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= Meitnerium =
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Introduction
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Meitnerium is a synthetic chemical element; it has symbol Mt and
atomic number 109. It is an extremely radioactive synthetic element
(an element not found in nature, but can be created in a laboratory).
The most stable known isotope, meitnerium-278, has a half-life of 4.5
seconds, although the unconfirmed meitnerium-282 may have a longer
half-life of 67 seconds. The element was first synthesized in August
1982 by the GSI Helmholtz Centre for Heavy Ion Research near
Darmstadt, Germany, and it was named after Lise Meitner in 1997.
In the periodic table, meitnerium is a d-block transactinide element.
It is a member of the 7th period and is placed in the group 9
elements, although no chemical experiments have yet been carried out
to confirm that it behaves as the heavier homologue to iridium in
group 9 as the seventh member of the 6d series of transition metals.
Meitnerium is calculated to have properties similar to its lighter
homologues, cobalt, rhodium, and iridium.
Discovery
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Meitnerium was first synthesized on August 29, 1982, by a German
research team led by Peter Armbruster and Gottfried Münzenberg at the
Institute for Heavy Ion Research (Gesellschaft für
Schwerionenforschung) in Darmstadt. The team bombarded a target of
bismuth-209 with accelerated nuclei of iron-58 and detected a single
atom of the isotope meitnerium-266:
: + → +
This work was confirmed three years later at the Joint Institute for
Nuclear Research at Dubna (then in the Soviet Union).
Naming
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Using Mendeleev's nomenclature for unnamed and undiscovered elements,
meitnerium should be known as 'eka-iridium'. In 1979, during the
Transfermium Wars (but before the synthesis of meitnerium), IUPAC
published recommendations according to which the element was to be
called 'unnilennium' (with the corresponding symbol of 'Une'), a
systematic element name as a placeholder, until the element was
discovered (and the discovery then confirmed) and a permanent name was
decided on. Although widely used in the chemical community on all
levels, from chemistry classrooms to advanced textbooks, the
recommendations were mostly ignored among scientists in the field, who
either called it "element 109", with the symbol of 'E109', '(109)' or
even simply '109', or used the proposed name "meitnerium".
The naming of meitnerium was discussed in the element naming
controversy regarding the names of elements 104 to 109, but
'meitnerium' was the only proposal and thus was never disputed. The
name 'meitnerium' (Mt) was suggested by the GSI team in September 1992
in honor of the Austrian physicist Lise Meitner, a co-discoverer of
protactinium (with Otto Hahn), and one of the discoverers of nuclear
fission. In 1994 the name was recommended by IUPAC, and was officially
adopted in 1997. It is thus the only element named specifically after
a non-mythological woman (curium being named for both Pierre and Marie
Curie).
Isotopes
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Meitnerium 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.
Eight different isotopes of meitnerium have been reported with mass
numbers 266, 268, 270, and 274-278, two of which, meitnerium-268 and
meitnerium-270, have unconfirmed metastable states. A ninth isotope
with mass number 282 is unconfirmed. Most of these decay predominantly
through alpha decay, although some undergo spontaneous fission.
Stability and half-lives
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{{Isotopes summary
|element=meitnerium
|reaction ref=
|isotopes=
}}
All meitnerium isotopes are extremely unstable and radioactive; in
general, heavier isotopes are more stable than the lighter. The most
stable known meitnerium isotope, 278Mt, is also the heaviest known; it
has a half-life of 4.5 seconds. The unconfirmed 282Mt is even heavier
and appears to have a longer half-life of 67 seconds. With a half-life
of 0.8 seconds, the next most stable known isotope is 270Mt. The
isotopes 276Mt and 274Mt have half-lives of 0.62 and 0.64 seconds
respectively.
The isotope 277Mt, created as the final decay product of 293Ts for the
first time in 2012, was observed to undergo spontaneous fission with a
half-life of 5 milliseconds. Preliminary data analysis considered the
possibility of this fission event instead originating from 277Hs, for
it also has a half-life of a few milliseconds, and could be populated
following undetected electron capture somewhere along the decay chain.
This possibility was later deemed very unlikely based on observed
decay energies of 281Ds and 281Rg and the short half-life of 277Mt,
although there is still some uncertainty of the assignment.
Regardless, the rapid fission of 277Mt and 277Hs is strongly
suggestive of a region of instability for superheavy nuclei with 'N' =
168-170. The existence of this region, characterized by a decrease in
fission barrier height between the deformed shell closure at 'N' = 162
and spherical shell closure at 'N' = 184, is consistent with
theoretical models.
Predicted properties
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Other than nuclear properties, no properties of meitnerium or its
compounds have been measured; this is due to its extremely limited and
expensive production and the fact that meitnerium and its parents
decay very quickly. Properties of meitnerium metal remain unknown and
only predictions are available.
Chemical
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Meitnerium is the seventh member of the 6d series of transition
metals, and should be much like the platinum group metals.
Calculations on its ionization potentials and atomic and ionic radii
are similar to that of its lighter homologue iridium, thus implying
that meitnerium's basic properties will resemble those of the other
group 9 elements, cobalt, rhodium, and iridium.
Prediction of the probable chemical properties of meitnerium has not
received much attention recently. Meitnerium is expected to be a noble
metal. The standard electrode potential for the Mt3+/Mt couple is
expected to be 0.8 V. Based on the most stable oxidation states of the
lighter group 9 elements, the most stable oxidation states of
meitnerium are predicted to be the +6, +3, and +1 states, with the +3
state being the most stable in aqueous solutions. In comparison,
rhodium and iridium show a maximum oxidation state of +6, while the
most stable states are +4 and +3 for iridium and +3 for rhodium. The
oxidation state +9, represented only by iridium in [IrO4]+, might be
possible for its congener meitnerium in the nonafluoride (MtF9) and
the [MtO4]+ cation, although [IrO4]+ is expected to be more stable
than these meitnerium compounds. The tetrahalides of meitnerium have
also been predicted to have similar stabilities to those of iridium,
thus also allowing a stable +4 state. It is further expected that the
maximum oxidation states of elements from bohrium (element 107) to
darmstadtium (element 110) may be stable in the gas phase but not in
aqueous solution.
Physical and atomic
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Meitnerium is expected to be a solid under normal conditions and
assume a face-centered cubic crystal structure, similarly to its
lighter congener iridium. It should be a very heavy metal with a
density of around 27-28 g/cm3, which would be among the highest of any
of the 118 known elements. Meitnerium is also predicted to be
paramagnetic.
Theoreticians have predicted the covalent radius of meitnerium to be 6
to 10 pm larger than that of iridium. The atomic radius of meitnerium
is expected to be around 128 pm.
Experimental chemistry
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Meitnerium is the first element on the periodic table whose chemistry
has not yet been investigated. Unambiguous determination of the
chemical characteristics of meitnerium has yet to have been
established due to the short half-lives of meitnerium isotopes and a
limited number of likely volatile compounds that could be studied on a
very small scale. One of the few meitnerium compounds that are likely
to be sufficiently volatile is meitnerium hexafluoride (), as its
lighter homologue iridium hexafluoride () is volatile above 60 °C and
therefore the analogous compound of meitnerium might also be
sufficiently volatile; a volatile octafluoride () might also be
possible. For chemical studies to be carried out on a transactinide,
at least four atoms must be produced, the half-life of the isotope
used must be at least 1 second, and the rate of production must be at
least one atom per week. Even though the half-life of 278Mt, the most
stable confirmed meitnerium isotope, is 4.5 seconds, long enough to
perform chemical studies, another obstacle is the need to increase the
rate of production of meitnerium isotopes and allow experiments to
carry on for weeks or months so that statistically significant results
can be obtained. Separation and detection must be carried out
continuously to separate out the meitnerium isotopes and have
automated systems experiment on the gas-phase and solution chemistry
of meitnerium, as the yields for heavier elements are predicted to be
smaller than those for lighter elements; some of the separation
techniques used for bohrium and hassium could be reused. However, the
experimental chemistry of meitnerium has not received as much
attention as that of the heavier elements from copernicium to
livermorium.
The Lawrence Berkeley National Laboratory attempted to synthesize the
isotope 271Mt in 2002-2003 for a possible chemical investigation of
meitnerium, because it was expected that it might be more stable than
nearby isotopes due to having 162 neutrons, a magic number for
deformed nuclei; its half-life was predicted to be a few seconds, long
enough for a chemical investigation. However, no atoms of 271Mt were
detected; this isotope of meitnerium is currently unknown.
An experiment determining the chemical properties of a transactinide
would need to compare a compound of that transactinide with analogous
compounds of some of its lighter homologues: for example, in the
chemical characterization of hassium, hassium tetroxide (HsO4) was
compared with the analogous osmium compound, osmium tetroxide (OsO4).
In a preliminary step towards determining the chemical properties of
meitnerium, the GSI attempted sublimation of the rhodium compounds
rhodium(III) oxide (Rh2O3) and rhodium(III) chloride (RhCl3). However,
macroscopic amounts of the oxide would not sublimate until 1000 °C and
the chloride would not until 780 °C, and then only in the presence of
carbon aerosol particles: these temperatures are far too high for such
procedures to be used on meitnerium, as most of the current methods
used for the investigation of the chemistry of superheavy elements do
not work above 500 °C.
Following the 2014 successful synthesis of seaborgium hexacarbonyl,
Sg(CO)6, studies were conducted with the stable transition metals of
groups 7 through 9, suggesting that carbonyl formation could be
extended to further probe the chemistries of the early 6d transition
metals from rutherfordium to meitnerium inclusive. Nevertheless, the
challenges of low half-lives and difficult production reactions make
meitnerium difficult to access for radiochemists, though the isotopes
278Mt and 276Mt are long-lived enough for chemical research and may be
produced in the decay chains of 294Ts and 288Mc respectively. 276Mt is
likely more suitable, since producing tennessine requires a rare and
rather short-lived berkelium target. The isotope 270Mt, observed in
the decay chain of 278Nh with a half-life of 0.69 seconds, may also be
sufficiently long-lived for chemical investigations, though a direct
synthesis route leading to this isotope and more precise measurements
of its decay properties would be required.
External links
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* [
http://www.periodicvideos.com/videos/109.htm Meitnerium] at 'The
Periodic Table of Videos' (University of Nottingham)
License
=========
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Original Article:
http://en.wikipedia.org/wiki/Meitnerium
