Hassium

Hassium, ₁₀₈Hs

Hassium
Pronunciation	/ˈhæsiəm/ ​(HASS-ee-əm)
Mass number	[271] (data not decisive)
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
Group	group 8
Period	period 7
Block	d-block
Electron configuration	[Rn] 5f14 6d6 7s2
Electrons per shell	2, 8, 18, 32, 32, 14, 2
Physical properties
Phase at STP	solid (predicted)
Density (near r.t.)	27–29 g/cm3 (predicted)
Atomic properties
Oxidation states	common: (none) (+3), (+4), (+6), (+8)
Ionization energies	1st: 730 kJ/mol 2nd: 1760 kJ/mol 3rd: 2830 kJ/mol (more) (predicted)
Atomic radius	empirical: 126 pm (estimated)
Covalent radius	134 pm (estimated)
Other properties
Natural occurrence	synthetic
Crystal structure	​hexagonal close-packed (hcp) (predicted)
CAS Number	54037-57-9
History
Naming	after Hassia, Latin for Hesse, Germany, where it was discovered
Discovery	Gesellschaft für Schwerionenforschung (1984)
Isotopes of hassium
Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct 269Hs synth 12 s α 265Sg 270Hs synth 7.6 s α 266Sg 271Hs synth 46 s α 267Sg 277mHs synth 130 s SF –

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 only been produced in a laboratory, in very small quantities. Natural occurrences of the element have been hypothesised, but none has ever been found.

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, 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 to the heaviest elements

"Transactinide element" redirects here. Not to be confused with Transuranium element.

Superheavy elements
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

Z ≥ 104 (Rf)

Superheavy elements, also known as transactinide elements, transactinides, or super-heavy elements, or superheavies for short, are the chemical elements with atomic number greater than 104. The superheavy elements are those beyond the actinides in the periodic table; the last actinide is lawrencium (atomic number 103). By definition, superheavy elements are also transuranium elements, i.e., having atomic numbers greater than that of uranium (92). Depending on the definition of group 3 adopted by authors, lawrencium may also be included to complete the 6d series.

Glenn T. Seaborg first proposed the actinide concept, which led to the acceptance of the actinide series. He also proposed a transactinide series ranging from element 104 to 121 and a superactinide series approximately spanning elements 122 to 153 (though more recent work suggests the end of the superactinide series to occur at element 157 instead). The transactinide seaborgium was named in his honor.

Superheavies are radioactive and have only been obtained synthetically in laboratories. No macroscopic sample of any of these elements has ever been produced. Superheavies are all named after physicists and chemists or important locations involved in the synthesis of the elements.

IUPAC defines an element to exist if its lifetime is longer than 10⁻¹⁴ seconds, which is the time it takes for the atom to form an electron cloud.

The known superheavies form part of the 6d and 7p series in the periodic table. Except for rutherfordium and dubnium (and lawrencium if it is included), even the longest-lived known isotopes of superheavies have half-lives of minutes or less. The element naming controversy involved elements 102–109. Some of these elements thus used systematic names for many years after their discovery was confirmed. (Usually the systematic names are replaced with permanent names proposed by the discoverers relatively soon after a discovery has been confirmed.)

Introduction

Synthesis of superheavy nuclei

See also: Nucleosynthesis and Nuclear 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 atomic nucleus is created in a nuclear reaction that combines two other nuclei of unequal size into one; roughly, the more unequal the two nuclei in terms of mass, the greater the possibility that the two react. 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. 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.

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. This happens because during the attempted formation of a single nucleus, electrostatic repulsion tears apart the nucleus that is being formed. 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. 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.

External videos
Visualization of unsuccessful nuclear fusion, based on calculations from the Australian National University

The resulting merger is an excited state—termed a compound nucleus—and thus it is very unstable. To reach a more stable state, the temporary merger may fission without formation of a more stable nucleus. 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. 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.

Decay and detection

See also: Gaseous ionization detector

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

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. 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. Superheavy nuclei are thus theoretically predicted and have so far been observed to predominantly decay via decay modes that are caused by such repulsion: alpha decay and spontaneous fission. Almost all alpha emitters have over 210 nucleons, and the lightest nuclide primarily undergoing spontaneous fission has 238. In both decay modes, nuclei are inhibited from decaying by corresponding energy barriers for each mode, but they can be tunneled through.

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.

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. Spontaneous fission is caused by electrostatic repulsion tearing the nucleus apart and produces various nuclei in different instances of identical nuclei fissioning. 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), and by 30 orders of magnitude from thorium (element 90) to fermium (element 100). 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. 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. 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. Experiments on lighter superheavy nuclei, as well as those closer to the expected island, have shown greater than previously anticipated stability against spontaneous fission, showing the importance of shell effects on nuclei.

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. (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.) 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). Spontaneous fission, however, produces various nuclei as products, so the original nuclide cannot be determined from its daughters.

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.

History

Early predictions

The heaviest element known at the end of the 19th century was uranium, with an atomic mass of about 240 (now known to be 238) amu. Accordingly, it was placed in the last row of the periodic table; this fueled speculation about the possible existence of elements heavier than uranium and why A = 240 seemed to be the limit. Following the discovery of the noble gases, beginning with argon in 1895, the possibility of heavier members of the group was considered. Danish chemist Julius Thomsen proposed in 1895 the existence of a sixth noble gas with Z = 86, A = 212 and a seventh with Z = 118, A = 292, the last closing a 32-element period containing thorium and uranium. In 1913, Swedish physicist Johannes Rydberg extended Thomsen's extrapolation of the periodic table to include even heavier elements with atomic numbers up to 460, but he did not believe that these superheavy elements existed or occurred in nature.

In 1914, German physicist Richard Swinne proposed that elements heavier than uranium, such as those around Z = 108, could be found in cosmic rays. He suggested that these elements may not necessarily have decreasing half-lives with increasing atomic number, leading to speculation about the possibility of some longer-lived elements at Z = 98–102 and Z = 108–110 (though separated by short-lived elements). Swinne published these predictions in 1926, believing that such elements might exist in Earth's core, iron meteorites, or the ice caps of Greenland where they had been locked up from their supposed cosmic origin.

Discoveries

Work performed from 1961 to 2013 at four labs – Lawrence Berkeley National Laboratory in the US, the Joint Institute for Nuclear Research in the USSR (later Russia), the GSI Helmholtz Centre for Heavy Ion Research in Germany, and Riken in Japan – identified and confirmed the elements lawrencium to oganesson according to the criteria of the IUPAC–IUPAP Transfermium Working Groups and subsequent Joint Working Parties. These discoveries complete the seventh row of the periodic table. The next two elements, ununennium (Z = 119) and unbinilium (Z = 120), have not yet been synthesized. They would begin an eighth period.

List of elements

• 103 Lawrencium, Lr, for Ernest Lawrence; sometimes but not always included
• 104 Rutherfordium, Rf, for Ernest Rutherford
• 105 Dubnium, Db, for the town of Dubna, near Moscow
• 106 Seaborgium, Sg, for Glenn T. Seaborg
• 107 Bohrium, Bh, for Niels Bohr
• 108 Hassium, Hs, for Hassia (Hesse), location of Darmstadt
• 109 Meitnerium, Mt, for Lise Meitner
• 110 Darmstadtium, Ds, for Darmstadt)
• 111 Roentgenium, Rg, for Wilhelm Röntgen
• 112 Copernicium, Cn, for Nicolaus Copernicus
• 113 Nihonium, Nh, for Nihon (Japan), location of the Riken institute
• 114 Flerovium, Fl, for Russian physicist Georgy Flyorov
• 115 Moscovium, Mc, for Moscow
• 116 Livermorium, Lv, for Lawrence Livermore National Laboratory
• 117 Tennessine, Ts, for Tennessee, location of Oak Ridge National Laboratory
• 118 Oganesson, Og, for Russian physicist Yuri Oganessian

Characteristics

Due to their short half-lives (for example, the most stable known isotope of seaborgium has a half-life of 14 minutes, and half-lives decrease with increasing atomic number) and the low yield of the nuclear reactions that produce them, new methods have had to be created to determine their gas-phase and solution chemistry based on very small samples of a few atoms each. Relativistic effects become very important in this region of the periodic table, causing the filled 7s orbitals, empty 7p orbitals, and filling 6d orbitals to all contract inward toward the atomic nucleus. This causes a relativistic stabilization of the 7s electrons and makes the 7p orbitals accessible in low excitation states.

Elements 103 to 112, lawrencium to copernicium, form the 6d series of transition elements. Experimental evidence shows that elements 103–108 behave as expected for their position in the periodic table, as heavier homologs of lutetium through osmium. They are expected to have ionic radii between those of their 5d transition metal homologs and their actinide pseudohomologs: for example, Rf⁴⁺ is calculated to have ionic radius 76 pm, between the values for Hf⁴⁺ (71 pm) and Th⁴⁺ (94 pm). Their ions should also be less polarizable than those of their 5d homologs. Relativistic effects are expected to reach a maximum at the end of this series, at roentgenium (element 111) and copernicium (element 112). Nevertheless, many important properties of the transactinides are still not yet known experimentally, though theoretical calculations have been performed.

Elements 113 to 118, nihonium to oganesson, should form a 7p series, completing the seventh period in the periodic table. Their chemistry will be greatly influenced by the very strong relativistic stabilization of the 7s electrons and a strong spin–orbit coupling effect "tearing" the 7p subshell apart into two sections, one more stabilized (7p_1/2, holding two electrons) and one more destabilized (7p_3/2, holding four electrons). Lower oxidation states should be stabilized here, continuing group trends, as both the 7s and 7p_1/2 electrons exhibit the inert-pair effect. These elements are expected to largely continue to follow group trends, though with relativistic effects playing an increasingly larger role. In particular, the large 7p splitting results in an effective shell closure at flerovium (element 114) and a hence much higher than expected chemical activity for oganesson (element 118).

Element 118 is the last element that has been synthesized. The next two elements, 119 and 120, should form an 8s series and be an alkali and alkaline earth metal respectively. The 8s electrons are expected to be relativistically stabilized, so that the trend toward higher reactivity down these groups will reverse and the elements will behave more like their period 5 homologs, rubidium and strontium. The 7p_3/2 orbital is still relativistically destabilized, potentially giving these elements larger ionic radii and perhaps even being able to participate chemically. In this region, the 8p electrons are also relativistically stabilized, resulting in a ground-state 8s²8p¹ valence electron configuration for element 121. Large changes are expected to occur in the subshell structure in going from element 120 to element 121: for example, the radius of the 5g orbitals should drop drastically, from 25 Bohr units in element 120 in the excited [Og] 5g¹ 8s¹ configuration to 0.8 Bohr units in element 121 in the excited [Og] 5g¹ 7d¹ 8s¹ configuration, in a phenomenon called "radial collapse". Element 122 should add either a further 7d or a further 8p electron to element 121's electron configuration. Elements 121 and 122 should be similar to actinium and thorium respectively.

At element 121, the superactinide series is expected to begin, when the 8s electrons and the filling 8p_1/2, 7d_3/2, 6f_5/2, and 5g_7/2 subshells determine the chemistry of these elements. Complete and accurate calculations are not available for elements beyond 123 because of the extreme complexity of the situation: the 5g, 6f, and 7d orbitals should have about the same energy level, and in the region of element 160 the 9s, 8p_3/2, and 9p_1/2 orbitals should also be about equal in energy. This will cause the electron shells to mix so that the block concept no longer applies very well, and will also result in novel chemical properties that will make positioning these elements in a periodic table very difficult.

Beyond superheavy elements

It has been suggested that elements beyond Z = 126 be called beyond superheavy elements. Other sources refer to elements around Z = 164 as hyperheavy elements.

See also

• Bose–Einstein condensate (also known as Superatom)
• Island of stability

References

Bibliography

• Audi, G.; Kondev, F. G.; Wang, M.; et al. (2017). "The NUBASE2016 evaluation of nuclear properties". Chinese Physics C. 41 (3). 030001. Bibcode:2017ChPhC..41c0001A. doi:10.1088/1674-1137/41/3/030001.
       pp. 030001-1–030001-17, pp. 030001-18–030001-138, Table I. The NUBASE2016 table of nuclear and decay properties
• Beiser, A. (2003). Concepts of modern physics (6th ed.). McGraw-Hill. ISBN 978-0-07-244848-1. OCLC 48965418.
• Hoffman, D. C.; Ghiorso, A.; Seaborg, G. T. (2000). The Transuranium People: The Inside Story. World Scientific. ISBN 978-1-78-326244-1.
• Kragh, H. (2018). From Transuranic to Superheavy Elements: A Story of Dispute and Creation. Springer. ISBN 978-3-319-75813-8.
• Zagrebaev, V.; Karpov, A.; Greiner, W. (2013). "Future of superheavy element research: Which nuclei could be synthesized within the next few years?". Journal of Physics: Conference Series. 420 (1). 012001. arXiv:1207.5700. Bibcode:2013JPhCS.420a2001Z. doi:10.1088/1742-6596/420/1/012001. ISSN 1742-6588.

Discovery

See also: Discovery of the chemical elements and 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.

Cold fusion

Nuclear reactions used in the 1960s resulted in high excitation energies that required expulsion of four or five neutrons; these reactions used targets made of elements with high atomic numbers to maximize the size difference between the two nuclei in a reaction. While this increased the chance of fusion due to the lower electrostatic repulsion between the target and the projectile, the formed compound nuclei often broke apart and did not survive to form a new element. Moreover, fusion processes inevitably produce neutron-poor nuclei, as heavier elements require more neutrons per proton to maximize stability; therefore, the necessary ejection of neutrons results in final products with typically have shorter lifetimes. As such, light beams (6 to 10 protons) only allowed synthesis of elements up to 106.

To advance to heavier elements, Soviet physicist Yuri Oganessian at the Joint Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast, Russian SFSR, Soviet Union, proposed a different mechanism, in which the bombarded nucleus would be lead-208, which has magic numbers of protons and neutrons, or one close to it. The magic number of protons and/or neutrons gives the nuclide additional stability, which requires more energy for an external nucleus to penetrate it. 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. This leaves less excitation energy for the newly created compound nucleus, which necessitates fewer neutron ejections to reach a stable state. Because of this energy difference, the former mechanism became known as "hot fusion" and the latter as "cold fusion".

Cold fusion was first declared successful in 1974 at JINR, when it was tested for synthesis of the yet-undiscovered element 106. These new nuclei were projected to decay via spontaneous fission. The physicists at JINR concluded those were not seen before because no fissioning nucleus known at the time 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. When asked about how far this new method could go and if lead targets were a physics' Klondike, Oganessian responded, "Klondike may be an exaggeration [...] But soon, we will try to get elements 107...108 in these reactions."

Reports

The synthesis of element 108 was first attempted in 1978 by a research team led by Oganessian at the JINR. The team used a reaction that would generate element 108, specifically, the isotope ²⁷⁰108, from fusion of radium (specifically, the isotope 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. 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.

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

In 1983, new experiments were performed at JINR. The experiments probably resulted in the synthesis of element 108; bismuth (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. These experiments were not claimed as a discovery and Oganessian announced them in a conference rather than in a written report.

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

Later in 1984, a research team led by Peter Armbruster and Gottfried Münzenberg at Gesellschaft für Schwerionenforschung (GSI; Institute for Heavy Ion Research) in Darmstadt, Hesse, West Germany, attempted to create element 108. The team bombarded a lead (²⁰⁸
₈₂Pb
) target with accelerated iron (⁵⁸
₂₆Fe
) nuclei. GSI's experiment to create element 108 was delayed until after their creation of element 109 in 1982, as prior calculations had suggested that even–even isotopes of element 108 would have spontaneous fission half-lives of less than one microsecond, making them difficult to detect and identify. The element 108 experiment finally went ahead after ²⁶⁶109 had been synthesized and was found to decay by alpha emission, suggesting that isotopes of element 108 would do likewise, and this was corroborated by an experiment aimed at synthesizing isotopes of element 106. GSI reported synthesis of three atoms of ²⁶⁵108. Two years later, they reported synthesis of one atom of the even–even ²⁶⁴108.

Arbitration

In 1985, the International Union of Pure and Applied Chemistry (IUPAC) and the International Union of Pure and Applied Physics (IUPAP) formed the Transfermium Working Group (TWG) to assess discoveries and establish final names for elements with atomic numbers greater than 100. The party held meetings with delegates from the three competing institutes; in 1990, they established criteria for recognition of an element and in 1991, they finished the work of assessing discoveries and disbanded. These results were published in 1993.

According to the report, the 1984 works from JINR and GSI simultaneously and independently established synthesis of element 108. Of the two 1984 works, the one from GSI was said to be sufficient as a discovery on its own. The JINR work, which preceded the GSI one, "very probably" displayed synthesis of element 108. However, that was determined in retrospect given the work from Darmstadt; the JINR work focused on chemically identifying remote granddaughters of element 108 isotopes (which could not exclude the possibility that these daughter isotopes had other progenitors), while the GSI work clearly identified the decay path of those element 108 isotopes. The report concluded that the major credit should be awarded to GSI. In written responses to this ruling, both JINR and GSI agreed with its conclusions. In the same response, GSI confirmed that they and JINR were able to resolve all conflicts between them; GSI also proposed a name for element 108 that had been officially presented at the facility three weeks earlier.

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

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

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.

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

The process of naming of element 108 was a part of a larger process of naming a number of elements starting with element 101; three teams—JINR, GSI, and the Lawrence Berkeley Laboratory (LBL; originally Radiation Laboratory, RL, and later Lawrence Berkeley National Laboratory, LBNL) of the University of California in Berkeley, California, United States—claimed discoveries of several elements and the right to name those elements. Sometimes, these claims clashed; since a discoverer was considered entitled to naming of an element, conflicts over priority of discovery often resulted in conflicts over names of these new elements. These conflicts became known as the Transfermium Wars. Different suggestions to name the whole set of elements from 101 onward and they occasionally assigned names suggested by one team to be used for elements discovered by another. However, not all suggestions were met with equal approval; the teams openly protested naming proposals on several occasions.

In 1994, IUPAC Commission on Nomenclature of Inorganic Chemistry recommended that element 108 be named "hahnium" (Hn) after the German physicist Otto Hahn so elements named after Hahn and Lise Meitner (it was recommended element 109 should be named meitnerium, following GSI's suggestion) would be next to each other, honouring their joint discovery of nuclear fission; IUPAC commented that they felt the German suggestion was obscure. GSI protested, saying this proposal contradicted the long-standing convention of giving the discoverer the right to suggest a name; the American Chemical Society supported GSI. The name "hahnium", albeit with the different symbol Ha, had already been proposed and used by the American scientists for element 105, for which they had a discovery dispute with JINR; they thus protested the confusing scrambling of names. Following the uproar, IUPAC formed an ad hoc committee of representatives from the national adhering organizations of the three countries home to the competing institutions; they produced a new set of names in 1995. Element 108 was again named hahnium; this proposal was also retracted. The final compromise was reached in 1996 and published in 1997; element 108 was named hassium (Hs). Simultaneously, the name dubnium (Db; from Dubna, the JINR location) was assigned to element 105, and the name hahnium was not used for any element.

The official justification for this naming, alongside that of darmstadtium for element 110, was that it completed a set of geographic names for the location of the GSI; this set had been initiated by 19th-century names europium and germanium. This set would serve as a response to earlier naming of americium, californium, and berkelium for elements discovered in Berkeley. Armbruster commented on this, "this bad tradition was established by Berkeley. We wanted to do it for Europe." Later, when commenting on the naming of element 112, Armbruster said, "I did everything to ensure that we do not continue with German scientists and German towns."

Isotopes

Main article: Isotopes of hassium

List of hassium isotopes

Isotope	Half-life		Decay mode	Discovery year	Discovery reaction
	Value	Ref
263Hs	760 µs		α, SF	2009	208Pb(56Fe,n)
264Hs	540 µs		α, SF	1986	207Pb(58Fe,n)
265Hs	1.96 ms		α, SF	1984	208Pb(58Fe,n)
265mHs	360 µs		α	1995	208Pb(56Fe,n)
266Hs	3.02 ms		α, SF	2001	270Ds(—,α)
266mHs	280 ms		α	2011	270mDs(—,α)
267Hs	55 ms		α	1995	238U(34S,5n)
267mHs	990 µs		α	2004	238U(34S,5n)
268Hs	1.42 s		α	2010	238U(34S,4n)
269Hs	16 s		α	1996	277Cn(—,2α)
270Hs	9 s		α	2003	248Cm(26Mg,4n)
271Hs	10 s		α	2008	248Cm(26Mg,3n)
273Hs	510 ms		α	2010	285Fl(—,3α)
275Hs	200 ms		α	2004	287Fl(—,3α)
277Hs	11 ms		α	2010	289Fl(—,3α)
277mHs	1.8 min		SF	2012	293mLv(—,4α)

Hassium has no stable or naturally occurring isotopes. Several radioactive isotopes have been synthesized in the laboratory, either by fusing two atoms or by observing the decay of heavier elements. As of 2019, the quantity of all hassium ever produced was on the order of hundreds of atoms. 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, although that of hassium-277 is unconfirmed. Most of these isotopes decay predominantly through alpha decay; this is the most common for all isotopes for which comprehensive decay characteristics are available, the only exception being hassium-277, which undergoes spontaneous fission. 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.

Atomic nuclei have well-established nuclear shells, and the existence of these shells provides nuclei with additional stability. If a nucleus has certain numbers of protons or neutrons, called magic numbers, that complete certain nuclear shells, then the nucleus is even more stable against decay. The highest known magic numbers are 82 for protons and 126 for neutrons. This notion is sometimes expanded to include additional numbers between those magic numbers, which also provide some additional stability and indicate closure of "sub-shells". In contrast to the better-known lighter nuclei, superheavy nuclei are deformed. Until the 1960s, it was thought that deformation made superheavy nuclei incapable of forming a nuclear structure because these nuclei would decay before they could form one; it thus appeared that nuclei with Z ≈ 103 were too heavy to exist for a considerable length of time. Subsequent theories proposed greater stability of heavier nuclei.

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

Theoretical models predict a region of instability for some hassium isotopes to lie around A = 275 and N = 168–170, which is between the predicted neutron shell closures at N = 162 for deformed nuclei and N = 184 for spherical nuclei. Nuclides within this region are predicted to have low fission barrier heights, resulting in short partial half-lives toward spontaneous fission. This prediction is supported by the observed 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.

In 1991, Polish physicists Zygmunt Patyk and Adam Sobiczewski predicted that 108 is a proton magic number for deformed nuclei and 162 is a neutron magic number for such nuclei. This means such nuclei are permanently deformed in their ground state but have high, narrow fission barriers to further deformation and hence relatively long life-times toward spontaneous fission. Computational prospects for shell stabilization for ²⁷⁰Hs made it a promising candidate for a deformed doubly magic nucleus. Experimental data is scarce, but the existing data is interpreted by the researchers to support the assignment of N = 162 as a magic number. In particular, this conclusion was drawn from the decay data of ²⁶⁹Hs, ²⁷⁰Hs, and ²⁷¹Hs. In 1997, Polish physicist Robert Smolańczuk calculated that the isotope ²⁹²Hs may be the most stable superheavy nucleus against alpha decay and spontaneous fission as a consequence of the predicted N = 184 shell closure.

Natural occurrence

Molybdenite

Hassium is not known to occur naturally on Earth; the half-lives of all 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.

In 1963, Soviet geologist and physicist Viktor Cherdyntsev, who had previously claimed the existence of primordial curium-247, claimed to have discovered element 108—specifically the ²⁶⁷108 isotope, which supposedly had a half-life of 400 to 500 million years—in natural molybdenite and suggested the provisional name sergenium (symbol Sg); this name takes its origin from the name for the Silk Road and was explained as "coming from Kazakhstan" for it. His rationale for claiming that sergenium was the heavier homologue to osmium was that minerals supposedly containing sergenium formed volatile oxides when boiled in nitric acid, similarly to osmium.

Cherdyntsev's findings were criticized by Soviet physicist Vladimir Kulakov on the grounds that some of the properties Cherdyntsev claimed sergenium had were inconsistent with the then-current nuclear physics. The chief questions raised by Kulakov were that the claimed alpha decay energy of sergenium was many orders of magnitude lower than expected and the half-life given was eight orders of magnitude shorter than what would be predicted for a nuclide alpha-decaying with the claimed decay energy. At the same time, a corrected half-life in the region of 10¹⁶ years would be impossible because it would imply the samples contained about 100 milligrams of sergenium. In 2003, it was suggested that the observed alpha decay with energy 4.5 MeV could be due to a low-energy and strongly enhanced transition between different hyperdeformed states of a hassium isotope around ²⁷¹Hs, thus suggesting that the existence of superheavy elements in nature was at least possible, although unlikely.

In 2006, Russian geologist Alexei Ivanov hypothesized that an isomer of ²⁷¹Hs might have a half-life of around (2.5±0.5)×10⁸ years, which would explain the observation of alpha particles with energies of around 4.4 MeV in some samples of molybdenite and osmiridium. This isomer of ²⁷¹Hs could be produced from the beta decay of ²⁷¹Bh and ²⁷¹Sg, which, being homologous to rhenium and molybdenum respectively, should occur in molybdenite along with rhenium and molybdenum if they occurred in nature. Because hassium is homologous to osmium, it should occur along with osmium in osmiridium if it occurs in nature. The decay chains of ²⁷¹Bh and ²⁷¹Sg are hypothetical and the predicted half-life of this hypothetical hassium isomer is not long enough for any sufficient quantity to remain on Earth. It is possible that more ²⁷¹Hs may be deposited on the Earth as the Solar System travels through the spiral arms of the Milky Way; this would explain excesses of plutonium-239 found on the ocean floors of the Pacific Ocean and the Gulf of Finland. However, minerals enriched with ²⁷¹Hs are predicted to have excesses of its daughters uranium-235 and lead-207; they would also have different proportions of elements that are formed during spontaneous fission, such as krypton, zirconium, and xenon. The natural occurrence of hassium in minerals such as molybdenite and osmiride is theoretically possible, but very unlikely.

In 2004, the Joint Institute for Nuclear Research started a search for natural hassium in the Modane Underground Laboratory in Modane, Auvergne-Rhône-Alpes, France; this was done underground to avoid interference and false positives from cosmic rays. In 2008–09, an experiment run in the laboratory resulted in detection of several registered events of neutron multiplicity (number of emitted free neutrons after a nucleus hit has been hit by a neutron and fissioned) above three in natural osmium, and in 2012–13, these findings were reaffirmed in another experiment run in the laboratory. These results hinted natural hassium could potentially exist in nature in amounts that allow its detection by the means of analytical chemistry, but this conclusion is based on an explicit assumption that there is a long-lived hassium isotope to which the registered events could be attributed.

Since ²⁹²Hs may be particularly stable against alpha decay and spontaneous fission, it was considered as a candidate to exist in nature. This nuclide, however, is predicted to be very unstable toward beta decay and any beta-stable isotopes of hassium such as ²⁸⁶Hs would be too unstable in the other decay channels to be observed in nature. A 2012 search for ²⁹²Hs in nature along with its homologue osmium at the Maier-Leibnitz Laboratory in Garching, Bavaria, Germany, was unsuccessful, setting an upper limit to its abundance at 3×10⁻¹⁵ grams of hassium per gram of osmium.

Predicted properties

Various calculations suggest 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.

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

Relativistic effects

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 d_3/2 and d_5/2 orbitals) in nonrelativistic calculations.

Relativistic effects on hassium should arise due to the high charge of its nuclei, which causes the electrons around the nucleus to move faster—so fast their velocity becomes comparable to the speed of light. There are three main effects: the direct relativistic effect, the indirect relativistic effect, and 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%.)

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 leads to contraction of the atomic orbitals, most specifically the s and p_1/2 orbitals. Their electrons become more closely attached to the atom and harder to pull from the nucleus. This is the direct relativistic effect. It was originally thought to be strong only for the innermost electrons, but was later established to significantly influence valence electrons as well.

Since the s and p_1/2 orbitals are closer to the nucleus, they take a bigger portion of the electric charge of the nucleus on themselves ("shield" it). This leaves less charge for attraction of the remaining electrons, whose orbitals therefore expand, making them easier to pull from the nucleus. This is the indirect relativistic effect. 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. The ionic radius (in oxidation state +8) of hassium is greater than that of osmium because of the relativistic expansion of the 6p_3/2 orbitals, which are the outermost orbitals for an Hs⁸⁺ ion (although in practice such highly charged ions would be too polarised in chemical environments to have much reality).

There are several kinds of electronic orbitals, denoted by the letters s, p, d, and f (g orbitals are expected to start being chemically active among elements after element 120). Each of these corresponds to an azimuthal quantum number l: s to 0, p to 1, d to 2, and f to 3. Every electron also corresponds to a spin quantum number s, which may equal either +1/2 or −1/2. Thus, the total angular momentum quantum number j = l + s is equal to j = l ± 1/2 (except for l = 0, for which for both electrons in each orbital j = 0 + 1/2 = 1/2). Spin of an electron 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): for instance, of the six 6p electrons, two become 6p_1/2 and four become 6p_3/2. This is the spin–orbit splitting (sometimes also referred to as subshell splitting or jj coupling). It is most visible with p electrons, which do not play an important role in the chemistry of hassium, but those for d and f electrons are within the same order of magnitude (quantitatively, spin–orbit splitting in expressed in energy units, such as electronvolts).

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. Relativistic effects decrease the atomization energies of the compounds of hassium because the spin–orbit splitting of the d orbital lowers binding energy between electrons and the nucleus and because relativistic effects decrease ionic character in bonding.

Physical and atomic

The previous members of group 8 have relatively high melting points: Fe, 1538 °C; Ru, 2334 °C; Os, 3033 °C. Much like them, hassium is predicted to be a solid at room temperature although its melting point has not been precisely calculated. Hassium should crystallize in the hexagonal close-packed structure (^c/_a = 1.59), similarly to its lighter congener osmium. Pure metallic hassium is calculated to have a bulk modulus (resistance to uniform compression) of 450 GPa, comparable with that of diamond, 442 GPa. Hassium is expected to have a bulk density of 41 g/cm³ 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 (22.6 g/cm³).

The atomic radius of hassium is expected to be around 126 pm. Due to the relativistic stabilization of the 7s orbital and destabilization of the 6d orbital, the Hs⁺ ion is predicted to have an electron configuration of [Rn] 5f¹⁴ 6d⁵ 7s², giving up a 6d electron instead of a 7s electron, which is the opposite of the behaviour of its lighter homologues. The Hs²⁺ ion is expected to have an electron configuration of [Rn] 5f¹⁴ 6d⁵ 7s¹, analogous to that calculated for the Os²⁺ ion. In chemical compounds, hassium is calculated to display bonding characteristic for a d-block element, whose bonding will be primarily executed by 6d_3/2 and 6d_5/2 orbitals; compared to the elements from the previous periods, 7s, 6p_1/2, 6p_3/2, and 7p_1/2 orbitals should be more important.

Chemical

Stable oxidation states in group 8

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

Hassium is the sixth member of the 6d series of transition metals and is expected to be much like the platinum group metals. Some of these properties were confirmed by gas-phase chemistry experiments. The group 8 elements portray a wide variety of oxidation states but ruthenium and osmium readily portray their group oxidation state of +8; this state becomes more stable down the group. This oxidation state is extremely rare: among stable elements, only ruthenium, osmium, and xenon are able to attain it in reasonably stable compounds. Hassium is expected to follow its congeners and have a stable +8 state, but like them it should show lower stable oxidation states such as +6, +4, +3, and +2. Hassium(IV) is expected to be more stable than hassium(VIII) in aqueous solution. Hassium should be a rather noble metal. The standard reduction potential for the Hs⁴⁺/Hs couple is expected to be 0.4 V.

The group 8 elements show a distinctive oxide chemistry. All of the lighter members have known or hypothetical tetroxides, MO₄. Their oxidizing power decreases as one descends the group. FeO₄ is not known due to its extraordinarily large electron affinity—the amount of energy released when an electron is added to a neutral atom or molecule to form a negative ion—which results in the formation of the well-known oxyanion ferrate(VI), FeO²⁻
₄. Ruthenium tetroxide, RuO₄, which is formed by oxidation of ruthenium(VI) in acid, readily undergoes reduction to ruthenate(VI), RuO²⁻
₄. Oxidation of ruthenium metal in air forms the dioxide, RuO₂. In contrast, osmium burns to form the stable tetroxide, OsO₄, which complexes with the hydroxide ion to form an osmium(VIII) -ate complex, [OsO₄(OH)₂]²⁻. Therefore, eka-osmium properties for hassium should be demonstrated by the formation of a stable, very volatile tetroxide HsO₄, which undergoes complexation with hydroxide to form a hassate(VIII), [HsO₄(OH)₂]²⁻. Ruthenium tetroxide and osmium tetroxide are both volatile due to their symmetrical tetrahedral molecular geometry and because they are charge-neutral; hassium tetroxide should similarly be a very volatile solid. The trend of the volatilities of the group 8 tetroxides is known to be RuO₄ < OsO₄ > HsO₄, which confirms the calculated results. In particular, the calculated enthalpies of adsorption—the energy required for the adhesion of atoms, molecules, or ions from a gas, liquid, or dissolved solid to a surface—of HsO₄, −(45.4 ± 1) kJ/mol on quartz, agrees very well with the experimental value of −(46 ± 2) kJ/mol.

Experimental chemistry

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

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

The first goal for chemical investigation was the formation of the tetroxide; it was chosen because ruthenium and osmium form volatile tetroxides, being the only transition metals to display a stable compound in the +8 oxidation state. Despite this selection for gas-phase chemical studies being clear from the beginning, chemical characterization of hassium was considered a difficult task for a long time. Although hassium isotopes were first synthesized in 1984, it was not until 1996 that a hassium isotope long-lived enough to allow chemical studies was synthesized. Unfortunately, this hassium isotope, ²⁶⁹Hs, was synthesized indirectly from the decay of ²⁷⁷Cn; not only are indirect synthesis methods not favourable for chemical studies, but the reaction that produced the isotope ²⁷⁷Cn had a low yield—its cross section was only 1 pb—and thus did not provide enough hassium atoms for a chemical investigation. Direct synthesis of ²⁶⁹Hs and ²⁷⁰Hs in the reaction ²⁴⁸Cm(²⁶Mg,xn)^274−xHs (x = 4 or 5) appeared more promising because the cross section for this reaction was somewhat larger at 7 pb. This yield was still around ten times lower than that for the reaction used for the chemical characterization of bohrium. New techniques for irradiation, separation, and detection had to be introduced before hassium could be successfully characterized chemically.

Ruthenium and osmium have very similar chemistry due to the lanthanide contraction but iron shows some differences from them; for example, although ruthenium and osmium form stable tetroxides in which the metal is in the +8 oxidation state, iron does not. In preparation for the chemical characterization of hassium, research focused on ruthenium and osmium rather than iron because hassium was expected to be similar to ruthenium and osmium, as the predicted data on hassium closely matched that of those two.

The first chemistry experiments were performed using gas thermochromatography in 2001, using the synthetic osmium radioisotopes ¹⁷²Os and ¹⁷³Os as a reference. During the experiment, seven hassium atoms were synthesized using the reactions ²⁴⁸Cm(²⁶Mg,5n)²⁶⁹Hs and ²⁴⁸Cm(²⁶Mg,4n)²⁷⁰Hs. They were then thermalized and oxidized in a mixture of helium and oxygen gases to form hassium tetroxide molecules.

Hs + 2 O₂ → HsO₄

The measured deposition temperature of hassium tetroxide was higher than that of osmium tetroxide, which indicated the former was the less volatile one, and this placed hassium firmly in group 8. The enthalpy of adsorption for HsO₄ measured, −46±2 kJ/mol, was significantly lower than the predicted value, −36.7±1.5 kJ/mol, indicating OsO₄ is more volatile than HsO₄, contradicting earlier calculations that implied they should have very similar volatilities. For comparison, the value for OsO₄ is −39±1 kJ/mol. (The calculations that yielded a closer match to the experimental data came after the experiment, in 2008.) It is possible hassium tetroxide interacts differently with silicon nitride than with silicon dioxide, the chemicals used for the detector; further research is required to establish whether there is a difference between such interactions and whether it has influenced the measurements. Such research would include more accurate measurements of the nuclear properties of ²⁶⁹Hs and comparisons with RuO₄ in addition to OsO₄.

In 2004, scientists reacted hassium tetroxide and sodium hydroxide to form sodium hassate(VIII), a reaction that is well known with osmium. This was the first acid-base reaction with a hassium compound, forming sodium hassate(VIII):

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. Scientists at GSI were hoping to use TASCA to study the synthesis and properties of the hassium(II) compound hassocene, Hs(C₅H₅)₂, using the reaction ²²⁶Ra(⁴⁸Ca,xn). This compound is analogous to the lighter compounds ferrocene, ruthenocene, and osmocene, and is expected to have the two cyclopentadienyl rings in an eclipsed conformation like ruthenocene and osmocene and not in a staggered conformation like ferrocene. Hassocene, which is expected to be a stable and highly volatile compound, was chosen because it has hassium in the low formal oxidation state of +2—although the bonding between the metal and the rings is mostly covalent in metallocenes—rather than the high +8 state that had previously been investigated, and relativistic effects were expected to be stronger in the lower oxidation state. The highly symmetrical structure of hassocene and its low number of atoms make relativistic calculations easier. As of 2019, there are no experimental reports of hassocene.

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