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=                            Mendelevium                             =
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                            Introduction
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Mendelevium is a synthetic chemical element; it has symbol Md
(formerly Mv) and atomic number 101. A metallic radioactive
transuranium element in the actinide series, it is the first element
by atomic number that currently cannot be produced in macroscopic
quantities by neutron bombardment of lighter elements. It is the
thirteenth actinide, the ninth transuranic element, and the first
transfermium; it is named after Dmitri Mendeleev, the father of the
periodic table.

Like all the transfermiums, it can only be produced in particle
accelerators by bombarding lighter elements with charged particles.
The element was first produced in 1955 by bombarding einsteinium with
alpha particles, the method still used today. Using commonly-available
microgram quantities of einsteinium-253, over a million mendelevium
atoms may be made each hour. The chemistry of mendelevium is typical
for the late actinides, with a dominant +3 oxidation state but also a
+2 oxidation state accessible in solution. All known isotopes of
mendelevium have short half-lives; there are currently no uses for it
outside basic scientific research, and only small amounts are
produced.


                             Discovery
======================================================================
Mendelevium was the ninth transuranic element to be synthesized. It
was first synthesized by Albert Ghiorso, Glenn T. Seaborg, Gregory
Robert Choppin, Bernard G. Harvey, and team leader Stanley G. Thompson
in early 1955 at the University of California, Berkeley. The team
produced 256Md (half-life 77.7 minutes) when they bombarded an 253Es
target consisting of only a billion (109) einsteinium atoms with alpha
particles (helium nuclei) in the Berkeley Radiation Laboratory's
60-inch cyclotron, thus increasing the target's atomic number by two.
256Md thus became the first isotope of any element to be synthesized
one atom at a time. In total, seventeen mendelevium atoms were
detected. This discovery was part of a program, begun in 1952, that
irradiated plutonium with neutrons to transmute it into heavier
actinides. This method was necessary because of a lack of known beta
decaying isotopes of fermium that might allow production by neutron
capture; it is now known that such production is impossible at any
possible reactor flux due to the very short half-life to spontaneous
fission of 258Fm and subsequent isotopes, which still do not beta
decay - the fermium gap that, as far as we know, sets a hard limit to
the success of neutron capture processes.


To predict if the production of mendelevium would be possible, the
team made use of a rough calculation. The number of atoms that would
be produced would be approximately equal to the product of the number
of atoms of target material, the target's cross section, the ion beam
intensity, and the time of bombardment; this last factor was related
to the half-life of the product when bombarding for a time on the
order of its half-life. This gave one atom per experiment. Thus under
optimum conditions, the preparation of only one atom of element 101
per experiment could be expected. This calculation demonstrated that
it was feasible to go ahead with the experiment. The target material,
einsteinium-253, could be produced readily from irradiating plutonium:
one year of irradiation would give a billion atoms, and its three-week
half-life meant that the element 101 experiments could be conducted in
one week after the produced einsteinium was separated and purified to
make the target. However, it was necessary to upgrade the cyclotron to
obtain the needed intensity of 1014 alpha particles per second;
Seaborg applied for the necessary funds.

While Seaborg applied for funding, Harvey worked on the einsteinium
target, while Thomson and Choppin focused on methods for chemical
isolation. Choppin suggested using α-hydroxyisobutyric acid to
separate the mendelevium atoms from those of the lighter actinides.
The initial separation was done by a recoil technique suggested by
Albert Ghiorso: the einsteinium was placed on the opposite side of the
target from the beam, so that the momentum of the recoiling
mendelevium atoms would allow them to leave the target and be caught
on a gold catcher foil behind it. This recoil target was made by an
electroplating technique, developed by Alfred Chetham-Strode. This
technique gave a very high yield, which was absolutely necessary when
working with such a rare and valuable product as the einsteinium
target material. The recoil target consisted of 109 atoms of 253Es
which were deposited electrolytically on a thin gold foil. It was
bombarded by 41 MeV alpha particles in the Berkeley cyclotron with a
very high beam density of 6×1013 particles per second over an area of
0.05 cm2. The target was cooled by water or liquid helium, and the
foil could be replaced.

Initial experiments were carried out in September 1954. No alpha decay
was seen from mendelevium atoms; thus, Ghiorso suggested that the
mendelevium had all decayed by electron capture to fermium-256,
correctly believed to decay primarily by fission, and that the
experiment should be repeated, this time searching for those
spontaneous fission events. This version of the experiment was
performed in February 1955.

On the day of discovery, 19 February, alpha irradiation of the
einsteinium target occurred in three three-hour sessions. The
cyclotron was in the University of California campus, while the
Radiation Laboratory was on the next hill. To deal with this
situation, a complex procedure was used: Ghiorso took the catcher
foils (there were three targets and three foils) from the cyclotron to
Harvey, who would use aqua regia to dissolve it and pass it through an
anion-exchange resin column to separate the transuranium elements from
the gold and other products. The resultant drops entered a test tube,
which Choppin and Ghiorso took in a car to get to the Radiation
Laboratory as soon as possible. Thompson and Choppin used a
cation-exchange resin column and the α-hydroxyisobutyric acid. The
solution drops were collected on platinum disks and dried under heat
lamps. The three disks were expected to contain, respectively, the
fermium, no new elements, and the mendelevium. Finally, they were
placed in their own counters, which were connected to recorders such
that spontaneous fission events would be recorded as huge deflections
in a graph showing the number and time of the decays. There thus was
no direct detection, but by observation of spontaneous fission events
arising from its electron-capture daughter 256Fm. The first one was
identified with a "hooray" followed by a "double hooray" and a "triple
hooray". The fourth one eventually officially proved the chemical
identification of the 101st element, mendelevium. In total, five
decays were reported up until 4 a.m. Seaborg was notified and the team
left to sleep. Additional analysis and further experimentation showed
the produced mendelevium isotope to have the expected mass of 256 and
decay by electron capture to fermium-256 (half-life 157.6 minutes),
the source of the observed fission.



Being the first of the second hundred of the chemical elements, it was
decided that the element would be named "mendelevium" after the
Russian chemist Dmitri Mendeleev, father of the periodic table.
Because this discovery came during the Cold War, Seaborg had to
request permission from the government of the United States to propose
that the element be named for a Russian, but it was granted. The name
"mendelevium" was accepted by the International Union of Pure and
Applied Chemistry (IUPAC) in 1955 with symbol "Mv", which was changed
to "Md" in the next IUPAC General Assembly (Paris, 1957).


Physical
==========
In the periodic table, mendelevium is located to the right of the
actinide fermium, to the left of the actinide nobelium, and below the
lanthanide thulium. Mendelevium metal has not yet been prepared in
bulk quantities, and bulk preparation is currently impossible.
Nevertheless, a number of predictions and some preliminary
experimental results have been done regarding its properties.

The lanthanides and actinides, in the metallic state, can exist as
either divalent (such as europium and ytterbium) or trivalent (most
other lanthanides) metals. The former have f'n's2 configurations,
whereas the latter have f'n'−1d1s2 configurations. In 1975, Johansson
and Rosengren examined the measured and predicted values for the
cohesive energies (enthalpies of crystallization) of the metallic
lanthanides and actinides, both as divalent and trivalent metals. The
conclusion was that the increased binding energy of the [Rn]5f126d17s2
configuration over the [Rn]5f137s2 configuration for mendelevium was
not enough to compensate for the energy needed to promote one 5f
electron to 6d, as is true also for the very late actinides: thus
einsteinium, fermium, mendelevium, and nobelium were expected to be
divalent metals. The increasing predominance of the divalent state
well before the actinide series concludes is attributed to the
relativistic stabilization of the 5f electrons, which increases with
increasing atomic number. Thermochromatographic studies with trace
quantities of mendelevium by Zvara and Hübener from 1976 to 1982
confirmed this prediction. In 1990, Haire and Gibson estimated
mendelevium metal to have an enthalpy of sublimation between 134 and
142 kJ/mol. Divalent mendelevium metal should have a metallic radius
of around . Like the other divalent late actinides (except the once
again trivalent lawrencium), metallic mendelevium should assume a
face-centered cubic crystal structure. Mendelevium's melting point has
been estimated at 800 °C, the same value as that predicted for the
neighbouring element nobelium. Its density is predicted to be around .


Chemical
==========
The chemistry of mendelevium is known largely in solution (as
available quantities do not allow the creation of pure compounds), in
which it can take on the +3 or +2 oxidation states. The +1 state has
also been reported, but has not yet been confirmed.

Before mendelevium's discovery, Seaborg and Katz predicted that it
should be predominantly trivalent in aqueous solution and hence should
behave similarly to other tripositive lanthanides and actinides. After
the synthesis of mendelevium in 1955, these predictions were
confirmed, first in the observation at its discovery that it eluted
just after fermium in the trivalent actinide elution sequence from a
cation-exchange column of resin, and later the 1967 observation that
mendelevium could form insoluble hydroxides and fluorides that
coprecipitated with trivalent lanthanide salts. Cation-exchange and
solvent extraction studies led to the conclusion that mendelevium was
a trivalent actinide with an ionic radius somewhat smaller than that
of the previous actinide, fermium. Mendelevium can form coordination
complexes with 1,2-cyclohexanedinitrilotetraacetic acid (DCTA).

In reducing conditions, mendelevium(III) can be easily reduced to
mendelevium(II), which is stable in aqueous solution. The standard
reduction potential of the 'E'°(Md3+→Md2+) couple was variously
estimated in 1967 as −0.10 V or −0.20 V: later 2013 experiments
established the value as .
In comparison, 'E'°(Md3+→Md0) should be around −1.74 V, and
'E'°(Md2+→Md0) should be around −2.5 V. Mendelevium(II)'s elution
behavior has been compared with that of strontium(II) and
europium(II).

In 1973, mendelevium(I) was reported to have been produced by Russian
scientists, who obtained it by reducing higher oxidation states of
mendelevium with samarium(II). It was found to be stable in neutral
water-ethanol solution and be homologous to caesium(I). However, later
experiments found no evidence for mendelevium(I) and found that
mendelevium behaved like divalent elements when reduced, not like the
monovalent alkali metals. Nevertheless, the Russian team conducted
further studies on the thermodynamics of cocrystallizing mendelevium
with alkali metal chlorides, and concluded that mendelevium(I) had
formed and could form mixed crystals with divalent elements, thus
cocrystallizing with them. The status of the +1 oxidation state is
still tentative.

The electrode potential 'E'°(Md4+→Md3+) was predicted in 1975 to be
+5.4 V; 1967 experiments with the strong oxidizing agent sodium
bismuthate were unable to oxidize mendelevium(III) to mendelevium(IV).


Atomic
========
A mendelevium atom has 101 electrons. They are expected to be arranged
in the configuration [Rn]5f137s2 (ground state term symbol 2F7/2),
although experimental verification of this electron configuration had
not yet been made as of 2006. The fifteen electrons in the 5f and 7s
subshells are valence electrons. In forming compounds, three valence
electrons may be lost, leaving behind a [Rn]5f12 core: this conforms
to the trend set by the other actinides with their [Rn] 5f'n' electron
configurations in the tripositive state. The first ionization
potential of mendelevium was measured to be at most (6.58 ± 0.07) eV
in 1974, based on the assumption that the 7s electrons would ionise
before the 5f ones; this value has not yet been refined further due to
the lack to larger samples of mendelevium. The ionic radius of
hexacoordinate Md3+ had been preliminarily estimated in 1978 to be
around 91.2 pm; 1988 calculations based on the logarithmic trend
between distribution coefficients and ionic radius produced a value of
89.6 pm, as well as an enthalpy of hydration of . Md2+ should have an
ionic radius of 115 pm and hydration enthalpy −1413 kJ/mol; Md+ should
have ionic radius 117 pm.


Isotopes
==========
Seventeen isotopes of mendelevium are known, with mass numbers from
244 to 260; all are radioactive. The longest-lived isotope is 258Md
with a half-life of 51.6 days. Nevertheless, the shorter-lived 256Md
(half-life 77.7 minutes) is more often used in chemical experiments
because it can be produced in larger quantities from einsteinium, as
258Md would require 255Es, of which significant quantities are
available only as a minor component of an isotopic mixture.

The half-lives of mendelevium isotopes mostly increase smoothly (apart
from odd/even effects) toward higher mass, up to 258Md, then decrease
(as indicated by what experimental data is available) as spontaneous
fission becomes the dominant decay mode; the second longest-living
isotope is 260Md, the heaviest known, with a half-life of 27.8 days.
Mendelevium is the last element that has any known isotope with a
half-life longer than a day.

Mendelevium-256, the currently most important isotope of mendelevium,
decays about 90% through electron capture and 10% through alpha decay.
It is most easily detected through the spontaneous fission of its
electron capture daughter fermium-256, but in the presence of other
nuclides that undergo spontaneous fission, alpha decays at the
characteristic energies for mendelevium-256 (7.205 and 7.139 MeV) can
provide more useful identification.


                      Production and isolation
======================================================================
The lightest isotopes (244Md to 247Md) are mostly produced through
bombardment of bismuth targets with argon ions, while slightly heavier
ones (248Md to 253Md) are produced by bombarding plutonium and
americium targets with ions of carbon and nitrogen. The most important
and most stable isotopes are in the range from 254Md to 258Md and are
produced through bombardment of einsteinium with alpha particles:
einsteinium-253, −254, and −255 can all be used. 259Md is produced as
a daughter of 259No, and 260Md can be produced in a transfer reaction
between einsteinium-254 and oxygen-18. Typically, the most commonly
used isotope 256Md is produced by bombarding either einsteinium-253 or
−254 with alpha particles: einsteinium-254 is preferred when available
because it has a longer half-life and therefore can be used as a
target for longer. Using available microgram quantities of
einsteinium, femtogram quantities of mendelevium-256 may be produced.

The recoil momentum of the produced mendelevium-256 atoms is used to
bring them physically far away from the einsteinium target from which
they are produced, bringing them onto a thin foil of metal (usually
beryllium, aluminium, platinum, or gold) just behind the target in a
vacuum. This eliminates the need for immediate chemical separation,
which is both costly and prevents reusing of the expensive einsteinium
target. The mendelevium atoms are then trapped in a gas atmosphere
(frequently helium), and a gas jet from a small opening in the
reaction chamber carries the mendelevium along. Using a long capillary
tube, and including potassium chloride aerosols in the helium gas, the
mendelevium atoms can be transported over tens of meters to be
chemically analysed and have their quantity determined. The
mendelevium can then be separated from the foil material and other
fission products by applying acid to the foil and then coprecipitating
the mendelevium with lanthanum fluoride, then using a cation-exchange
resin column with a 10% ethanol solution saturated with hydrochloric
acid, acting as an eluant. However, if the foil is made of gold and
thin enough, it is enough to simply dissolve the gold in aqua regia
before separating the trivalent actinides from the gold using
anion-exchange chromatography, the eluant being 6 M hydrochloric acid.

Mendelevium can finally be separated from the other trivalent
actinides using selective elution from a cation-exchange resin column,
the eluant being ammonia α-HIB.< Using the gas-jet method often
renders the first two steps unnecessary.

Another possible way to separate the trivalent actinides is via
solvent extraction chromatography using bis-(2-ethylhexyl) phosphoric
acid (abbreviated as HDEHP) as the stationary organic phase and nitric
acid as the mobile aqueous phase. The actinide elution sequence is
reversed from that of the cation-exchange resin column, so that the
heavier actinides elute later. The mendelevium separated by this
method has the advantage of being free of organic complexing agent
compared to the resin column; the disadvantage is that mendelevium
then elutes very late in the elution sequence, after fermium.

Another method to isolate mendelevium exploits the distinct elution
properties of Md2+ from those of Es3+ and Fm3+. The initial steps are
the same as above, and employs HDEHP for extraction chromatography,
but coprecipitates the mendelevium with terbium fluoride instead of
lanthanum fluoride. Then, 50 mg of chromium is added to the
mendelevium to reduce it to the +2 state in 0.1 M hydrochloric acid
with zinc or mercury. The solvent extraction then proceeds, and while
the trivalent and tetravalent lanthanides and actinides remain on the
column, mendelevium(II) does not and stays in the hydrochloric acid.
It is then reoxidized to the +3 state using hydrogen peroxide and then
isolated by selective elution with 2 M hydrochloric acid (to remove
impurities, including chromium) and finally 6 M hydrochloric acid (to
remove the mendelevium). It is also possible to use a column of
cationite and zinc amalgam, using 1 M hydrochloric acid as an eluant,
to effect the reduction. Thermochromatographic chemical isolation
could be achieved using the volatile mendelevium
hexafluoroacetylacetonate: the analogous fermium compound is known and
similar.


                              Toxicity
======================================================================
Though few people come in contact with mendelevium, the International
Commission on Radiological Protection has set annual exposure limits
for the most stable isotope. For mendelevium-258, the ingestion limit
was set at 9×105 becquerels (1 Bq = 1 decay per second). Given the
half-life of this isotope, this is only 2.48 ng (nanograms). The
inhalation limit is at 6000 Bq or 16.5 pg (picogram).


                          Further reading
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* Hoffman, D.C., Ghiorso, A., Seaborg, G. T. The transuranium people:
the inside story, (2000), 201-229
* Morss, L. R., Edelstein, N. M., Fuger, J., The chemistry of the
actinide and transactinide element, 3, (2006), 1630-1636
* 'A Guide to the Elements - Revised Edition', Albert Stwertka,
(Oxford University Press; 1998)


                           External links
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*[https://periodic.lanl.gov/101.shtml Los Alamos National Laboratory -
Mendelevium]
*[https://education.jlab.org/itselemental/ele101.html It's Elemental -
Mendelevium]
* [http://www.periodicvideos.com/videos/101.htm Mendelevium] at 'The
Periodic Table of Videos' (University of Nottingham)
*[https://environmentalchemistry.com/yogi/periodic/Md.html
Environmental Chemistry - Md info]


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