 via ^{208}Pb(^{56}Fe, n)^{263}Hs, with six correlated decay chains providing robust independent verification of hassium's nuclear characteristics. These efforts not only replicated key production methods but also expanded knowledge of hassium's isotopic landscape, removing any lingering doubts about element 108.

Naming process

The naming of hassium took place during the "," a period of international disputes over the discovery credits and nomenclature for superheavy elements beyond (atomic number 100), involving competing teams from the , , and . To resolve these conflicts and prevent nationalistic naming, the International Union of Pure and Applied Chemistry (IUPAC) prioritized honoring the institutions where the elements were undisputedly synthesized. The German team at the Gesellschaft für Schwerionenforschung (GSI Helmholtz Centre for Heavy Ion Research) in first expressed their intent to name element 108 "hassium" in 1992, deriving the name from the Latin "Hassia," referring to the state of , , home to their facility. This proposal aimed to recognize the location of the breakthrough synthesis achieved in 1984 by Peter Armbruster and Gottfried Münzenberg. In contrast, a 1994 provisional IUPAC recommendation suggested "hahnium" (symbol Hn) for element 108, honoring German chemist , as part of a set of contested names intended to balance international contributions but criticized for overlooking specific discoverers. Following arbitration by the IUPAC Transfermium Working Group, which confirmed the GSI team's priority in the discovery, the name "hassium" was formally proposed and accepted to avoid further disputes over names like "hahnium," which had been rejected for elements 105 and 108 to promote neutrality. The IUPAC Commission on Nomenclature of approved "hassium" (symbol Hs) at its August 1996 meeting in , aligning with traditions of geographic naming, and this was ratified in the official 1997 recommendations published in Pure and Applied Chemistry. The adoption emphasized the uncontested nature of the GSI synthesis and marked the end of naming controversies for element 108.

Synthesis Methods

Cold fusion reactions

Cold fusion reactions represent the principal approach for synthesizing hassium, leveraging the fusion of heavy target nuclei with medium-mass projectiles to form a compound nucleus at low excitation energies, thereby enhancing the probability of neutron evaporation over fission. These reactions exploit the doubly nature of the ^{208}Pb target to minimize the and promote compact fusion. The canonical reaction for hassium production is ^{208}\mathrm{Pb} + ^{58}\mathrm{Fe} \to ^{266}\mathrm{Hs}^{*} \to ^{265}\mathrm{Hs} + n (1n channel) or ^{264}\mathrm{Hs} + 2n (2n channel), with possible evaporation of up to a few additional neutrons in higher excitation scenarios. This process occurs at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt, Germany, where the Universal Linear Accelerator (UNILAC) delivers ^{58}Fe beams at energies of approximately 4.5-5.5 MeV/u to a ^{208}Pb target, optimized near the Bass interaction barrier to balance fusion probability and survival yield. The initial synthesis in 1984 produced three atoms of ^{265}Hs via this route, confirming the reaction's viability. The measured cross section for the 1n channel is approximately 20 pb, and for the 2n channel about 2.8 pb, underscoring the challenges in production due to competition from quasi-fission and incomplete fusion processes. These values were determined through excitation function measurements at GSI's SHIP velocity filter, which separates evaporation residues for further analysis.

Hot fusion reactions

Hot fusion reactions provide an alternative method for synthesizing hassium, using targets and lighter projectiles to access more neutron-rich isotopes, though with higher excitation energies leading to increased fission competition. These reactions typically yield cross sections on the order of a few picobarns and are performed at facilities like GSI and the (JINR). A key reaction is ^{248}\mathrm{Cm} + ^{26}\mathrm{Mg} \to ^{274}\mathrm{Hs}^{} \to ^{269}\mathrm{Hs} + 5n or ^{270}\mathrm{Hs} + 4n, optimized at beam energies around 8 MeV/u. This route was used in 2002 to produce seven atoms of ^{269}Hs and ^{270}Hs at GSI for the first chemical studies of hassium, with calculated cross sections of about 4 pb for ^{270}Hs and 6 pb for ^{269}Hs. More recently, in 2023, the isotope ^{272}Hs was synthesized at JINR's Superheavy Element Factory using the reaction ^{238}\mathrm{U} + ^{34}\mathrm{S} \to ^{272}\mathrm{Hs}^{}, demonstrating ongoing advancements in hot fusion techniques for elements.

Detection and decay analysis

Hassium atoms produced in fusion-evaporation reactions are isolated from the intense beam of projectiles and scattered target atoms using gas-filled recoil separators, such as the Separator for Heavy Ion Reaction Products (SHIP) at the GSI Helmholtz Centre for Heavy Ion Research in , . These devices exploit the magnetic rigidity of the heavy evaporation residues, which travel at velocities of about 5-10% of the , to separate them from lighter particles within microseconds after formation; the residues are then implanted into a position-sensitive detector array for subsequent . Detection of hassium relies primarily on the observation of correlated alpha decay chains, where the implantation of a single hassium nucleus is followed by sequential alpha emissions from the parent and daughter nuclides, allowing genetic linkage through time and position correlations in the detector. For instance, the isotope 269\Hs^{269}\Hs decays via alpha emission to 265\Sg^{265}\Sg with an energy of approximately 9.23 MeV, followed by further alpha decays to 261\Rf^{261}\Rf and subsequent daughters, often terminating in spontaneous fission. These alpha particles are measured using silicon detectors with high energy resolution (typically 20-30 keV full width at half maximum), enabling precise spectroscopy to distinguish the decay signatures from background events. Given the extremely low production cross-sections (on the order of picobarns), hassium synthesis yields only a few atoms per experiment, necessitating single-event analysis where each observed serves as an independent data point. Confirmation of hassium's identification requires the repeated observation of identical decay sequences across multiple independent experiments, ensuring statistical reliability and ruling out random coincidences, with background rates minimized to less than 10^{-4} events per hour per detector through shielding and systems.

Isotopes

Known isotopes and production

Hassium has 13 confirmed isotopes with mass numbers from 263 to 277, including up to six isomers, all produced artificially in heavy-ion fusion reactions due to the element's complete absence in . These isotopes are highly neutron-deficient and unstable, with production primarily occurring via methods at the GSI Helmholtz Centre for Heavy Ion Research, where beams of iron or isotopes are accelerated onto lead or targets, followed by neutron evaporation from the excited compound nucleus. For instance, the neutron-deficient ^{265}Hs is synthesized in the 1n evaporation channel of the reaction ^{208}Pb(^{58}Fe,1n)^{265}Hs, marking the initial discovery of the element. Other isotopes in this range, such as ^{263}Hs, ^{264}Hs, ^{266}Hs, ^{267}Hs, ^{268}Hs, ^{269}Hs, and ^{270}Hs, have been observed in similar channels (e.g., 3n to 0n evaporation), with cross-sections typically in the picobarn range, allowing for the detection of only a few atoms per experiment. As of 2025, over 130 hassium atoms have been produced across all isotopes, primarily through at GSI but with recent contributions from hot fusion reactions and at the (JINR), enabling detailed studies of their nuclear properties despite low yields. Heavier isotopes such as ^{271}Hs (via ^{248}Cm(^{26}Mg,3n)), ^{272}Hs, ^{273}Hs, ^{275}Hs, and ^{277}Hs have been identified in hot fusion experiments, for example, as in channels of ^{232}Th + ^{48}Ca at JINR's Superheavy Element Factory. Recent discoveries have extended the known isotopic range using these methods: in 2023, the ^{272}Hs was identified as a in the 4n channel of the reaction ^{232}Th + ^{48}Ca (yielding ^{276}Ds), with a measured production cross-section of approximately 250 pb and detection of three atoms. In 2024, the long-lived metastable ^{269m}Hs, with a of 2.8 s, was observed in the of ^{273}Ds produced via the same ^{232}Th + ^{48}Ca reaction, providing new insights into isomeric states in nuclei.

Decay properties and chains

All known isotopes of hassium undergo radioactive decay predominantly via alpha emission, reflecting the high fission barriers and repulsion in these nuclei; spontaneous fission branches are observed in several cases, but no or has been detected. Half-lives span from sub-millisecond for the lightest isotopes to nearly a minute for those near the neutron-rich deformed shell closure at N=162, indicating enhanced stability in the mid-mass region around A≈270–271. This trend aligns with theoretical predictions of a deformed doubly magic configuration at 270Hs, where increased suppresses decay rates. Representative examples illustrate these properties. The isotope ^{269}Hs, produced in fusion reactions such as ^{26}Mg + ^{248}Cm, has a half-life of 13^{+10}{-4} s and decays almost entirely by alpha emission with an energy of 9.34 MeV to ^{265}Sg. Similarly, ^{270}Hs exhibits a half-life of 7.6 \pm 4.9 s and Q\alpha = 9.07 MeV, decaying to ^{266}Sg, which frequently branches to spontaneous fission with a half-life of approximately 1.2 s. The most stable known isotope, ^{271}Hs, achieves a half-life of 46^{+56}{-16} s through alpha decay (Q\alpha \approx 9.48 MeV) to ^{267}Sg, with a spontaneous fission branch limited to less than 15%. Lighter isotopes, such as ^{263}Hs, are far less stable, with a half-life of 0.74 \pm 0.48 ms and higher alpha energy of 10.73 MeV leading to ^{259}Rf. Decay chains for hassium isotopes typically consist of 3–5 successive alpha decays, linking to known daughters in , , , and , before terminating in or long-lived actinides. For instance, a chain initiated by ^{268}Hs proceeds via ( 0.4 \pm 0.2 s, Q_\alpha = 9.62 MeV) to ^{264}Sg, which has a 30% SF branch ( 0.39^{+0.34}_{-0.14} s) or alpha decays further to ^{260}Rf (half-life 15 ms) and subsequent daughters. These chains, genetically correlated by time and position in detectors, provide critical confirmation of isotope assignments and reveal systematic decreases in alpha energies along the sequence, consistent with Q-value .

Occurrence

Terrestrial absence

Hassium exhibits complete terrestrial absence as a primordial element, owing to the extreme instability of its isotopes. All known isotopes of hassium have half-lives ranging from microseconds to about 22 seconds for the longest confirmed isotope, ^{270}Hs, precluding any survival from the formation of Earth approximately 4.54 billion years ago. A single unconfirmed observation suggests a spontaneous fission half-life of ~11 minutes for ^{277}Hs, but even this would result in over 10^{14} half-lives elapsed since planetary formation and thus total decay of any initial inventory. Geochemical searches for superheavy elements, including hassium, in diverse terrestrial samples such as ores, minerals, and heavy metal concentrates (e.g., mercury, thallium, lead, and bismuth) have established stringent upper limits on their abundance. These limits are below 10^{-12} g/g throughout the Earth's crust, corresponding to negligible atomic concentrations far below detectable levels. Investigations of potential production sites, including the Oklo natural nuclear reactor in Gabon—where uranium fission occurred about 2 billion years ago—have yielded no evidence of superheavy elements beyond atomic number 100, with hassium precluded by its rapid decay even if transiently formed. Similarly, cosmic ray spallation in the atmosphere and meteoritic material has not produced or preserved superheavy nuclei, with upper limits below 10^{-12} g/g in chondrites confirming non-detection. The consistent failure to detect hassium or related superheavy elements in natural samples underscores its exclusively synthetic origin, with no viable geochemical or nuclear pathways sustaining its presence on . These null results from tracking, alpha spectrometry, and mass analyses reinforce the understanding that hassium's instability renders it undetectable and absent in the terrestrial environment.

Astrophysical formation

Hassium, with 108, is theoretically produced in astrophysical environments through the rapid neutron-capture process (r-process), where seed nuclei rapidly capture neutrons in high-flux conditions, followed by beta decays to form heavier elements. This process is hypothesized to occur primarily in mergers, where the collision of compact objects ejects neutron-rich material, enabling the synthesis of superheavy nuclei up to Z ≈ 110. Explosive in core-collapse supernovae has also been proposed as a potential site, though simulations indicate it may be less efficient for elements beyond A ≈ 130 due to insufficient neutron availability. In neutron star mergers, such as the observed event , the dynamic ejecta and neutrino-driven winds provide the extreme conditions—a neutron-to-seed of approximately —necessary for r-process pathways to reach superheavy elements like hassium. Models based on the extended Thomas-Fermi-Strutinsky (ETFSI) framework predict that the r-process can extend to mass numbers A > 270, potentially forming hassium isotopes in the neutron-rich region of the nuclear chart. However, fission barriers and subsequent neutron-induced fission limit the survival of these nuclei, with only trace amounts expected in the heavy-element-rich ejecta. Despite these predictions, no direct observational for hassium or other superheavy elements from astrophysical sources has been found, as searches in cosmic rays, terrestrial samples, and stellar spectra yield no confirmed signatures. Theoretical models suggest that hassium isotopes near the —particularly those in the β-stability valley—could have half-lives up to ~10^8 years, allowing potential persistence in metal-poor stars or the early Solar System, though rapid decay dominates for most predicted yields. Ongoing simulations and observations of kilonovae continue to refine these estimates, emphasizing the role of nuclear data uncertainties in assessing feasibility.

Predicted Properties

Physical and atomic characteristics

Hassium's atomic structure is described by the electronic configuration [Rn] 5f^{14} 6d^6 7s^2, consistent with its position in group 8 of the periodic table. This configuration arises from relativistic stabilization of the 7s orbital and destabilization of the 6d orbitals, influencing the element's overall atomic properties. Theoretical calculations using relativistic (DFT) predict an of approximately 126 pm for hassium, reflecting the contraction due to relativistic effects in superheavy elements. The first is estimated at ~733 kJ/mol, lower than expected for lighter homologues due to these relativistic influences on binding. Macroscopic physical properties of hassium have been extrapolated from DFT models, yielding a predicted of ~41 g/cm³, making it one of the densest elements. Hassium is expected to exhibit refractory metal behavior similar to .

Chemical behavior

Hassium is predicted to belong to group 8 of the periodic table, exhibiting chemical properties akin to its homologues iron, ruthenium, and osmium, with a preference for high oxidation states due to its d-block electron configuration. Theoretical calculations indicate that the +8 oxidation state is expected to be the most stable for hassium, similar to osmium, where it readily forms the volatile tetroxide OsO₄; accordingly, hassium is predicted to form HsO₄, a highly volatile compound suitable for gas-phase studies. This +8 state aligns with experimental observations confirming hassium's reactivity toward oxygen, producing a tetroxide with adsorption behavior comparable to OsO₄. In group 8, the stability of the +8 in tetroxides increases down the group: iron forms unstable FeO₄²⁻ species, while and yield stable, volatile RuO₄ and OsO₄, respectively, and hassium is forecasted to follow this trend with a stable HsO₄ of volatility similar to OsO₄, with a sublimation of approximately 60 kJ/mol. Beyond oxides, theoretical models predict hassium's potential to form organometallic compounds, including hassocene, Hs(C₈H₈)₂, as a to in iron chemistry, reflecting similar bonding capabilities in group 8 elements.

Relativistic influences

In hassium, relativistic effects arise from the high nuclear charge (Z = 108), causing inner electrons to approach relativistic velocities, which in turn influences the valence orbitals through scalar relativistic contraction and spin-orbit . The 7s orbital experiences significant stabilization due to increased effective nuclear attraction and relativistic mass increase of the electrons, while the 6d orbitals are destabilized, with the 6d_{3/2} level splitting further from the 6d_{5/2} due to spin-orbit interaction. This orbital reconfiguration leads to enhanced s-d hybridization and overall contraction of the near the nucleus, resulting in shorter interatomic bonds in hassium compounds compared to lighter homologs like , and a predicted exceeding 40 g/cm³ for metallic hassium—substantially higher than non-relativistic estimates. For example, in hassium tetroxide (HsO₄), relativistic calculations predict bond lengths shortened by approximately 0.1 relative to non-relativistic models, strengthening the M-O bonds. The destabilization of the 6d orbitals reduces the in hassium compounds, lowering atomization energies and thereby enhancing volatility relative to non-relativistic predictions; this effect is particularly notable in the metallic state and oxides, where spin-orbit splitting weakens . In contrast, for HsO₄ specifically, relativistic strengthening of bonds slightly reduces volatility compared to , but the compound remains highly volatile overall, facilitating gas-phase transport in experiments. Predicted electronic transitions in gaseous HsO₄ are blue-shifted due to the contracted orbitals, suggesting a more intense yellow color than the pale yellow of OsO₄.

Experimental Chemistry

Early volatility studies

The first experimental investigation of hassium's chemical properties was conducted at the Gesellschaft für Schwerionenforschung (GSI) in , , in 2002, focusing on its volatility in the gas phase to assess its placement in group 8 of the periodic table. Hassium isotopes ^{269}Hs and ^{270}Hs were produced via the fusion reaction ^{248}Cm(^{26}Mg,5n/4n) using a rotating target wheel bombarded by a magnesium beam, yielding cross-sections on the order of picobarns and resulting in only seven detected hassium atoms over the course of the experiment. These hassium atoms were immediately oxidized in a helium-oxygen gas mixture (1% O_2) within the In situ Volatilization and On-line Detection (IVO) setup, forming a highly volatile species presumed to be hassium tetroxide (HsO_4), which was transported through a heated column maintained at 600°C. The volatile HsO_4 was then separated and directed to the On-Line Detector (), a thermochromatographic device where it adsorbed onto a silicon nitride surface along a from -20°C to -170°C, allowing measurement of adsorption behavior through subsequent detection. This process confirmed efficient transport of hassium as an oxide, with no evidence of less volatile dominating. The volatility of HsO_4 was found to be similar to that of its lighter homolog (OsO_4), as evidenced by comparable adsorption enthalpies: ΔH_{ads} = -46 ± 2 kJ/mol for HsO_4 versus -39 ± 1 kJ/mol for OsO_4, and deposition temperature maxima of -44 ± 6°C for HsO_4 compared to -82 ± 7°C for OsO_4. This alignment supported predictions of hassium's homology to , indicating group 8 congeners' characteristic formation of volatile tetroxides under oxidative conditions, though relativistic effects were anticipated to slightly reduce HsO_4's volatility relative to OsO_4. Due to the extremely low production rates (5-10 atoms total), no bulk compound was isolated, and the study served primarily as proof-of-principle for gas-phase chemistry.

Recent compound investigations

Following the foundational volatility studies of the early 2000s, advancements in hassium chemistry since 2010 have primarily involved preparatory and theoretical efforts rather than new experimental data, due to the element's low production rates of only a few atoms per experiment and half-lives on the order of seconds. A notable development was the adaptation of the (Short-lived Isotopes Studied in the SISAK system) continuous liquid-liquid extraction apparatus for potential studies of hassium tetroxide (HsO₄), aimed at probing its solubility and reactivity in organic solvents like diisobutyl ketone. This system was successfully tested with homologs, achieving high separation efficiencies (>95%) for OsO₄, but could not be applied to hassium owing to the limited number of available atoms—typically fewer than 10 per irradiation campaign at GSI. Between 2011 and 2016, theoretical calculations further validated hassium's chemical homology to , predicting similar volatility for oxides but highlighting relativistic effects that could weaken bonds in potential s. These predictions supported exploratory concepts for hydride formation using reactive gases like H₂ or HCl in gas-phase setups, analogous to methods used for , but no experimental attempts were executed for hassium during this period. No hassium atoms have been allocated for chemical investigations since 2016, as resources at GSI were redirected toward facility upgrades, including the new HELIAC linear accelerator, which was commissioned in 2024, to boost beam intensities for production. As a result, the field relies on the 2002 confirmation of HsO₄ as the only experimentally observed compound, with its adsorption enthalpy on measured at −46 ± 2 kJ/mol, comparable to OsO₄ (−39 ± 1 kJ/mol). As of 2025, no additional hassium compounds have been confirmed, underscoring persistent challenges in single-atom chemistry. Looking ahead, planned experiments at GSI's TASCA separator and collaborations with Dubna's Flerov Laboratory aim to employ matrix isolation techniques, embedding atoms in inert matrices (e.g., or at cryogenic temperatures) to prolong observation times and enable spectroscopic characterization of compounds like potential hydrides or halides. These methods, already demonstrated for lighter transactinides, promise to bridge the gap between theory and experiment for hassium and beyond, with initial tests expected post-HELIAC commissioning in the late .

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