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= Promethium =
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Introduction
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Promethium is a chemical element; it has symbol Pm and atomic number
61. All of its isotopes are radioactive; it is extremely rare, with
only about 500-600 grams naturally occurring in the Earth's crust at
any given time. Promethium is one of the only two radioactive elements
that are both preceded and followed in the periodic table by elements
with stable forms, the other being technetium. Chemically, promethium
is a lanthanide. Promethium shows only one stable oxidation state of
+3.
In 1902 Bohuslav Brauner suggested that there was a then-unknown
element with properties intermediate between those of the known
elements neodymium (60) and samarium (62); this was confirmed in 1914
by Henry Moseley, who, having measured the atomic numbers of all the
elements then known, found that the element with atomic number 61 was
missing. In 1926, two groups (one Italian and one American) claimed to
have isolated a sample of element 61; both "discoveries" were soon
proven to be false. In 1938, during a nuclear experiment conducted at
Ohio State University, a few radioactive nuclides were produced that
certainly were not radioisotopes of neodymium or samarium, but there
was a lack of chemical proof that element 61 was produced, and the
discovery was not much recognized. Promethium was first produced and
characterized at Oak Ridge National Laboratory in 1945 by the
separation and analysis of the fission products of uranium fuel
irradiated in a graphite reactor. The discoverers proposed the name
"prometheum" (the spelling was subsequently changed), derived from
Prometheus, the Titan in Greek mythology who stole fire from Mount
Olympus and brought it down to humans, to symbolize "both the daring
and the possible misuse of mankind's intellect". A sample of the metal
was made only in 1963.
The two sources of natural promethium are rare alpha decays of natural
europium-151 (producing promethium-147) and spontaneous fission of
uranium (various isotopes). Promethium-145 is the most stable
promethium isotope, but the only isotope with practical applications
is promethium-147, chemical compounds of which are used in luminous
paint, atomic batteries and thickness-measurement devices. Because
natural promethium is exceedingly scarce, it is typically synthesized
by bombarding uranium-235 (enriched uranium) with thermal neutrons to
produce promethium-147 as a fission product.
Physical properties
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A promethium atom has 61 electrons, arranged in the configuration [Xe]
4f5 6s2. The seven 4f and 6s electrons are valence electrons. In
forming compounds, the atom loses its two outermost electrons and one
4f-electron, which belongs to an open subshell. The element's atomic
radius is the second largest among all the lanthanides but is only
slightly greater than those of the neighboring elements. It is the
most notable exception to the general trend of the contraction of
lanthanide atoms with the increase of their atomic numbers (lanthanide
contraction). Many properties of promethium rely on its position among
lanthanides and are intermediate between those of neodymium and
samarium. For example, the melting point, the first three ionization
energies, and the hydration energy are greater than those of neodymium
and lower than those of samarium; similarly, the estimate for the
boiling point, ionic (Pm3+) radius, and standard heat of formation of
monatomic gas are greater than those of samarium and less than those
of neodymium.
Promethium has a double hexagonal close packed (dhcp) structure and a
hardness of 63 kg/mm2. This low-temperature alpha form converts into a
beta, body-centered cubic (bcc) phase upon heating to 890 °C.
Chemical properties and compounds
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Promethium belongs to the cerium group of lanthanides and is
chemically very similar to the neighboring elements. Because of its
instability, chemical studies of promethium are incomplete. Even
though a few compounds have been synthesized, they are not fully
studied; in general, they tend to be pink or red in color. In May
2024, a promethium coordination complex with neutral PyDGA ligands was
characterized in aqueous solution. Treatment of acidic solutions
containing ions with ammonia results in a gelatinous light-brown
sediment of hydroxide, Promethium hydroxide, which is insoluble in
water. When dissolved in hydrochloric acid, a water-soluble yellow
salt, Promethium(III) chloride, is produced; similarly, when dissolved
in nitric acid, a nitrate results, Promethium(III) nitrate. The latter
is also well-soluble; when dried, it forms pink crystals, similar to
Neodymium(III) nitrate. The electron configuration for is [Xe] 4f4,
and the color of the ion is pink. The ground state term symbol is 5I4.
The sulfate is slightly soluble, like the other cerium group sulfates.
Cell parameters have been calculated for its octahydrate; they lead to
conclusion that the density of is 2.86 g/cm3. The oxalate, , has the
lowest solubility of all lanthanide oxalates.
Unlike the nitrate, the oxide is similar to the corresponding samarium
salt and not the neodymium salt. As-synthesized, e.g. by heating the
oxalate, it is a white or lavender-colored powder with disordered
structure. This powder crystallizes in a cubic lattice upon heating to
600 °C. Further annealing at 800 °C and then at 1750 °C irreversibly
transforms it to monoclinic and hexagonal phases, respectively, and
the last two phases can be interconverted by adjusting the annealing
time and temperature.
Formula symmetry space group No Pearson symbol 'a' (pm) 'b' (pm) 'c'
(pm) 'Z' density, g/cm3
α-Pm dhcp P63/mmc 194 hP4 365 365 1165 4 7.26
β-Pm bcc Fmm 225 cF4 410 410 410 4 6.99
Pm2O3 cubic Ia 206 cI80 1099 1099 1099 16 6.77
Pm2O3 monoclinic C2/m 12 mS30 1422 365 891 6 7.40
Pm2O3 hexagonal Pm1 164 hP5 380.2 380.2 595.4 1 7.53
Promethium forms only one stable oxidation state, +3, in the form of
ions; this is in line with other lanthanides. Promethium can also form
the +2 oxidation state. Thermodynamic properties of Pm2+ suggests that
the dihalides are stable, similar to NdCl2 and SmCl2.
Promethium halides
Formula color coordination number symmetry space group No Pearson
symbol m.p. (°C)
|PmF3 |Purple-pink |11 |hexagonal |Pc1 |165 |hP24 |1338
|PmCl3 |Lavender |9 |hexagonal |P63/mc |176 |hP8 |655
|PmBr3 |Red |8 |orthorhombic |Cmcm |63 |oS16 |624
|α-PmI3 |Red |8 |orthorhombic |Cmcm |63 |oS16 |α→β
|β-PmI3 |Red |6 |rhombohedral |R |148 |hR24 |695
Isotopes
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Promethium is the only lanthanide and one of only two elements among
the first 82 with no stable or long-lived (primordial) isotopes. This
is a result of a rarely occurring effect of the liquid drop model and
stabilities of neighbor element isotopes; it is also the least stable
element of the first 84. The primary decay products are neodymium and
samarium isotopes (promethium-146 decays to both, the lighter isotopes
generally to neodymium via positron decay and electron capture, and
the heavier isotopes to samarium via beta decay). Promethium nuclear
isomers may decay to other promethium isotopes and one isotope (145Pm)
has a very rare alpha decay mode to stable praseodymium-141.
The most stable isotope of the element is promethium-145, which has a
specific activity of 139 Ci/g and a half-life of 17.7 years via
electron capture. Because it has 84 neutrons (two more than 82, which
is a magic number corresponding to a stable neutron configuration), it
may emit an alpha particle (which has 2 neutrons) to form
praseodymium-141 with 82 neutrons. Thus it is the only promethium
isotope with an experimentally observed alpha decay. Its partial
half-life for alpha decay is about 6.3 years, and the relative
probability for a 145Pm nucleus to decay in this way is 2.8 %. Several
other promethium isotopes such as 144Pm, 146Pm, and 147Pm also have a
positive energy release for alpha decay; their alpha decays are
predicted to occur but have not been observed. In total, 41 isotopes
of promethium are known, ranging from 126Pm to 166Pm.
The element also has 18 nuclear isomers, with mass numbers of 133 to
142, 144, 148, 149, 152, and 154 (some mass numbers have more than one
isomer). The most stable of them is promethium-148m, with a half-life
of 43.1 days; this is longer than the half-lives of the ground states
of all promethium isotopes, except for promethium-143 to 147. In fact,
promethium-148m has a longer half-life than its ground state,
promethium-148.
Occurrence
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In 1934, Willard Libby reported that he had found weak beta activity
in pure neodymium, which was attributed to a half-life over 1012
years. Almost 20 years later, it was claimed that the element occurs
in natural neodymium in equilibrium in quantities below 10−20 grams of
promethium per one gram of neodymium. However, these observations were
disproved by newer investigations, because for all seven naturally
occurring neodymium isotopes, any single beta decays (which can
produce promethium isotopes) are forbidden by energy conservation. In
particular, careful measurements of atomic masses show that the mass
difference between 150Nd and 150Pm is negative (−87 keV), which
absolutely prevents the single beta decay of 150Nd to 150Pm.
In 1965, Olavi Erämetsä separated out traces of 147Pm from a rare
earth concentrate purified from apatite, resulting in an upper limit
of 10−21 for the abundance of promethium in nature; this may have been
produced by the natural nuclear fission of uranium, or by cosmic ray
spallation of 146Nd.
Both isotopes of natural europium have larger mass excesses than sums
of those of their potential alpha daughters plus that of an alpha
particle; therefore, they (stable in practice) may alpha decay to
promethium. Research at Laboratori Nazionali del Gran Sasso showed
that europium-151 decays to promethium-147 with the half-life of 5
years; later measurements gave the half-life as (4.62 ± 0.95(stat.) ±
0.68(syst.)) × 1018 years.
It has been shown that europium is "responsible" for about 12 grams
of promethium in the Earth's crust. Alpha decays for europium-153 have
not been found yet, and its theoretically calculated half-life is so
high (due to low energy of decay) that this process will probably not
be observed in the near future.
Promethium can also be formed in nature as a product of spontaneous
fission of uranium-238. Only trace amounts can be found in naturally
occurring ores: a sample of pitchblende has been found to contain
promethium at a concentration of four parts per quintillion (4) by
mass. Uranium is thus "responsible" for 560 g of promethium in Earth's
crust.
Promethium has also been identified in the spectrum of the star HR 465
in Andromeda; it also has been found in HD 101065 (Przybylski's star)
and HD 965. Because of the short half-life of promethium isotopes,
they should be formed near the surface of those stars.
Searches for element 61
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In 1902, Czech chemist Bohuslav Brauner found out that the differences
in properties between neodymium and samarium were the largest between
any two consecutive lanthanides in the sequence then known; as a
conclusion, he suggested there was an element with intermediate
properties between them. This prediction was supported in 1914 by
Henry Moseley who, having discovered that atomic number was an
experimentally measurable property of elements, found that a few
atomic numbers had no known corresponding elements: the gaps were 43,
61, 72, 75, 85, and 87. With the knowledge of a gap in the periodic
table several groups started to search for the predicted element among
other rare earths in the natural environment.
The first claim of a discovery was published by Luigi Rolla and
Lorenzo Fernandes of Florence, Italy. After separating a mixture of a
few rare earth elements nitrate concentrate from the Brazilian mineral
monazite by fractionated crystallization, they yielded a solution
containing mostly samarium. This solution gave x-ray spectra
attributed to samarium and element 61. In honor of their city, they
named element 61 "florentium". The results were published in 1926, but
the scientists claimed that the experiments were done in 1924. Also in
1926, a group of scientists from the University of Illinois at
Urbana-Champaign, Smith Hopkins and Len Yntema published the discovery
of element 61. They named it "illinium", after the university. Both of
these reported discoveries were shown to be erroneous because the
spectrum line that "corresponded" to element 61 was identical to that
of didymium; the lines thought to belong to element 61 turned out to
belong to a few impurities (barium, chromium, and platinum).
In 1934, Josef Mattauch finally formulated the isobar rule. One of the
indirect consequences of this rule was that element 61 was unable to
form stable isotopes. From 1938, a nuclear experiment was conducted by
H. B. Law et al. at the Ohio State University. Nuclides were produced
in 1941 which were not radioisotopes of neodymium or samarium, and the
name "cyclonium" was proposed, but there was a lack of chemical proof
that element 61 was produced and the discovery was not largely
recognized.
Discovery and synthesis of promethium metal
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Promethium was first produced and characterized at Oak Ridge National
Laboratory (Clinton Laboratories at that time) in 1945 by Jacob A.
Marinsky, Lawrence E. Glendenin and Charles D. Coryell by separation
and analysis of the fission products of uranium fuel irradiated in the
graphite reactor; however, being too busy with military-related
research during World War II, they did not announce their discovery
until 1947. The original proposed name was "clintonium", after the
laboratory where the work was conducted; however, the name
"prometheum" was suggested by Grace Mary Coryell, the wife of one of
the discoverers. It is derived from Prometheus, the Titan in Greek
mythology who stole fire from Mount Olympus and brought it down to
humans and symbolizes "both the daring and the possible misuse of the
mankind intellect". The spelling was then changed to "promethium", as
this was in accordance with most other metals.
File:Jacob A Marinsky.jpg|Jacob A. Marinsky
File:Larry E Glendenin.jpg|Lawrence E. Glendenin
File:Charles D. Coryell M.I.T. May 1947.png|Charles D. Coryell
In 1963, promethium(III) fluoride was used to make promethium metal.
Provisionally purified from impurities of samarium, neodymium, and
americium, it was put into a tantalum crucible which was located in
another tantalum crucible; the outer crucible contained lithium metal
(10 times excess compared to promethium). After creating a vacuum, the
chemicals were mixed to produce promethium metal:
:PmF3 + 3 Li → Pm + 3 LiF
The promethium sample produced was used to measure a few of the
metal's properties, such as its melting point.
In 1963, ion-exchange methods were used at ORNL to prepare about ten
grams of promethium from nuclear reactor fuel processing wastes.
Promethium can be either recovered from the byproducts of uranium
fission or produced by bombarding 146Nd with neutrons, turning it into
147Nd, which decays into 147Pm through beta decay with a half-life of
11 days.
Production
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The production methods for different isotopes vary, and only those for
promethium-147 are given because it is the only isotope with
industrial applications. Promethium-147 is produced in large
quantities (compared to other isotopes) by bombarding uranium-235 with
thermal neutrons. The output is relatively high, at 2.6% of the total
product. Another way to produce promethium-147 is via neodymium-147,
which decays to promethium-147 with a short half-life. Neodymium-147
can be obtained either by bombarding enriched neodymium-146 with
thermal neutrons or by bombarding a uranium carbide target with
energetic protons in a particle accelerator. Another method is to
bombard uranium-238 with fast neutrons to cause fast fission, which,
among multiple reaction products, creates promethium-147.
As early as the 1960s, Oak Ridge National Laboratory could produce 650
grams of promethium per year and was the world's only large-volume
synthesis facility. Gram-scale production of promethium was
discontinued in the U.S. in the early 1980s, but will possibly be
resumed after 2010 at the High Flux Isotope Reactor. In 2010, Russia
was the only country producing promethium-147 on a relatively large
scale.
Applications
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Only promethium-147 has uses outside laboratories. It is obtained as
the oxide or chloride, in milligram quantities. This isotope has a
relatively long half-life, does not emit gamma rays, and its radiation
has a relatively small penetration depth in matter.
Some signal lights use a luminous paint containing a phosphor that
absorbs the beta radiation emitted by promethium-147 and emits light.
This isotope does not cause aging of the phosphor, as alpha emitters
do, and therefore the light emission is stable for a few years.
Originally, radium-226 was used for the purpose, but it was later
replaced by promethium-147 and tritium (hydrogen-3). Promethium may be
favored over tritium for nuclear safety.
In atomic batteries, the beta particles emitted by promethium-147 are
converted into electric current by sandwiching a small promethium
source between two semiconductor plates. These batteries have a useful
lifetime of about five years. The first promethium-based battery was
assembled in 1964 and generated "a few milliwatts of power from a
volume of about 2 cubic inches, including shielding".
Promethium is also used to measure the thickness of materials by
measuring the amount of radiation from a promethium source that passes
through the sample. It has possible future uses in portable X-ray
sources, and as auxiliary heat or power sources for space probes and
satellites (although the alpha emitter plutonium-238 has become
standard for most space-exploration-related uses).
Promethium-147 is also used, albeit in very small quantities (less
than 330nCi), in some Philips CFL (Compact Fluorescent Lamp) glow
switches in the PLC 22W/28W 15mm CFL range.
Precautions
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Promethium has no biological role. Promethium-147 can emit gamma rays,
which are dangerous for all lifeforms, during its beta decay.
Interactions with tiny quantities of promethium-147 are not hazardous
if certain precautions are observed. In general, gloves, footwear
covers, safety glasses, and an outer layer of easily removed
protective clothing should be used.
It is not known what human organs are affected by interaction with
promethium; a possible candidate is the bone tissues. Sealed
promethium-147 is not dangerous. However, if the packaging is damaged,
then promethium becomes dangerous to the environment and humans. If
radioactive contamination is found, the contaminated area should be
washed with water and soap, but, even though promethium mainly affects
the skin, the skin should not be abraded. If a promethium leak is
found, the area should be identified as hazardous and evacuated, and
emergency services must be contacted. No dangers from promethium aside
from the radioactivity are known.
Bibliography
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*
*
*2013, E.R. Scerri,'A tale of seven elements,' Oxford University
Press, Oxford,
External links
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*[
http://education.jlab.org/itselemental/ele061.html It's Elemental -
Promethium]
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