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= Hassium =
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Introduction
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Hassium is a synthetic chemical element; it has symbol Hs and atomic
number 108. It is highly radioactive: its most stable known isotopes
have half-lives of about ten seconds. One of its isotopes, (270)Hs,
has magic numbers of protons and neutrons for deformed nuclei, giving
it greater stability against spontaneous fission. Hassium is a
superheavy element; it has been produced in a laboratory in very small
quantities by fusing heavy nuclei with lighter ones. Natural
occurrences of hassium have been hypothesized but never found.
In the periodic table, hassium is a transactinide element, a member of
period 7 and group 8; it is thus the sixth member of the 6d series of
transition metals. Chemistry experiments have confirmed that hassium
behaves as the heavier homologue to osmium, reacting readily with
oxygen to form a volatile tetroxide. The chemical properties of
hassium have been only partly characterized, but they compare well
with the chemistry of the other group 8 elements.
The main innovation that led to the discovery of hassium was cold
fusion, where the fused nuclei do not differ by mass as much as in
earlier techniques. It relied on greater stability of target nuclei,
which in turn decreased excitation energy. This decreased the number
of neutrons ejected during synthesis, creating heavier, more stable
resulting nuclei. The technique was first tested at Joint Institute
for Nuclear Research (JINR) in Dubna, Moscow Oblast, Russian SFSR,
Soviet Union, in 1974. JINR used this technique to attempt synthesis
of element 108 in 1978, in 1983, and in 1984; the latter experiment
resulted in a claim that element 108 had been produced. Later in 1984,
a synthesis claim followed from the Gesellschaft für
Schwerionenforschung (GSI) in Darmstadt, Hesse, West Germany. The 1993
report by the Transfermium Working Group, formed by the International
Union of Pure and Applied Chemistry (IUPAC) and the International
Union of Pure and Applied Physics (IUPAP), concluded that the report
from Darmstadt was conclusive on its own whereas that from Dubna was
not, and major credit was assigned to the German scientists. GSI
formally announced they wished to name the element 'hassium' after the
German state of Hesse (Hassia in Latin), home to the facility in 1992;
this name was accepted as final in 1997.
Cold fusion
=============
Nuclear reactions used in the 1960s resulted in high excitation
energies that required expulsion of four or five neutrons; these
reactions used targets made of elements with high atomic numbers to
maximize the size difference between the two nuclei in a reaction.
While this increased the chance of fusion due to the lower
electrostatic repulsion between target and projectile, the formed
compound nuclei often broke apart and did not survive to form a new
element. Moreover, fusion inevitably produces neutron-poor nuclei, as
heavier elements need more neutrons per proton for stability;
therefore, the necessary ejection of neutrons results in final
products that are typically shorter-lived. As such, light beams (six
to ten protons) allowed synthesis of elements only up to 106.
To advance to heavier elements, Soviet physicist Yuri Oganessian at
Joint Institute for Nuclear Research (JINR) in Dubna, Moscow Oblast,
Russian SFSR, Soviet Union, proposed a different mechanism, in which
the bombarded nucleus would be lead-208, which has magic numbers of
protons and neutrons, or another nucleus close to it. Each proton and
neutron has a fixed rest energy; those of all protons are equal and so
are those of all neutrons. In a nucleus, some of this energy is
diverted to binding protons and neutrons; if a nucleus has a magic
number of protons and/or neutrons, then even more of its rest energy
is diverted, which makes the nuclide more stable. This additional
stability requires more energy for an external nucleus to break the
existing one and penetrate it. More energy diverted to binding
nucleons means less rest energy, which in turn means less mass (mass
is proportional to rest energy). More equal atomic numbers of the
reacting nuclei result in greater electrostatic repulsion between
them, but the lower mass excess of the target nucleus balances it.
This leaves less excitation energy for the new compound nucleus, which
necessitates fewer neutron ejections to reach a stable state. Due to
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 element106. These new
nuclei were projected to decay via spontaneous fission. The physicists
at JINR concluded element 106 was produced in the experiment because
no fissioning nucleus known at the time showed parameters of fission
similar to what was observed during the experiment and because
changing either of the two nuclei in the reactions negated the
observed effects. Physicists at Lawrence Berkeley Laboratory (LBL;
originally Radiation Laboratory, RL, and later Lawrence Berkeley
National Laboratory, LBNL) of the University of California in
Berkeley, California, United States, also expressed great interest in
the new technique. 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
=========
Synthesis of element108 was first attempted in 1978 by a team led by
Oganessian at JINR. The team used a reaction that would generate
element108, specifically, the isotope (270)108, from fusion of radium
(specifically, the isotope and calcium . 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
element108 in reactions between lead and iron ; they were uncertain
in interpreting the data, suggesting the possibility that element108
had not been created.
In 1983, new experiments were performed at JINR. The experiments
probably resulted in the synthesis of element108; bismuth was
bombarded with manganese to obtain (263)108, lead ((207, 208)Pb) was
bombarded with iron ((58)Fe) to obtain (264)108, and californium was
bombarded with neon to obtain (270)108. 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 manganese and iron, respectively. Twenty-one
spontaneous fission events were recorded; the researchers concluded
they were caused by (264)108.
Later in 1984, a research team led by Peter Armbruster and Gottfried
Münzenberg at Gesellschaft für Schwerionenforschung (GSI; 'Institute
for Heavy Ion Research') in Darmstadt, Hesse, West Germany, tried to
create element108. The team bombarded a lead ((208)Pb) target with
accelerated iron ((58)Fe) nuclei. GSI's experiment to create
element108 was delayed until after their creation of element109 in
1982, as prior calculations had suggested that even-even isotopes of
element108 would have spontaneous fission half-lives of less than one
microsecond, making them difficult to detect and identify. The
element108 experiment finally went ahead after (266)109 had been
synthesized and was found to decay by alpha emission, suggesting that
isotopes of element108 would do likewise, and this was corroborated by
an experiment aimed at synthesizing isotopes of element106. GSI
reported synthesis of three atoms of (265)108. Two years later, they
reported synthesis of one atom of the even-even (264)108.
Arbitration
=============
In 1985, the International Union of Pure and Applied Chemistry (IUPAC)
and the International Union of Pure and Applied Physics (IUPAP) formed
the Transfermium Working Group (TWG) to assess discoveries and
establish final names for elements with atomic numbers greater than
100. The party held meetings with delegates from the three competing
institutes; in 1990, they established criteria for recognition of an
element and in 1991, they finished the work of assessing discoveries
and disbanded. These results were published in 1993.
According to the report, the 1984 works from JINR and GSI
simultaneously and independently established synthesis of element108.
Of the two 1984 works, the one from GSI was said to be sufficient as a
discovery on its own. The JINR work, which preceded the GSI one, "very
probably" displayed synthesis of element108. However, that was
determined in retrospect given the work from Darmstadt; the JINR work
focused on chemically identifying remote granddaughters of element108
isotopes (which could not exclude the possibility that these daughter
isotopes had other progenitors), while the GSI work clearly identified
the decay path of those element108 isotopes. The report concluded that
the major credit should be awarded to GSI. In written responses to
this ruling, both JINR and GSI agreed with its conclusions. In the
same response, GSI confirmed that they and JINR were able to resolve
all conflicts between them.
Naming
========
Historically, a newly discovered element was named by its discoverer.
The first regulation came in 1947, when IUPAC decided naming required
regulation in case there are conflicting names. 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.
In Mendeleev's nomenclature for unnamed and undiscovered elements,
hassium would be called "eka-osmium", as in "the first element below
osmium in the periodic table" (from Sanskrit 'eka' meaning "one"). In
1979, IUPAC published recommendations according to which the element
was to be called "unniloctium" (symbol "Uno"), a systematic element
name as a placeholder until the element was discovered and the
discovery then confirmed, and a permanent name was decided. Although
these recommendations were widely followed in the chemical community,
the competing physicists in the field ignored them. They either called
it "element108", with the symbols 'E108', '(108)' or '108', or used
the proposed name "hassium".
In 1990, in an attempt to break a deadlock in establishing priority of
discovery and naming of several elements, IUPAC reaffirmed in its
nomenclature of inorganic chemistry that after existence of an element
was established, the discoverers could propose a name. (Also, the
Commission of Atomic Weights was excluded from the naming process.)
The first publication on criteria for an element discovery, released
in 1991, specified the need for recognition by TWG.
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 7September
1992. For element108, the scientists proposed the name "hassium". It
is derived from the Latin name 'Hassia' for the German state of Hesse
where the institute is located. This name was proposed to IUPAC in a
written response to their ruling on priority of discovery claims of
elements, signed 29 September 1992.
The process of naming of element 108 was a part of a larger process of
naming a number of elements starting with element 101; three
teams--JINR, GSI, and LBL--claimed discovery of several elements and
the right to name those elements. Sometimes, these claims clashed;
since a discoverer was considered entitled to naming of an element,
conflicts over priority of discovery often resulted in conflicts over
names of these new elements. These conflicts became known as the
Transfermium Wars. Different suggestions to name the whole set of
elements from 101 onward and they occasionally assigned names
suggested by one team to be used for elements discovered by another.
However, not all suggestions were met with equal approval; the teams
openly protested naming proposals on several occasions.
In 1994, IUPAC Commission on Nomenclature of Inorganic Chemistry
recommended that element108 be named "hahnium" (Hn) after German
physicist Otto Hahn so elements named after Hahn and Lise Meitner (it
was recommended element109 should be named meitnerium, following GSI's
suggestion) would be next to each other, honouring their joint
discovery of nuclear fission; IUPAC commented that they felt the
German suggestion was obscure. GSI protested, saying this proposal
contradicted the long-standing convention of giving the discoverer the
right to suggest a name; the American Chemical Society supported GSI.
The name "hahnium", albeit with the different symbol Ha, had already
been proposed and used by the American scientists for element105, 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. Element108 was
again named 'hahnium'; this proposal was also retracted. The final
compromise was reached in 1996 and published in 1997; element108 was
named 'hassium' (Hs). Simultaneously, the name 'dubnium' (Db; from
Dubna, the JINR location) was assigned to element105, and the name
'hahnium' was not used for any element.
The official justification for this naming, alongside that of
darmstadtium for element110, was that it completed a set of geographic
names for the location of the GSI; this set had been initiated by
19th-century names europium and germanium. This set would serve as a
response to earlier naming of americium, californium, and berkelium
for elements discovered in Berkeley. Armbruster commented on this,
"this bad tradition was established by Berkeley. We wanted to do it
for Europe." Later, when commenting on the naming of element112,
Armbruster said, "I did everything to ensure that we do not continue
with German scientists and German towns."
Isotopes
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{{Isotopes summary
|element=hassium
|other_notes={{efn|Few nuclei of each hassium isotope have been
synthesized, and thus half-lives of these isotopes cannot be
determined very precisely. Therefore, a half-life may be given as the
most likely value alongside a confidence interval that corresponds to
one standard deviation (such an interval based on future experiments,
whose result is yet unknown, contains the true value with a
probability of ~68.3%): for example, the value of 1.42s in the isotope
table obtained for 268Hs was listed in the source as 1.42±1.13s, and
this value is a modification of the value of .}}
|year_ref=
|reaction_ref=
|isotopes=
}}
Hassium has no stable or naturally occurring isotopes. Several
radioisotopes have been synthesized in the lab, either by fusing two
atoms or by observing the decay of heavier elements. As of 2019, the
quantity of all hassium ever produced was on the order of hundreds of
atoms. Thirteen isotopes with mass numbers 263 through 277 (except for
274 and 276) have been reported, six of which--(265, 266, 267, 269,
271, 277)Hs--have known metastable states, though that of (277)Hs is
unconfirmed. Most of these isotopes decay mainly through alpha decay;
this is the most common for all isotopes for which comprehensive decay
characteristics are available; the only exception is (277)Hs, which
undergoes spontaneous fission. Lighter isotopes were usually
synthesized by direct fusion of two nuclei, whereas heavier isotopes
were typically observed as decay products of nuclei with larger atomic
numbers.
Atomic nuclei have well-established nuclear shells, which make nuclei
more stable. If a nucleus has certain numbers (magic numbers) of
protons or neutrons, that complete a nuclear shell, then the nucleus
is even more stable against decay. The highest known magic numbers are
82 for protons and 126 for neutrons. This notion is sometimes expanded
to include additional numbers between those magic numbers, which also
provide some additional stability and indicate closure of
"sub-shells". Unlike the better-known lighter nuclei, superheavy
nuclei are deformed. Until the 1960s, the liquid drop model was the
dominant explanation for nuclear structure. It suggested that the
fission barrier would disappear for nuclei with ~280nucleons. It was
thus thought that spontaneous fission would occur nearly instantly
before nuclei could form a structure that could stabilize them; it
appeared that nuclei with Z≈103 were too heavy to exist for a
considerable length of time.
The later nuclear shell model suggested that nuclei with ~300 nucleons
would form an island of stability where nuclei will be more resistant
to spontaneous fission and will mainly undergo alpha decay with longer
half-lives, and the next doubly magic nucleus (having magic numbers of
both protons and neutrons) is expected to lie in the center of the
island of stability near 'Z'=110-114 and the predicted magic neutron
number 'N'=184. Subsequent discoveries suggested that the predicted
island might be further than originally anticipated. They also showed
that nuclei intermediate between the long-lived actinides and the
predicted island are deformed, and gain additional stability from
shell effects, against alpha decay and especially against spontaneous
fission. The center of the region on a chart of nuclides that would
correspond to this stability for deformed nuclei was determined as
(270)Hs, 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 in this region are predicted to
have low fission barrier heights, resulting in short partial
half-lives toward spontaneous fission. This prediction is supported by
the observed 11-millisecond half-life of (277)Hs and the 5-millisecond
half-life of the neighbouring isobar (277)Mt because the hindrance
factors from the odd nucleon were shown to be much lower than
otherwise expected. The measured half-lives are even lower than those
originally predicted for the even-even (276)Hs and (278)Ds, 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
spontaneous-fission half-lives. Computational prospects for shell
stabilization for (270)Hs made it a promising candidate for a deformed
doubly magic nucleus. Experimental data is scarce, but the existing
data is interpreted by the researchers to support the assignment of
'N'=162 as a magic number. In particular, this conclusion was drawn
from the decay data of (269)Hs, (270)Hs, and (271)Hs. In 1997, Polish
physicist Robert Smolańczuk calculated that the isotope (292)Hs 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
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Hassium is not known to occur naturally on Earth; all its known
isotopes are so short-lived that no primordial hassium would survive
to today. This does not rule out the possibility of unknown,
longer-lived isotopes or nuclear isomers, some of which could still
exist in trace quantities if they are long-lived enough. As early as
1914, German physicist Richard Swinne proposed element108 as a source
of X-rays in the Greenland ice sheet. Though Swinne was unable to
verify this observation and thus did not claim discovery, he proposed
in 1931 the existence of "regions" of long-lived transuranic elements,
including one around 'Z'=108.
In 1963, Soviet geologist and physicist Viktor Cherdyntsev, who had
previously claimed the existence of primordial curium-247, claimed to
have discovered element108--specifically the (267)108 isotope, which
supposedly had a half-life of 400 to 500million years--in natural
molybdenite and suggested the provisional name 'sergenium' (symbol
Sg); this name comes 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.
Soviet physicist Vladimir Kulakov criticized Cherdyntsev's findings on
the grounds that some of the properties Cherdyntsev claimed sergenium
had, were inconsistent with then-current nuclear physics. The chief
questions Kulakov raised were that the claimed alpha decay energy of
sergenium was many orders of magnitude lower than expected and the
half-life given was eight orders of magnitude shorter than what would
be predicted for a nuclide alpha-decaying with the claimed decay
energy. At the same time, a corrected half-life in the region of
10(16)years would be impossible because it would imply the samples
contained ~100 milligrams of sergenium. In 2003, it was suggested that
the observed alpha decay with energy 4.5MeV could be due to a
low-energy and strongly enhanced transition between different
hyperdeformed states of a hassium isotope around (271)Hs, thus
suggesting that the existence of superheavy elements in nature was at
least possible, but unlikely.
In 2006, Russian geologist Alexei Ivanov hypothesized that an isomer
of (271)Hs might have a half-life of ~ years, which would explain the
observation of alpha particles with energies of ~4.4MeV in some
samples of molybdenite and osmiridium. This isomer of (271)Hs could be
produced from the beta decay of (271)Bh and (271)Sg, 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
(271)Bh and (271)Sg 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 (271)Hs 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 (271)Hs are predicted to have
excesses of its daughters uranium-235 and lead-207; they would also
have different proportions of elements that are formed by spontaneous
fission, such as krypton, zirconium, and xenon. The natural occurrence
of hassium in minerals such as molybdenite and osmiride is
theoretically possible, but very unlikely.
In 2004, JINR started a search for natural hassium in the Modane
Underground Laboratory in Modane, Auvergne-Rhône-Alpes, France; this
was done underground to avoid interference and false positives from
cosmic rays. In 2008-09, an experiment run in the laboratory resulted
in detection of several registered events of neutron multiplicity
(number of emitted free neutrons after a nucleus is hit by a neutron
and fissioned) above three in natural osmium, and in 2012-13, these
findings were reaffirmed in another experiment run in the laboratory.
These results hinted natural hassium could potentially exist in nature
in amounts that allow its detection by the means of analytical
chemistry, but this conclusion is based on an explicit assumption that
there is a long-lived hassium isotope to which the registered events
could be attributed.
Since (292)Hs may be particularly stable against alpha decay and
spontaneous fission, it was considered as a candidate to exist in
nature. This nuclide, however, is predicted to be very unstable toward
beta decay and any beta-stable isotopes of hassium such as (286)Hs
would be too unstable in the other decay channels to be observed in
nature. A 2012 search for (292)Hs in nature along with its homologue
osmium at the Maier-Leibnitz Laboratory in Garching, Bavaria, Germany,
was unsuccessful, setting an upper limit to its abundance at of
hassium per gram of osmium.
Predicted properties
======================================================================
Various calculations suggest hassium should be the heaviest group 8
element so far, consistently with the periodic law. Its properties
should generally match those expected for a heavier homologue of
osmium; as is the case for all transactinides, a few deviations are
expected to arise from relativistic effects.
Very few properties of hassium or its compounds have been measured;
this is due to its extremely limited and expensive production and the
fact that hassium (and its parents) decays very quickly. A few
singular chemistry-related properties have been measured, such as
enthalpy of adsorption of hassium tetroxide, but properties of hassium
metal remain unknown and only predictions are available.
Relativistic effects
======================
Relativistic effects in hassium should arise due to the high charge of
its nuclei, which causes the electrons around the nucleus to move
faster--so fast their speed is comparable to the speed of light. There
are three main effects: the direct relativistic effect, the indirect
relativistic effect, and spin-orbit splitting. (The existing
calculations do not account for Breit interactions, but those are
negligible, and their omission can only result in an uncertainty of
the current calculations of no more than 2%.)
As atomic number increases, so does the electrostatic attraction
between an electron and the nucleus. This causes the velocity of the
electron to increase, which leads to an increase in its mass. This in
turn leads to contraction of the atomic orbitals, most specifically
the s and p orbitals. Their electrons become more closely attached to
the atom and harder to pull from the nucleus. This is the direct
relativistic effect. It was originally thought to be strong only for
the innermost electrons, but was later established to significantly
influence valence electrons as well.
Since the s and p orbitals are closer to the nucleus, they take a
bigger portion of the electric charge of the nucleus on themselves
("shield" it). This leaves less charge for attraction of the remaining
electrons, whose orbitals therefore expand, making them easier to pull
from the nucleus. This is the indirect relativistic effect. As a
result of the combination of the direct and indirect relativistic
effects, the Hs(+) ion, compared to the neutral atom, lacks a 6d
electron, rather than a 7s electron. In comparison, Os(+) lacks a 6s
electron compared to the neutral atom. The ionic radius (in oxidation
state +8) of hassium is greater than that of osmium because of the
relativistic expansion of the 6p orbitals, which are the outermost
orbitals for an Hs(8+) ion (although in practice such highly charged
ions would be too polarized in chemical environments to have much
reality).
There are several kinds of electron orbitals, denoted s, p, d, and f
(g orbitals are expected to start being chemically active among
elements after element 120). Each of these corresponds to an azimuthal
quantum number 'l': s to 0, p to 1, d to 2, and f to 3. Every electron
also corresponds to a spin quantum number 's', which may equal either
+1/2 or −1/2. Thus, the total angular momentum quantum number 'j = l'
+ 's' is equal to 'j' = 'l' ± 1/2 (except for 'l' = 0, for which for
both electrons in each orbital 'j =' 0 + 1/2 = 1/2). Spin of an
electron relativistically interacts with its orbit, and this
interaction leads to a split of a subshell into two with different
energies (the one with 'j' = 'l' − 1/2 is lower in energy and thus
these electrons more difficult to extract): for instance, of the six
6p electrons, two become 6p and four become 6p. This is the spin-orbit
splitting (also called subshell splitting or 'jj' coupling). It is
most visible with p electrons, which do not play an important role in
the chemistry of hassium, but those for d and f electrons are within
the same order of magnitude (quantitatively, spin-orbit splitting in
expressed in energy units, such as electronvolts).
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 hassium compounds because the
spin-orbit splitting of the d orbital lowers binding energy between
electrons and the nucleus and because relativistic effects decrease
ionic character in bonding.
Physical and atomic
=====================
The previous members of group8 have high melting points: Fe, 1538°C;
Ru, 2334°C; Os, 3033°C. Like them, hassium is predicted to be a solid
at room temperature though its melting point has not been precisely
calculated. Hassium should crystallize in the hexagonal close-packed
structure (('c')/=1.59), similarly to its lighter congener osmium.
Pure metallic hassium is calculated to have a bulk modulus (resistance
to uniform compression) of 450GPa, comparable with that of diamond,
442GPa. Hassium is expected to be one of the densest of the 118 known
elements, with a predicted density of 27-29 g/cm(3) vs. the 22.59
g/cm(3) measured for osmium.
Hassium's atomic radius is expected to be ≈126pm. Due to relativistic
stabilization of the 7s orbital and destabilization of the 6d orbital,
the Hs(+) ion is predicted to have an electron configuration of
[Rn]5f(14)6d(5)7s(2), giving up a 6d electron instead of a 7s
electron, which is the opposite of the behaviour of its lighter
homologues. The Hs(2+) ion is expected to have electron configuration
[Rn]5f(14)6d(5)7s(1), analogous to that calculated for the Os(2+) ion.
In chemical compounds, hassium is calculated to display bonding
characteristic for a d-block element, whose bonding will be primarily
executed by 6d and 6d orbitals; compared to the elements from the
previous periods, 7s, 6p, 6p, and 7p orbitals should be more
important.
Chemical
==========
Stable oxidation states in group 8
Element !colspan="8"| 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
group8 elements portray a wide variety of oxidation states but
ruthenium and osmium readily portray their group oxidation state of
+8; this state becomes more stable down the group. This oxidation
state is extremely rare: among stable elements, only ruthenium,
osmium, and xenon are able to attain it in reasonably stable
compounds. Hassium is expected to follow its congeners and have a
stable +8 state, 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 0.4V.
The group 8 elements show a distinctive oxide chemistry. All the
lighter members have known or hypothetical tetroxides, MO. Their
oxidizing power decreases as one descends the group. FeO is not known
due to its extraordinarily large electron affinity--the amount of
energy released when an electron is added to a neutral atom or
molecule to form a negative ion--which results in the formation of the
well-known oxyanion ferrate(VI), . Ruthenium tetroxide, RuO4, which is
formed by oxidation of ruthenium(VI) in acid, readily undergoes
reduction to ruthenate(VI), . Oxidation of ruthenium metal in air
forms the dioxide, RuO2. In contrast, osmium burns to form the stable
tetroxide, OsO4, which complexes with the hydroxide ion to form an
osmium(VIII) -'ate' complex, [OsO4(OH)2]2−. Therefore, hassium should
behave as a heavier homologue of osmium by forming of a stable, very
volatile tetroxide HsO4, which undergoes complexation with hydroxide
to form a hassate(VIII), [HsO4(OH)2]2−. 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 group8 tetroxides is experimentally known to be
RuO44>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
======================================================================
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 was first synthesized in 1984, it was not until
1996 that a hassium isotope long-lived enough to allow chemical
studies was synthesized. Unfortunately, this isotope, (269)Hs, was
synthesized indirectly from the decay of (277)Cn; not only are
indirect synthesis methods not favourable for chemical studies, but
the reaction that produced the isotope (277)Cn had a low yield--its
cross section was only 1pb--and thus did not provide enough hassium
atoms for a chemical investigation. Direct synthesis of (269)Hs and
(270)Hs in the reaction (248)Cm((26)Mg,'x'n)(274−'x')Hs ('x'=4 or 5)
appeared more promising because the cross section for this reaction
was somewhat larger at 7pb. 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
(172, 173)Os as a reference. During the experiment, seven hassium
atoms were synthesized using the reactions (248)Cm((26)Mg,5n)(269)Hs
and (248)Cm((26)Mg,4n)(270)Hs. They were then thermalized and oxidized
in a mixture of helium and oxygen gases to form hassium tetroxide
molecules.
:Hs + 2 O → HsO
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 HsO measured, , was significantly lower than the
predicted value, , indicating OsO is more volatile than HsO,
contradicting earlier calculations that implied they should have very
similar volatilities. For comparison, the value for OsO is . (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
(269)Hs and comparisons with RuO in addition to OsO.
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):
: + 2 NaOH →
The team from the University of Mainz planned in 2008 to study the
electrodeposition of hassium atoms using the new TASCA facility at
GSI. Their aim was to use the reaction (226)Ra((48)Ca,4n)(270)Hs.
Scientists at GSI were hoping to use TASCA to study the synthesis and
properties of the hassium(II) compound hassocene, Hs(CH), using the
reaction (226)Ra((48)Ca,'x'n). 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. , there are
no experimental reports of hassocene.
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Original Article:
http://en.wikipedia.org/wiki/Hassium
