Hassium

Hassium is a chemical element with the symbol Hs and the atomic number 108. Hassium is highly radioactive; the most stable known isotope, 269Hs, has a half-life of approximately 16 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 has been made only in laboratories in minuscule quantities; its possible occurrence in nature has been hypothesized but no natural hassium has been found so far.

Hassium, 108Hs
Hassium
Pronunciation/ˈhæsiəm/ (listen)[1] ​(HASS-ee-əm)
Mass number[269]
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
↓
(Uhb)
bohrium ← hassium → meitnerium
Atomic number (Z)108
Groupgroup 8
Periodperiod 7
Blockd-block
Element category  Transition metal
Electron configuration[Rn] 5f14 6d6 7s2[2]
Electrons per shell2, 8, 18, 32, 32, 14, 2
Physical properties
Phase at STPsolid (predicted)[3]
Density (near r.t.)41 g/cm3 (predicted)[4]
Atomic properties
Oxidation states(+2), (+3), (+4), (+6), +8[4][5][6] (brackets: prediction)
Ionization energies
  • 1st: 730 kJ/mol
  • 2nd: 1760 kJ/mol
  • 3rd: 2830 kJ/mol
  • (more) (predicted)[7]
Atomic radiusempirical: 126 pm (estimated)[4]
Covalent radius134 pm (estimated)[8]
Other properties
Natural occurrencesynthetic
Crystal structure ​hexagonal close-packed (hcp)

(predicted)[3]
CAS Number54037-57-9
History
Namingafter Hassia, Latin for Hesse, Germany, where it was discovered[9]
DiscoveryGesellschaft für Schwerionenforschung (1984)
Main isotopes of hassium
Iso­tope Abun­dance Half-life (t1/2) Decay mode Pro­duct
269Hs syn 16 s α 265Sg
270Hs syn 9 s α 266Sg
271Hs syn 10 s α 267Sg

The first attempts to synthesize element 108 were made in two different experiments at the Joint Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast, Russian SFSR, Soviet Union, in 1978. More attempts were made at the same venue in 1983 and then in 1984; the latter resulted in a claim that element 108 had been produced. Later in 1984, 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 the report from Darmstadt was more conclusive on its own and the major credit was assigned to the German scientists. Following the recognition, GSI formally announced they wished to name the element hassium after the German state of Hesse home to the facility. Two rulings from the International Union of Pure and Applied Chemistry (IUPAC) followed in 1994 and 1995 to establish permanent names of a series of elements that included element 108; both were met with rejection by competing scientists and scrapped. A third ruling, published in 1997, named the element hassium per the original suggestion by GSI; it was accepted by the scientific community and the name was established as final.

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, in group 8, reacting readily with oxygen to form a volatile tetroxide. The chemical properties of hassium have only been partly characterized but they compare well with the chemistry of the other group 8 elements.

Introduction
See also: Superheavy element § Introduction
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.
External video
Visualization of unsuccessful nuclear fusion, based on calculations by the Australian National University[10]

The heaviest[lower-alpha 1] atomic nuclei are created in nuclear reactions that combines two other nuclei of unequal size[lower-alpha 2] into one; roughly, the more unequal the two nuclei in terms of mass, the greater the possibility that the two react.[16] 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.[17] Coming close alone is not enough for two nuclei to fuse: when two nuclei approach each other, they usually remain together for approximately 10−20 seconds and then part ways (not necessarily in the same composition as before the reaction) rather than form a single nucleus.[17][18] If fusion does occur, the temporary merger—termed a compound nucleus—is an excited state. To lose its excitation energy and reach a more stable state, a compound nucleus either fissions or ejects one or several neutrons,[lower-alpha 3] which carry away the energy. This occurs in approximately 10−16 seconds after the initial collision.[19][lower-alpha 4]

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.[22] In the separator, the newly produced nucleus is separated from other nuclides (that of the original beam and any other reaction products)[lower-alpha 5] 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.[22] The transfer takes about 10−6 seconds; in order to be detected, the nucleus must survive this long.[25] The nucleus is recorded again once its decay is registered, and the location, the energy, and the time of the decay are measured.[22]

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, as it has unlimited range.[26] Nuclei of the heaviest elements are thus theoretically predicted[27] and have so far been observed[28] to primarily decay via decay modes that are caused by such repulsion: alpha decay and spontaneous fission;[lower-alpha 6] these modes are predominant for nuclei of superheavy elements. 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.[lower-alpha 7] Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.[lower-alpha 8]

The information available to physicists aiming to synthesize one of the heaviest elements 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.[lower-alpha 9]

Discovery

Discoverers of transuranium elements prior to element 108
See also: Timeline of chemical element discoveries
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]

The chemical element with the highest atomic number that exists in nature in significant quantities is uranium. The atomic number is the number of protons in an atomic nucleus. Such a number constitutes an exhaustive definition of an element, so an element can be referred to by its atomic number; for example, uranium is element 92. All elements with atomic number above 92 were discovered by synthesis rather than by observation in nature. The first such element, element 93, later named neptunium, was discovered in 1940 at the University of California in Berkeley, California, United States.[41][lower-alpha 10] Elements through 101 were discovered at this university's Radiation Laboratory (RL; later named Lawrence Berkeley Laboratory, LBL, and now Lawrence Berkeley National Laboratory, LBNL). Starting with element 102, another major facility emerged that claimed discoveries of new elements: the Joint Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast, Russian SFSR, Soviet Union, which first reported synthesis of a new element in 1964[44] (the scientific team reported their first attempt in 1956 from the Institute of Atomic Energy of the Academy of Sciences of the USSR in Moscow).[35][45] Another major venue—Gesellschaft für Schwerionenforschung (GSI; Institute for Heavy Ion Research) in Darmstadt, Hesse, West Germany—first reported synthesis of a new element (element 107) in 1981.[46] These facilities often claimed discoveries of new 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.[47]

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;[lower-alpha 11] therefore, the necessary ejection of neutrons results in final products with typically have shorter lifetimes. As such, light beams (6 to 10 protons) only allowed synthesis of elements up to 106.[49]

To advance to heavier elements, Soviet physicist Yuri Oganessian at JINR hypothesized a different mechanism, in which the bombarded nucleus would be lead-208, which has magic numbers of protons and neutrons, or one close to it.[50] The magic number of protons and/or neutrons gives the nuclide additional stability, which requires more energy for an external nucleus to penetrate it.[34] More equal atomic numbers of the reacting nuclei result in greater electrostatic repulsion between them, but the greater mass excess of the target nucleus balances it.[50] This leaves less excitation energy for the newly created compound nucleus, which necessitates fewer neutron ejections to reach a stable state.[34] Because of this energy difference, the former mechanism became known as "hot fusion" and the latter as "cold fusion".

Cold fusion was first declared initially successful in 1974 at JINR, when it was tested for synthesis of the yet undiscovered element 106.[34] These new nuclei were projected to decay via spontaneous fission. The physicists at JINR concluded those were not seen before because no then-known fissioning nucleus showed similar parameters of fission and because changing either of the two nuclei in the reactions negated the seen effects. Physicists at LBL also expressed great interest in the new technique.[34] 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."[34]

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,[lower-alpha 12] 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.[51] 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.[52]

GSI's linear particle accelerator UNILAC, where hassium was discovered[53] and where its chemistry was first observed[54]

In 1983, new experiments were performed at JINR.[55] The experiments probably resulted in the synthesis of element 108; bismuth (209
83Bi
) was bombarded with manganese (55
25Mn
) to obtain 263108, lead (207
82Pb
, 208
82Pb
) was bombarded with iron (58
26Fe
) to obtain 264108, and californium (249
98Cf
) was bombarded with neon (22
10Ne
) to obtain 270108.[9] These experiments were not claimed as a discovery and Oganessian announced them in a conference rather than in a written report.[55]

In 1984, JINR researchers in Dubna performed experiments set up identically to the previous ones; they bombarded bismuth and lead targets with ions of lighter elements manganese and iron, respectively. Twenty-one spontaneous fission events were recorded; these were assigned to 264108.[56]

Later in 1984, a research team led by Peter Armbruster and Gottfried Münzenberg at GSI attempted to create element 108. The team bombarded a lead (208
82Pb
) target with accelerated iron (58
26Fe
) nuclei.[15] 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.[57] 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.[57]

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.[58]

According to the report, the 1984 works from JINR and GSI simultaneously and independently established synthesis of element 108. Of the two 1984 works, the one from GSI was said to be sufficient as a discovery on its own. The JINR work, which preceded the GSI one, "very probably" displayed synthesis of element 108 but that was determined in retrospect given the work from Darmstadt; it 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.[56] 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; GSI also proposed a name for element 108 that had been officially presented at the facility three weeks earlier.[38]

Naming
See also: Transfermium Wars

Historically, a newly discovered element was named by its discoverer. The first regulation came in 1947, when IUPAC decided that naming required regulation in case there are conflicting names.[59][lower-alpha 13] 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.[59] The discoverers held the right to name an element, but their name would be a subject to approval by IUPAC.[59] The Commission of Atomic Weights distanced itself from element naming in most cases.[59]

According to Mendeleev's nomenclature for unnamed and undiscovered elements, hassium should be known as "eka-osmium". In 1979, IUPAC published recommendations according to which the element was to be called "unniloctium" and assigned the corresponding symbol of "Uno",[60] a systematic element name as a placeholder until the element was discovered and the discovery then confirmed, and a permanent name was decided. Although these recommendations were widely followed in the chemical community, most scientists in the field ignored them.[61] They either called it "element 108", with the symbols E108, (108) or 108, or used the proposed name "hassium".[62]

Hessentag festival in 2011, celebrating the culture 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.[59]

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".[63] It is derived from the Latin name Hassia for the German state of Hesse where the institute is located.[9][38] This name was proposed to IUPAC in a written response to their ruling on priority of discovery claims of elements, signed 29 September 1992.[38]

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;[64] IUPAC commented that they felt the German suggestion was obscure.[65] GSI protested, saying this proposal contradicted the long-standing convention of giving the discoverer the right to suggest a name;[66] the American Chemical Society supported GSI.[9] 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.[67] 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.[68] The final compromise was reached in 1996 and published in 1997; element 108 was named hassium (Hs).[69] 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.[39][lower-alpha 14]

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[lower-alpha 15] was established by Berkeley. We wanted to do it for Europe."[71] 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."[71]

Isotopes
Main article: Isotopes of hassium
List of hassium isotopes
Isotope Half-life[lower-alpha 16] Decay
mode		 Discovery
year[72]		 Discovery
reaction[73]
Value Ref
263Hs760 µs[72]α, SF2009208Pb(56Fe,n)
264Hs540 µs[72]α, SF1986207Pb(58Fe,n)
265Hs1.96 ms[72]α, SF1984208Pb(58Fe,n)
265mHs360 µs[72]α1995208Pb(56Fe,n)
266Hs3.02 ms[72]α, SF2001270Ds(—,α)
266mHs280 ms[72]α2011270mDs(—,α)
267Hs55 ms[74]α1995238U(34S,5n)
267mHs990 µs[74]α2004238U(34S,5n)
268Hs1.42 s[74]α2010238U(34S,4n)
269Hs16 s[74]α1996277Cn(—,2α)
270Hs9 s[74]α2003248Cm(26Mg,4n)
271Hs10 s[lower-alpha 17][75]α2008248Cm(26Mg,3n)
273Hs510 ms[76] α2010285Fl(—,3α)
275Hs200 ms[77]α2004287Fl(—,3α)
277Hs11 ms[78]α2010289Fl(—,3α)
277mHs1.8 min[lower-alpha 18][78]SF2012293mLv(—,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. Twelve isotopes with mass numbers ranging from 263 to 277 (with the exceptions of 272, 274, and 276) have been reported, four of which—hassium-265, -267, -269, and -277—have known metastable states,[79] although that of hassium-277 is unconfirmed.[80] 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.[79] The lightest isotopes, which usually have shorter half-lives, were synthesized by direct fusion between two lighter nuclei and as decay products. The heaviest isotope produced by direct fusion is 271Hs; heavier isotopes have only been observed as decay products of elements with larger atomic numbers.

Superheavy nuclei are deformed as opposed to regular spherical nuclei. Until the 1960s, it was thought that deformation made these nuclei incapable of forming a nuclear structure and thus very unstable.[81] 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.[82][83] 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.[82][83] 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.[83] 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.[83] 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 being a magic number for protons in deformed nuclei and 162 being a magic neutron number.[84] Experiments on lighter superheavy nuclei,[85] as well as those closer to the expected island,[81] 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[86] and N = 168–170, which is between the predicted neutron shell closures at N = 162 for deformed nuclei and N = 184 for spherical nuclei.[87] Nuclides within this region are predicted to have low fission barrier heights, resulting in short partial half-lives toward spontaneous fission. This prediction is supported by the observed 11 millisecond half-life of 277Hs and that of the neighbouring isobar 277Mt because the expected hindrance factors from the odd nucleon were shown to be much lower than expected. The measured half-lives are even lower than those predicted for the even–even 276Hs and 278Ds, which suggests a gap in stability away from the shell closures and perhaps a weakening of the shell closures in this region.[87]

In 1991, Polish physicists Zygmunt Patyk and Adam Sobiczewski predicted[88] that 108 is a proton magic number for deformed nuclei—nuclei that are far from spherical—and 162 is a neutron magic number for deformed nuclei. This means such nuclei are permanently deformed in their ground state but have high, narrow fission barriers to further deformation and hence relatively long life-times toward spontaneous fission.[89][90] Computational prospects for shell stabilization for 270Hs made it a promising candidate for a deformed doubly magic nucleus.[91] 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.[lower-alpha 19]

Natural occurrence
Molybdenite

Hassium is not known to occur naturally on Earth; the half-lives of all of its known isotopes are short enough that no primordial hassium would have survived to the present day. This does not rule out the possibility of the existence of unknown, longer-lived isotopes or nuclear isomers, some of which could still exist in trace quantities if they are long-lived enough. As early as 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.[94][lower-alpha 20]

In 1963, Soviet geologist and physicist Viktor Cherdyntsev, who had previously claimed the existence of primordial curium-247,[95] 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);[96][lower-alpha 21] this name takes its origin from the name for the Silk Road and was explained as "coming from Kazakhstan" for it.[96] 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.[97]

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 100 milligrams of sergenium.[97] 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.[98]

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.[99] 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.[99] 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.[99]

In 2007, the Joint Institute for Nuclear Research started a search for natural hassium; this was done underground to avoid interference and false positives from cosmic rays. In 2010, Oganessian reaffirmed that the search was still underway.[100] No results have been released.

Atomic nuclei show additional stability if they have specific numbers of protons or neutrons called magic numbers. The nuclear shell model describes this as a consequence of closed "shells" of protons and neutrons, whose closures create especially stable configurations. 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". There are various predictions for higher magic numbers; 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", which is theorized to contain longer-lived superheavy nuclides in the vicinity of Z = 110–114 and the predicted magic neutron number N = 184. 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.[101][102] As such, it was considered as a candidate to exist in nature. This nucleus, however, is predicted to be very unstable toward beta decay and any beta-stable isotopes of hassium such as 286Hs would be too unstable in the other decay channels to be observed in nature.[102] Indeed, a 2012 search for 292Hs in nature along with its homologue osmium was unsuccessful, setting an upper limit to its abundance at 3×10−15 grams of hassium per gram of osmium.[103]

Predicted properties

Various calculations suggest that 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 6d metals, a few deviations are expected to arise from relativistic effects.[104]

Very few properties of hassium or its compounds have been measured; this is due to its extremely limited and expensive production[16] 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
Main article: Relativistic quantum chemistry
Energy levels of outermost orbitals of hassium and osmium atoms in electronvolts, with and without taking in account relativistic effects. 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 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.[105] There are three main effects: the direct relativistic effect, the indirect relativistic effect, and the 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%.)[106]

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 its mass. This in turn increases the gravitational attraction between the electron and the nucleus, which 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.[107]

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.[108] The combination of the two leads to that 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.[2] 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.[109]

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.[110] 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).[110] Spin of an electron interacts with its orbit, which 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):[111] for instance, of the six 6p electrons, two become 6p1/2 and four become 6p3/2. This is the spin–orbit splitting (sometimes also referred to as subshell splitting or jj coupling).[112] It is most visible with p electrons,[106] which do not play an important role in the chemistry of hassium,[7] but those for d and f electrons are within the same order of magnitude[106] (quantitatively, spin–orbit splitting in expressed in energy units, such as electronvolts).[110]

Densities of the group 4-12 elements, including predictions for period 7. Hassium is predicted to be nearly twice as dense as the densest measured elements, osmium and iridium.

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.[113] 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.[113]

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[3] although its melting point has not been precisely calculated. Hassium should crystallize in the hexagonal close-packed structure (c/a = 1.59),[3] similarly to its lighter congener osmium.[3] Pure metallic hassium is calculated[3][114] to have a bulk modulus (resistance to uniform compression) of 450 GPa, comparable with that of diamond, 442 GPa.[115] Hassium is expected to have a bulk density of 41 g/cm3[116] at standard pressure and temperature, the highest of any of the 118 known elements and nearly twice the highest density of an element observed to this day at 22.6 g/cm3.[lower-alpha 22]

The atomic radius of hassium is expected to be around 126 pm.[4] 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.[2] 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.[120]

Chemical
Stable oxidation states in group 8[121]
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.[122] Some of these properties were confirmed by gas-phase chemistry experiments.[123][124][125] 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.[121][126][127] This oxidation state is extremely rare: among stable elements, only ruthenium, osmium, and xenon are able to attain it in reasonably stable compounds.[lower-alpha 23] Hassium is expected to follow its congeners and have a stable +8 state,[124] but like them it should show lower stable oxidation states such as +6, +4, +3, and +2.[4][6] Hassium(IV) is expected to be more stable than hassium(VIII) in aqueous solution.[130] Hassium should be a rather noble metal;[131] the standard reduction potential for the Hs4+/Hs couple is expected to be 0.4 V,[4] which is more than that for the Cu2+/Cu couple of copper (0.3419 V), but less than that for the Ru2+/Ru couple for ruthenium (0.455 V).[132]

The group 8 elements show a distinctive oxide chemistry. All of the lighter members have known or hypothetical tetroxides, MO4.[133] 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[134]—which results in the formation of the well-known oxyanion ferrate(VI), FeO2−
4.[135] Ruthenium tetroxide, RuO4, which is formed by oxidation of ruthenium(VI) in acid, readily undergoes reduction to ruthenate(VI), RuO2−
4.[136][137] Oxidation of ruthenium metal in air forms the dioxide, RuO2.[138] In contrast, osmium burns to form the stable tetroxide, OsO4,[139][140] which complexes with the hydroxide ion to form an osmium(VIII) -ate complex, [OsO4(OH)2]2−.[141] Therefore, eka-osmium properties for hassium should be demonstrated by the formation of a stable, very volatile tetroxide HsO4,[9][123][125][126][142] which undergoes complexation with hydroxide to form a hassate(VIII), [HsO4(OH)2]2−.[143] Ruthenium tetroxide and osmium tetroxide are both volatile due to their symmetrical tetrahedral molecular geometry and because they are charge-neutral; hassium tetroxide should similarly be a very volatile solid. The trend of the volatilities of the group 8 tetroxides is known to be RuO4 < OsO4 > HsO4, which confirms the calculated results. In particular, the calculated enthalpies of adsorption—the energy required for the adhesion of atoms, molecules, or ions from a gas, liquid, or dissolved solid to a surface—of HsO4, −(45.4 ± 1) kJ/mol on quartz, agrees very well with the experimental value of −(46 ± 2) kJ/mol.[144]

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 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.[145] Despite this selection for gas-phase chemical studies being clear from the beginning,[126] chemical characterization of hassium was considered a difficult task for a long time.[126] 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;[126] not only are indirect synthesis methods not favourable for chemical studies,[146] but the reaction that produced the isotope 277Cn had a low yield—its cross section was only 1 pb[126]—and thus did not provide enough hassium atoms for a chemical investigation.[122] 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.[126] This yield was still around ten times lower than that for the reaction used for the chemical characterization of bohrium.[126] New techniques for irradiation, separation, and detection had to be introduced before hassium could be successfully characterized chemically.[126]

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.[126][133] In preparation for the chemical characterization of hassium, research focused on ruthenium and osmium rather than iron[126] because hassium was expected to be similar to ruthenium and osmium, as the predicted data on hassium closely matched that of those two.[147][148]

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.[123][125][149]

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.[123][125][150] 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.[151] (The calculations that yielded a closer match to the experimental data came after the experiment, in 2008.)[144] It is possible hassium tetroxide interacts differently with silicon nitride than with silicon dioxide, the chemicals used for the detector; further research is required, including more accurate measurements of the nuclear properties of 269Hs and comparisons with RuO4 in addition to OsO4.[150]

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):[143]

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.[152] 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.[6] 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.[6] As of 2019, there are no experimental reports of hassocene.[153]

Notes
  1. In nuclear physics, an element is called heavy if its atomic number is high; lead (element 82) is one example of such a heavy element. The term "superheavy elements" typically refers to elements with atomic number greater than 103 (although there are other definitions, such as atomic number greater than 100[11] or 112;[12] sometimes, the term is presented an equivalent to the term "transactinide", which puts an upper limit before the beginning of the hypothetical superactinide series).[13] Terms "heavy isotopes" (of a given element) and "heavy nuclei" mean what could be understood in the common language—isotopes of high mass (for the given element) and nuclei of high mass, respectively.
  2. In 2009, a team at JINR led by Oganessian published results of their attempt to create hassium in a symmetric 136Xe + 136Xe reaction. They failed to observe a single atom in such a reaction, putting the upper limit on the cross section, the measure of probability of a nuclear reaction, as 2.5 pb.[14] In comparison, the reaction that resulted in hassium discovery, 208Pb + 58Fe, had a cross section of ~20 pb (more specifically, 19+19
    −11     pb), as estimated by the discoverers.[15]
  3. The greater the excitation energy, the more neutrons are ejected. If the excitation energy is lower than energy binding each neutron to the rest of the nucleus, neutrons are not emitted; instead, the compound nucleus de-excites by emitting a gamma ray.[19]
  4. The definition by the IUPAC/IUPAP Joint Working Party 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 its outer electrons and thus display its chemical properties.[20] This figure also marks the generally accepted upper limit for lifetime of a compound nucleus.[21]
  5. This separation is based on that the resulting nuclei move past the target more slowly then the unreacted beam nuclei. The separator contains electric and magnetic fields whose effects on a moving particle cancel out for a specific velocity of a particle.[23] Such separation can also be aided by a time-of-flight measurement and a recoil energy measurement; a combination of the two may allow to estimate the mass of a nucleus.[24]
  6. Not all decay modes are caused by electrostatic repulsion. For example, beta decay is caused by the weak interaction.[29]
  7. Since mass of a nucleus is not measured directly but is rather calculated from that of another nucleus, such measurement is called indirect. Direct measurements are also possible, but for the most part they have remained unavailable for heaviest nuclei.[30] The first direct measurement of mass of a superheavy nucleus was reported in 2018 at LBNL.[31] Mass was determined from the location of a nucleus after the transfer (the location helps determine its trajectory, which is linked to the mass-to-charge ratio of the nucleus, since the transfer was done in presence of a magnet).[32]
  8. Spontaneous fission was discovered by Soviet physicist Georgy Flerov,[33] a leading scientist at JINR, and thus it was a "hobbyhorse" for the facility.[34] In contrast, the LBL scientists believed fission information was not sufficient for a claim of synthesis of an element. They believed spontaneous fission had not been studied enough to use it for identification of a new element, since there was a difficulty of establishing that a compound nucleus had only ejected neutrons and not charged particles like protons or alpha particles.[21] They thus preferred to link new isotopes to the already known ones by successive alpha decays.[33]
  9. For instance, element 102 was mistakenly identified in 1957 at the Nobel Institute of Physics in Stockholm, Stockholm County, Sweden.[35] There were no earlier definitive claims of creation of this element, and the element was assigned a name by its Swedish, American, and British discoverers, nobelium. It was later shown that the identification was incorrect.[36] The following year, RL was unable to reproduce the Swedish results and announced instead their synthesis of the element; that claim was also disproved later.[36] JINR insisted that they were the first to create the element and suggested a name of their own for the new element, joliotium;[37] the Soviet name was also not accepted (JINR later referred to the naming of element 102 as "hasty").[38] The name "nobelium" remained unchanged on account of its widespread usage.[39]
  10. Neptunium was discovered after bombardment of uranium with singular neutrons; the reaction did not produce neptunium, but it created an isotope of uranium that decayed to it. This reaction does not necessitate formation of a temporary compound nucleus and is referred to as a direct reaction as opposed to the previously described compound nucleus reactions. The two are differentiated by the number of interactions: it is equal to one for direct reactions and is greater (usually, significantly) than one for compound nucleus reactions.[42] Such production is possible for all elements up to fermium.[43]
  11. Generally, heavier nuclei require more neutrons because as the number of protons increases, so does electrostatic repulsion between them. This repulsion is balanced by the strong interaction between quarks within nucleons; it is enough to hold the quarks together in a nucleon together and some of it is left for binding of different nucleons. The more nucleons there are in a nucleus, the more energy there is for binding the nucleons.[48] However, having too many neutrons per proton results in beta decay.
  12. The superscript number to the left of a chemical symbol refers to the mass of a given nuclide; for instance, 48Ca is the notation for calcium-48. In superheavy element research, elements that have not been assigned a name and a symbol, are often referred to by their atomic numbers in lieu of symbols; if a symbol has been assigned and the number is to be displayed, it is written in subscript to the left of the symbol. 270108 would be 270Hs or 270
    108    Hs
     in modern nomenclature (or hassium-270 if spelled out).
  13. This was intended not only to resolve any future conflicts, but also a number of ones that existed back then: beryllium/glucinium, niobium/columbium, lutecium/cassiopeium, hafnium/celtium, tungsten/wolfram, and protoactinium/brevium.
  14. American physicist Glenn T. Seaborg suggested that name for element 110 on behalf of LBNL in November 1997 after IUPAC surveyed the three main collaborations (GSI, JINR/LLNL, and LBNL) how they think the element should be named.[70]
  15. Similarly, there are names of ruthenium, moscovium, and dubnium for JINR. The only element discovered by RIKEN in Wakō, Saitama Prefecture, Japan, is named nihonium after a Japanese name of Japan.
  16. Different sources give different values for half-lives; the most recently published values are listed.
  17. Half-life not precisely measured
  18. Isotope unconfirmed
  19. In particular, the low decay energy for 270Hs matches calculations.[90] The conclusion for 269Hs was made after its decay data was compared to that of 273Ds; the decay of the latter into the former has an energy sufficiently greater than the decay of the former (11.2 MeV and 9.2 MeV, respectively). The great value of the former energy was explained as a right-to-left crossing of N = 162 (273Ds has 163 neutrons and 269Hs has 161).[92] A similar observation and conclusion were made after measurement of decay energy of 271Hs and 267Sg.[93]
  20. The symbol Z refers to the atomic number—number of protons in an atomic nucleus. The symbol N refers to the neutron number—number of neutrons in a nucleus. The symbol A refers to the mass number—number of neutrons and protons in a nucleus combined.
  21. At the time, this symbol had not yet been taken by seaborgium.
  22. The two densest elements whose densities have been measured so far are osmium and iridium, both from the sixth period. There have been different records on which is denser; different texts published different results.[117][118] More precise measurements from the 1990s onward established that osmium was slightly denser at 22.589 ± 0.005 g/cm3 at the standard conditions (iridium may be the denser one at high pressures).[119]
  23. While iridium is known to show a +8 state in iridium tetroxide, as well as a unique +9 state in the iridium tetroxide cation IrO+
    4    , the former is only known in matrix isolation and the latter in the gas phase, and no iridium compounds in such high oxidation states have been synthesized in macroscopic amounts.[128][129]

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