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= Seaborgium =
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Introduction
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Seaborgium is a synthetic chemical element; it has symbol Sg and
atomic number 106. It is named after the American nuclear chemist
Glenn T. Seaborg. As a synthetic element, it can be created in a
laboratory but is not found in nature. It is also radioactive; the
most stable known isotopes have half-lives on the order of several
minutes.
In the periodic table of the elements, it is a d-block transactinide
element. It is a member of the 7th period and belongs to the group 6
elements as the fourth member of the 6d series of transition metals.
Chemistry experiments have confirmed that seaborgium behaves as the
heavier homologue to tungsten in group 6. The chemical properties of
seaborgium are characterized only partly, but they compare well with
the chemistry of the other group 6 elements.
In 1974, a few atoms of seaborgium were produced in laboratories in
the Soviet Union and in the United States. The priority of the
discovery and therefore the naming of the element was disputed between
Soviet and American scientists, and it was not until 1997 that the
International Union of Pure and Applied Chemistry (IUPAC) established
seaborgium as the official name for the element. It is one of only two
elements named after a living person at the time of naming, the other
being oganesson, element 118.
History
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Following claims of the observation of elements 104 and 105 in 1970 by
Albert Ghiorso et al. at the Lawrence Livermore National Laboratory, a
search for element 106 using oxygen-18 projectiles and the previously
used californium-249 target was conducted. Several 9.1 MeV alpha
decays were reported and are now thought to originate from element
106, though this was not confirmed at the time. In 1972, the HILAC
accelerator received equipment upgrades, preventing the team from
repeating the experiment, and data analysis was not done during the
shutdown. This reaction was tried again several years later, in 1974,
and the Berkeley team realized that their new data agreed with their
1971 data, to the astonishment of Ghiorso. Hence, element 106 could
have actually been discovered in 1971 if the original data was
analyzed more carefully.
Two groups claimed discovery of the element. Evidence of element 106
was first reported in 1974 by a Russian research team in Dubna led by
Yuri Oganessian, in which targets of lead-208 and lead-207 were
bombarded with accelerated ions of chromium-54. In total, fifty-one
spontaneous fission events were observed with a half-life between four
and ten milliseconds. After having ruled out nucleon transfer
reactions as a cause for these activities, the team concluded that the
most likely cause of the activities was the spontaneous fission of
isotopes of element 106. The isotope in question was first suggested
to be seaborgium-259, but was later corrected to seaborgium-260.
: + → + 2
: + → +
A few months later in 1974, researchers including Glenn T. Seaborg,
Carol Alonso and Albert Ghiorso at the University of California,
Berkeley, and E. Kenneth Hulet from the Lawrence Livermore National
Laboratory, also synthesized the element by bombarding a
californium-249 target with oxygen-18 ions, using equipment similar to
that which had been used for the synthesis of element 104 five years
earlier, observing at least seventy alpha decays, seemingly from the
isotope seaborgium-263m with a half-life of seconds. The alpha
daughter rutherfordium-259 and granddaughter nobelium-255 had
previously been synthesised and the properties observed here matched
with those previously known, as did the intensity of their production.
The cross-section of the reaction observed, 0.3 nanobarns, also agreed
well with theoretical predictions. These bolstered the assignment of
the alpha decay events to seaborgium-263m.
: + → + 4 → + → +
A dispute thus arose from the initial competing claims of discovery,
though unlike the case of the synthetic elements up to element 105,
neither team of discoverers chose to announce proposed names for the
new elements, thus averting an element naming controversy temporarily.
The dispute on discovery, however, dragged on until 1992, when the
IUPAC/IUPAP Transfermium Working Group (TWG), formed to put an end to
the controversy by making conclusions regarding discovery claims for
elements 101 to 112, concluded that the Soviet synthesis of
seaborgium-260 was not convincing enough, "lacking as it is in yield
curves and angular selection results", whereas the American synthesis
of seaborgium-263 was convincing due to its being firmly anchored to
known daughter nuclei. As such, the TWG recognised the Berkeley team
as official discoverers in their 1993 report.
Seaborg had previously suggested to the TWG that if Berkeley was
recognised as the official discoverer of elements 104 and 105, they
might propose the name 'kurchatovium' (symbol Kt) for element 106 to
honour the Dubna team, which had proposed this name for element 104
after Igor Kurchatov, the former head of the Soviet nuclear research
programme. However, due to the worsening relations between the
competing teams after the publication of the TWG report (because the
Berkeley team vehemently disagreed with the TWG's conclusions,
especially regarding element 104), this proposal was dropped from
consideration by the Berkeley team. After being recognized as official
discoverers, the Berkeley team started deciding on a name in earnest:
Seaborg's son Eric remembered the naming process as follows:
The name 'seaborgium' and symbol 'Sg' were announced at the 207th
national meeting of the American Chemical Society in March 1994 by
Kenneth Hulet, one of the co-discovers. However, IUPAC resolved in
August 1994 that an element could not be named after a living person,
and Seaborg was still alive at the time. Thus, in September 1994,
IUPAC recommended a set of names in which the names proposed by the
three laboratories (the third being the GSI Helmholtz Centre for Heavy
Ion Research in Darmstadt, Germany) with competing claims to the
discovery for elements 104 to 109 were shifted to various other
elements, in which 'rutherfordium' (Rf), the Berkeley proposal for
element 104, was shifted to element 106, with 'seaborgium' being
dropped entirely as a name.
Summary of element naming proposals and final decisions for elements
101-112 (those covered in the TWG report)
Atomic number !! Systematic !! American !! Russian !! German !!
Compromise 92 !! IUPAC 94 !! ACS 94 !! IUPAC 95 !! IUPAC 97 !! Present
101 unnilunium mendelevium mendelevium mendelevium
mendelevium mendelevium mendelevium mendelevium
102 unnilbium nobelium joliotium joliotium nobelium
nobelium flerovium nobelium nobelium
103 unniltrium lawrencium rutherfordium lawrencium
lawrencium lawrencium lawrencium lawrencium lawrencium
104 unnilquadium rutherfordium kurchatovium meitnerium
dubnium rutherfordium dubnium rutherfordium rutherfordium
105 unnilpentium hahnium nielsbohrium kurchatovium
joliotium hahnium joliotium dubnium dubnium
106 unnilhexium seaborgium rutherfordium rutherfordium
seaborgium seaborgium seaborgium seaborgium
107 unnilseptium nielsbohrium nielsbohrium bohrium
nielsbohrium nielsbohrium bohrium bohrium
108 unniloctium hassium hassium hahnium hassium
hahnium hassium hassium
109 unnilennium meitnerium hahnium meitnerium
meitnerium meitnerium meitnerium meitnerium
110 ununnilium hahnium becquerelium darmstadtium
darmstadtium
111 unununium roentgenium roentgenium
112 ununbium copernicium copernicium
This decision ignited a firestorm of worldwide protest for
disregarding the historic discoverer's right to name new elements, and
against the new retroactive rule against naming elements after living
persons; the American Chemical Society stood firmly behind the name
'seaborgium' for element 106, together with all the other American and
German naming proposals for elements 104 to 109, approving these names
for its journals in defiance of IUPAC. At first, IUPAC defended
itself, with an American member of its committee writing: "Discoverers
don't have a right to name an element. They have a right to suggest a
name. And, of course, we didn't infringe on that at all." However,
Seaborg responded:
Bowing to public pressure, IUPAC proposed a different compromise in
August 1995, in which the name 'seaborgium' was reinstated for element
106 in exchange for the removal of all but one of the other American
proposals, which met an even worse response. Finally, IUPAC rescinded
these previous compromises and made a final, new recommendation in
August 1997, in which the American and German proposals for elements
104 to 109 were all adopted, including 'seaborgium' for element 106,
with the single exception of element 105, named 'dubnium' to recognise
the contributions of the Dubna team to the experimental procedures of
transactinide synthesis. This list was finally accepted by the
American Chemical Society, which wrote:
Seaborg commented regarding the naming:
{{blockquote|I am, needless to say, proud that U.S. chemists
recommended that element 106, which is placed under tungsten (74), be
called 'seaborgium.' I was looking forward to the day when chemical
investigators will refer to such compounds as seaborgous chloride,
seaborgic nitrate, and perhaps, sodium seaborgate. This is the
greatest honor ever bestowed upon me--even better, I think, than
winning the Nobel Prize. Future students of chemistry, in learning
about the periodic table, may have reason to ask why the element was
named for me, and thereby learn more about my work.|author=Glenn
Seaborg}}
Seaborg died a year and a half later, on 25 February 1999, at the age
of 86.
Isotopes
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{{Isotopes summary
|element=seaborgium
|isotopes=
}}
Superheavy elements such as seaborgium are produced by bombarding
lighter elements in particle accelerators that induces fusion
reactions. Whereas most of the isotopes of seaborgium can be
synthesized directly this way, some heavier ones have only been
observed as decay products of elements with higher atomic numbers.
Depending on the energies involved, fusion reactions that generate
superheavy elements are separated into "hot" and "cold". In hot fusion
reactions, very light, high-energy projectiles are accelerated toward
very heavy targets (actinides), giving rise to compound nuclei at high
excitation energy (~40-50 MeV) that may either fission or evaporate
several (3 to 5) neutrons. In cold fusion reactions, the produced
fused nuclei have a relatively low excitation energy (~10-20 MeV),
which decreases the probability that these products will undergo
fission reactions. As the fused nuclei cool to the ground state, they
require emission of only one or two neutrons, and thus, allows for the
generation of more neutron-rich products. The latter is a distinct
concept from that of where nuclear fusion claimed to be achieved at
room temperature conditions (see cold fusion).
Seaborgium 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.
Fourteen different isotopes of seaborgium have been reported with mass
numbers 257-269 and 271, four of which, seaborgium-261, −263, −265,
and −267, have known metastable states. All of these decay only
through alpha decay and spontaneous fission, with the single exception
of seaborgium-261 that can also undergo electron capture to
dubnium-261.
There is a trend toward increasing half-lives for the heavier
isotopes, though even-odd isotopes are generally more stable than
their neighboring even-even isotopes, because the odd neutron leads to
increased hindrance of spontaneous fission; among known seaborgium
isotopes, alpha decay is the predominant decay mode in even-odd nuclei
whereas fission dominates in even-even nuclei. Three of the heaviest
known isotopes, 267Sg, 269Sg, and 271Sg, are also the longest-lived,
having half-lives on the order of several minutes. Some other isotopes
in this region are predicted to have comparable or even longer
half-lives. Additionally, 263Sg, 265Sg, 265mSg, and 268Sg have
half-lives measured in seconds. All the remaining isotopes have
half-lives measured in milliseconds, with the exception of the
shortest-lived isotope, 261mSg, with a half-life of only 9.3
microseconds.
The proton-rich isotopes from 257Sg to 261Sg were directly produced by
cold fusion; all heavier isotopes were produced from the repeated
alpha decay of the heavier elements hassium, darmstadtium, and
flerovium, with the exceptions of the isotopes 263mSg, 264Sg, 265Sg,
and 265mSg, which were directly produced by hot fusion through
irradiation of actinide targets.
Predicted properties
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Very few properties of seaborgium or its compounds have been measured;
this is due to its extremely limited and expensive production and the
fact that seaborgium (and its parents) decays very quickly. A few
singular chemistry-related properties have been measured, but
properties of seaborgium metal remain unknown and only predictions are
available.
Physical
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Seaborgium is expected to be a solid under normal conditions and
assume a body-centered cubic crystal structure, similar to its lighter
congener tungsten. Early predictions estimated that it should be a
very heavy metal with density around 35.0 g/cm3, but calculations in
2011 and 2013 predicted a somewhat lower value of 23-24 g/cm3.
Chemical
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Seaborgium is the fourth member of the 6d series of transition metals
and the heaviest member of group 6 in the periodic table, below
chromium, molybdenum, and tungsten. All the members of the group form
a diversity of oxoanions. They readily portray their group oxidation
state of +6, although this is highly oxidising in the case of
chromium, and this state becomes more and more stable to reduction as
the group is descended: indeed, tungsten is the last of the 5d
transition metals where all four 5d electrons participate in metallic
bonding. As such, seaborgium should have +6 as its most stable
oxidation state, both in the gas phase and in aqueous solution, and
this is the only positive oxidation state that is experimentally known
for it; the +5 and +4 states should be less stable, and the +3 state,
the most common for chromium, would be the least stable for
seaborgium.
This stabilisation of the highest oxidation state occurs in the early
6d elements because of the similarity between the energies of the 6d
and 7s orbitals, since the 7s orbitals are relativistically stabilised
and the 6d orbitals are relativistically destabilised. This effect is
so large in the seventh period that seaborgium is expected to lose its
6d electrons before its 7s electrons (Sg, [Rn]5f146d47s2; Sg+,
[Rn]5f146d37s2; Sg2+, [Rn]5f146d37s1; Sg4+, [Rn]5f146d2; Sg6+,
[Rn]5f14). Because of the great destabilisation of the 7s orbital,
SgIV should be even more unstable than WIV and should be very readily
oxidised to SgVI. The predicted ionic radius of the hexacoordinate
Sg6+ ion is 65 pm, while the predicted atomic radius of seaborgium is
128 pm. Nevertheless, the stability of the highest oxidation state is
still expected to decrease as LrIII > RfIV > DbV > SgVI. Some
predicted standard reduction potentials for seaborgium ions in aqueous
acidic solution are as follows:
: 2 SgO3 + 2 H+ + 2 e− Sg2O5 + H2O E0 = −0.046 V
Sg2O5 + 2 H+ + 2 e− 2 SgO2 + H2O E0 = +0.11 V
SgO2 + 4 H+ + e− Sg3+ + 2 H2O E0 = −1.34 V
Sg3+ + e− Sg2+ E0 = −0.11 V
Sg3+ + 3 e− Sg E0 = +0.27 V
Seaborgium should form a very volatile hexafluoride (SgF6) as well as
a moderately volatile hexachloride (SgCl6), pentachloride (SgCl5), and
oxychlorides SgO2Cl2 and SgOCl4. SgO2Cl2 is expected to be the most
stable of the seaborgium oxychlorides and to be the least volatile of
the group 6 oxychlorides, with the sequence MoO2Cl2 > WO2Cl2 >
SgO2Cl2. The volatile seaborgium(VI) compounds SgCl6 and SgOCl4 are
expected to be unstable to decomposition to seaborgium(V) compounds at
high temperatures, analogous to MoCl6 and MoOCl4; this should not
happen for SgO2Cl2 due to the much higher energy gap between the
highest occupied and lowest unoccupied molecular orbitals, despite the
similar Sg-Cl bond strengths (similarly to molybdenum and tungsten).
Molybdenum and tungsten are very similar to each other and show
important differences to the smaller chromium, and seaborgium is
expected to follow the chemistry of tungsten and molybdenum quite
closely, forming an even greater variety of oxoanions, the simplest
among them being seaborgate, , which would form from the rapid
hydrolysis of , although this would take place less readily than with
molybdenum and tungsten as expected from seaborgium's greater size.
Seaborgium should hydrolyse less readily than tungsten in hydrofluoric
acid at low concentrations, but more readily at high concentrations,
also forming complexes such as SgO3F− and : complex formation competes
with hydrolysis in hydrofluoric acid.
Experimental chemistry
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Experimental chemical investigation of seaborgium has been hampered
due to the need to produce it one atom at a time, its short half-life,
and the resulting necessary harshness of the experimental conditions.
The isotope 265Sg and its isomer 265mSg are advantageous for
radiochemistry: they are produced in the 248Cm(22Ne,5n) reaction.
In the first experimental chemical studies of seaborgium in 1995 and
1996, seaborgium atoms were produced in the reaction
248Cm(22Ne,4n)266Sg, thermalised, and reacted with an O2/HCl mixture.
The adsorption properties of the resulting oxychloride were measured
and compared with those of molybdenum and tungsten compounds. The
results indicated that seaborgium formed a volatile oxychloride akin
to those of the other group 6 elements, and confirmed the decreasing
trend of oxychloride volatility down group 6:
:Sg + + 2 HCl → +
In 2001, a team continued the study of the gas phase chemistry of
seaborgium by reacting the element with O2 in a H2O environment. In a
manner similar to the formation of the oxychloride, the results of the
experiment indicated the formation of seaborgium oxide hydroxide, a
reaction well known among the lighter group 6 homologues as well as
the pseudohomologue uranium.
:2 Sg + 3 → 2
: + →
Predictions on the aqueous chemistry of seaborgium have largely been
confirmed. In experiments conducted in 1997 and 1998, seaborgium was
eluted from cation-exchange resin using a HNO3/HF solution, most
likely as neutral SgO2F2 or the anionic complex ion [SgO2F3]− rather
than . In contrast, in 0.1 M nitric acid, seaborgium does not elute,
unlike molybdenum and tungsten, indicating that the hydrolysis of
[Sg(H2O)6]6+ only proceeds as far as the cationic complex
[Sg(OH)4(H2O)]2+ or [SgO(OH)3(H2O)2]+, while that of molybdenum and
tungsten proceed to neutral [MO2(OH)2].
The only other oxidation state known for seaborgium other than the
group oxidation state of +6 is the zero oxidation state. Similarly to
its three lighter congeners, forming chromium hexacarbonyl, molybdenum
hexacarbonyl, and tungsten hexacarbonyl, seaborgium has been shown in
2014 to also form seaborgium hexacarbonyl, Sg(CO)6. Like its
molybdenum and tungsten homologues, seaborgium hexacarbonyl is a
volatile compound that reacts readily with silicon dioxide.
Absence in nature
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Searches for long-lived primordial nuclides of seaborgium in nature
have all yielded negative results. One 2022 study estimated the
concentration of seaborgium atoms in natural tungsten (its chemical
homolog) is less than atom(Sg)/atom(W).
External links
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* [
http://www.rsc.org/periodic-table/podcast Chemistry in its element
podcast] (MP3) from the Royal Society of Chemistry's Chemistry World:
[
http://www.rsc.org/periodic-table/element/106/seaborgium#podcast
Seaborgium]
* [
http://www.periodicvideos.com/videos/106.htm Seaborgium] at 'The
Periodic Table of Videos' (University of Nottingham)
* [
http://www.webelements.com/webelements/elements/text/Sg/index.html
WebElements.com - Seaborgium]
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Seaborgium
