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= Actinium =
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Introduction
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Actinium is a chemical element; it has symbol Ac and atomic number 89.
It was discovered by Friedrich Oskar Giesel in 1902, who gave it the
name 'emanium'; the element got its name by being wrongly identified
with a substance André-Louis Debierne found in 1899 and called
actinium. The actinide series, a set of 15 elements between actinium
and lawrencium in the periodic table, are named for actinium. Together
with polonium, radium, and radon, actinium was one of the first
non-primordial radioactive elements to be discovered.
A soft, silvery-white radioactive metal, actinium reacts rapidly with
oxygen and moisture in air forming a white coating of actinium oxide
that prevents further oxidation. As with most lanthanides and many
actinides, actinium assumes oxidation state +3 in nearly all its
chemical compounds. Actinium is found only in traces in uranium and
thorium ores as the isotope 227Ac, which decays with a half-life of
21.772 years, predominantly emitting beta and sometimes alpha
particles, and 228Ac, which is beta active with a half-life of 6.15
hours. One tonne of natural uranium in ore contains about 0.2
milligrams of actinium-227, and one tonne of thorium contains about 5
nanograms of actinium-228. The close similarity of physical and
chemical properties of actinium and lanthanum makes separation of
actinium from the ore impractical. Instead, the element is prepared,
in milligram amounts, by the neutron irradiation of
{{chem2|^{226}Ra|link=radium}} in a nuclear reactor. Owing to its
scarcity, high price and radioactivity, actinium has no significant
industrial use. Its current applications include a neutron source and
an agent for radiation therapy.
History
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André-Louis Debierne, a French chemist, announced the discovery of a
new element in 1899. He separated it from pitchblende residues left by
Marie and Pierre Curie after they had extracted radium. In 1899,
Debierne described the substance as similar to titanium and (in 1900)
as similar to thorium. Friedrich Oskar Giesel found in 1902 a
substance similar to lanthanum and called it "emanium" in 1904. After
a comparison of the substances' half-lives determined by Debierne,
Harriet Brooks in 1904, and Otto Hahn and Otto Sackur in 1905,
Debierne's chosen name for the new element was retained because it had
seniority, despite the contradicting chemical properties he claimed
for the element at different times.
Articles published in the 1970s and later suggest that Debierne's
results published in 1904 conflict with those reported in 1899 and
1900. Furthermore, the now-known chemistry of actinium precludes its
presence as anything other than a minor constituent of Debierne's 1899
and 1900 results; in fact, the chemical properties he reported make it
likely that he had, instead, accidentally identified protactinium,
which would not be discovered for another fourteen years, only to have
it disappear due to its hydrolysis and adsorption onto his laboratory
equipment. This has led some authors to advocate that Giesel alone
should be credited with the discovery. A less confrontational vision
of scientific discovery is proposed by Adloff. He suggests that
hindsight criticism of the early publications should be mitigated by
the then nascent state of radiochemistry: highlighting the prudence of
Debierne's claims in the original papers, he notes that nobody can
contend that Debierne's substance did not contain actinium. Debierne,
who is now considered by the vast majority of historians as the
discoverer, lost interest in the element and left the topic. Giesel,
on the other hand, can rightfully be credited with the first
preparation of radiochemically pure actinium and with the
identification of its atomic number 89.
The name actinium originates from the Ancient Greek 'aktis, aktinos'
(ακτίς, ακτίνος), meaning beam or ray. Its symbol Ac is also used in
abbreviations of other compounds that have nothing to do with
actinium, such as acetyl, acetate and sometimes acetaldehyde.
Properties
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Actinium is a soft, silvery-white, radioactive, metallic element. Its
estimated shear modulus is similar to that of lead. Owing to its
strong radioactivity, actinium glows in the dark with a pale blue
light, which originates from the surrounding air ionized by the
emitted energetic particles. Actinium has similar chemical properties
to lanthanum and other lanthanides, and therefore these elements are
difficult to separate when extracting from uranium ores. Solvent
extraction and ion chromatography are commonly used for the
separation.
The first element of the actinides, actinium gave the set its name,
much as lanthanum had done for the lanthanides. The actinides are much
more diverse than the lanthanides and therefore it was not until 1945
that the most significant change to Dmitri Mendeleev's periodic table
since the recognition of the lanthanides, the introduction of the
actinides, was generally accepted after Glenn T. Seaborg's research on
the transuranium elements (although it had been proposed as early as
1892 by British chemist Henry Bassett).
Actinium reacts rapidly with oxygen and moisture in air forming a
white coating of actinium oxide that impedes further oxidation. As
with most lanthanides and actinides, actinium exists in the oxidation
state +3, and the Ac3+ ions are colorless in solutions. The oxidation
state +3 originates from the [Rn] 6d17s2 electronic configuration of
actinium, with three valence electrons that are easily donated to give
the stable closed-shell structure of the noble gas radon. Although the
5f orbitals are unoccupied in an actinium atom, it can be used as a
valence orbital in actinium complexes and hence it is generally
considered the first 5f element by authors working on it. Ac3+ is the
largest of all known tripositive ions and its first coordination
sphere contains approximately 10.9 ± 0.5 water molecules.
Chemical compounds
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Due to actinium's intense radioactivity, only a limited number of
actinium compounds are known. These include: AcF3, AcCl3, AcBr3, AcOF,
AcOCl, AcOBr, Ac2S3, Ac2O3, AcPO4 and Ac(NO3)3. They all contain
actinium in the oxidation state +3. In particular, the lattice
constants of the analogous lanthanum and actinium compounds differ by
only a few percent.
Formula color symmetry space group No Pearson symbol 'a' (pm) 'b'
(pm) 'c' (pm) 'Z' density, g/cm3
Ac silvery 'fcc' Fmm 225 cF4 531.1 531.1 531.1 4 10.07
AcH2 |unknown cubic Fmm 225 cF12 567 567 567 4 8.35
Ac2O3 white trigonal Pm1 164 hP5 408 408 630 1 9.18
Ac2S3 black cubic I3d 220 cI28 778.56 778.56 778.56 4 6.71
AcF3 white hexagonal Pc1 165 hP24 741 741 755 6 7.88
AcCl3 |white hexagonal P63/m 165 hP8 764 764 456 2 4.8
AcBr3 white hexagonal P63/m 165 hP8 764 764 456 2 5.85
AcOF white cubic Fmm 593.1 8.28
AcOCl |white tetragonal 424 424 707 7.23
AcOBr |white tetragonal 427 427 740 7.89
AcPO4·0.5H2O |unknown hexagonal 721 721 664 5.48
Here 'a', 'b' and 'c' are lattice constants, No is space group number
and 'Z' is the number of formula units per unit cell. Density was not
measured directly but calculated from the lattice parameters.
Oxides
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Actinium oxide (Ac2O3) can be obtained by heating the hydroxide at or
the oxalate at , in vacuum. Its crystal lattice is isotypic with the
oxides of most trivalent rare-earth metals.
Halides
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Actinium trifluoride can be produced either in solution or in solid
reaction. The former reaction is carried out at room temperature, by
adding hydrofluoric acid to a solution containing actinium ions. In
the latter method, actinium metal is treated with hydrogen fluoride
vapors at in an all-platinum setup. Treating actinium trifluoride
with ammonium hydroxide at yields oxyfluoride AcOF. Whereas lanthanum
oxyfluoride can be easily obtained by burning lanthanum trifluoride in
air at for an hour, similar treatment of actinium trifluoride yields
no AcOF and only results in melting of the initial product.
:AcF3 + 2 NH3 + H2O → AcOF + 2 NH4F
Actinium trichloride is obtained by reacting actinium hydroxide or
oxalate with carbon tetrachloride vapors at temperatures above .
Similarly to the oxyfluoride, actinium oxychloride can be prepared by
hydrolyzing actinium trichloride with ammonium hydroxide at . However,
in contrast to the oxyfluoride, the oxychloride could well be
synthesized by igniting a solution of actinium trichloride in
hydrochloric acid with ammonia.
Reaction of aluminium bromide and actinium oxide yields actinium
tribromide:
:Ac2O3 + 2 AlBr3 → 2 AcBr3 + Al2O3
and treating it with ammonium hydroxide at results in the oxybromide
AcOBr.
Other compounds
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Actinium hydride was obtained by reduction of actinium trichloride
with potassium at , and its structure was deduced by analogy with the
corresponding LaH2 hydride. The source of hydrogen in the reaction was
uncertain.
Mixing monosodium phosphate (NaH2PO4) with a solution of actinium in
hydrochloric acid yields white-colored actinium phosphate hemihydrate
(AcPO4·0.5H2O), and heating actinium oxalate with hydrogen sulfide
vapors at for a few minutes results in a black actinium sulfide
Ac2S3. It may possibly be produced by acting with a mixture of
hydrogen sulfide and carbon disulfide on actinium oxide at .
Isotopes
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Naturally occurring actinium is principally composed of two
radioactive isotopes; {{chem2|^{227}Ac}} (from the radioactive family
of {{chem2|^{235}U}}) and {{chem2|^{228}Ac}} (a granddaughter of
{{chem2|^{232}Th}}). {{chem2|^{227}Ac}} decays mainly as a beta
emitter with a very small energy, but in 1.38% of cases it emits an
alpha particle, so it can readily be identified through alpha
spectrometry. Thirty-three radioisotopes have been identified, the
most stable being {{chem2|^{227}Ac}} with a half-life of 21.772 years,
^{225}Ac}} with a half-life of 10.0 days and {{chem2|^{226}Ac}} with a
half-life of 29.37 hours. All remaining radioactive isotopes have
half-lives that are less than 10 hours and the majority of them have
half-lives shorter than one minute. The shortest-lived known isotope
of actinium is {{chem2|^{217}Ac}} (half-life of 69 nanoseconds) which
decays through alpha decay. Actinium also has two known meta states.
The most significant isotopes for chemistry are {{chem2|^{225}Ac}},
{{chem2|^{227}Ac}}, and {{chem2|^{228}Ac}}.
Purified {{chem2|^{227}Ac}} comes into equilibrium with its decay
products after about a half of year. It decays according to its
21.772-year half-life emitting mostly beta (98.62%) and some alpha
particles (1.38%); the successive decay products are part of the
actinium series. Owing to the low available amounts, low energy of its
beta particles (maximum 44.8 keV) and low intensity of alpha
radiation, {{chem2|^{227}Ac}} is difficult to detect directly by its
emission and it is therefore traced via its decay products. The
isotopes of actinium range in atomic weight from ({{chem2|^{203}Ac}})
to ({{chem2|^{236}Ac}}).
!Isotope !Production !Decay !Half-life |- |221Ac |align=right
|232Th(d,9n)→225Pa(α)→221Ac |α |52 ms |- |222Ac |align=right
|232Th(d,8n)→226Pa(α)→222Ac |α |5.0 s |- |223Ac |align=right
|232Th(d,7n)→227Pa(α)→223Ac |α |2.1 min |- |224Ac |align=right
|232Th(d,6n)→228Pa(α)→224Ac |α |2.78 hours |- |225Ac |align=right
|232Th(n,γ)→233Th(β−)→233Pa(β−)→233U(α)→229Th(α)→225Ra(β−)→225Ac |α
|10 days |- |226Ac |align=right |226Ra(d,2n)→226Ac |α, β−
electron capture |29.37 hours |- |227Ac |align=right
|235U(α)→231Th(β−)→231Pa(α)→227Ac |α, β− |21.77 years |- |228Ac
|align=right |232Th(α)→228Ra(β−)→228Ac |β− |6.15 hours |- |229Ac
|align=right |228Ra(n,γ)→229Ra(β−)→229Ac |β− |62.7 min |- |230Ac
|align=right |232Th(d,α)→230Ac |β− |122 s |- |231Ac |align=right
|232Th(γ,p)→231Ac |β− |7.5 min |- |232Ac |align=right
|232Th(n,p)→232Ac |β− |119 s
Occurrence and synthesis
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Actinium is found only in traces in uranium ores - one tonne of
uranium in ore contains about 0.2 milligrams of 227Ac - and in thorium
ores, which contain about 5 nanograms of 228Ac per one tonne of
thorium. The actinium isotope 227Ac is a transient member of the
uranium-actinium series decay chain, which begins with the parent
isotope 235U (or 239Pu) and ends with the stable lead isotope 207Pb.
The isotope 228Ac is a transient member of the thorium series decay
chain, which begins with the parent isotope 232Th and ends with the
stable lead isotope 208Pb. Another actinium isotope (225Ac) is
transiently present in the neptunium series decay chain, beginning
with 237Np (or 233U) and ending with thallium (205Tl) and near-stable
bismuth (209Bi); even though all primordial 237Np has decayed away, it
is continuously produced by neutron knock-out reactions on natural
238U.
The low natural concentration, and the close similarity of physical
and chemical properties to those of lanthanum and other lanthanides,
which are always abundant in actinium-bearing ores, render separation
of actinium from the ore impractical. The most concentrated actinium
sample prepared from raw material consisted of 7 micrograms of 227Ac
in less than 0.1 milligrams of La2O3, and complete separation was
never achieved. Instead, actinium is prepared, in milligram amounts,
by the neutron irradiation of {{chem2|^{226}Ra|link=Radium-226}} in a
nuclear reactor.
:^{226}_{88}Ra + ^{1}_{0}n -> ^{227}_{88}Ra ->[\beta^-][42.2 \
\ce{min}] ^{227}_{89}Ac
The reaction yield is about 2% of the radium weight. 227Ac can further
capture neutrons resulting in small amounts of 228Ac. After the
synthesis, actinium is separated from radium and from the products of
decay and nuclear fusion, such as thorium, polonium, lead and bismuth.
The extraction can be performed with thenoyltrifluoroacetone-benzene
solution from an aqueous solution of the radiation products, and the
selectivity to a certain element is achieved by adjusting the pH (to
about 6.0 for actinium). An alternative procedure is anion exchange
with an appropriate resin in nitric acid, which can result in a
separation factor of 1,000,000 for radium and actinium vs. thorium in
a two-stage process. Actinium can then be separated from radium, with
a ratio of about 100, using a low cross-linking cation exchange resin
and nitric acid as eluant.
225Ac was first produced artificially at the Institute for
Transuranium Elements (ITU) in Germany using a cyclotron and at St
George Hospital in Sydney using a linac in 2000. This rare isotope has
potential applications in radiation therapy and is most efficiently
produced by bombarding a radium-226 target with 20-30 MeV deuterium
ions. This reaction also yields 226Ac which however decays with a
half-life of 29 hours and thus does not contaminate 225Ac.
Actinium metal has been prepared by the reduction of actinium fluoride
with lithium vapor in vacuum at a temperature between . Higher
temperatures resulted in evaporation of the product and lower ones
lead to an incomplete transformation. Lithium was chosen among other
alkali metals because its fluoride is most volatile.
Applications
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Owing to its scarcity, high price and radioactivity, 227Ac currently
has no significant industrial use, but 225Ac is currently being
studied for use in cancer treatments such as targeted alpha therapies.
227Ac is highly radioactive and was therefore studied for use as an
active element of radioisotope thermoelectric generators, for example
in spacecraft. The oxide of 227Ac pressed with beryllium is also an
efficient neutron source with the activity exceeding that of the
standard americium-beryllium and radium-beryllium pairs. In all those
applications, 227Ac (a beta source) is merely a progenitor which
generates alpha-emitting isotopes upon its decay. Beryllium captures
alpha particles and emits neutrons owing to its large cross-section
for the (α,n) nuclear reaction:
: ^{9}_{4}Be + ^{4}_{2}He -> ^{12}_{6}C + ^{1}_{0}n + \gamma
The 227AcBe neutron sources can be applied in a neutron probe - a
standard device for measuring the quantity of water present in soil,
as well as moisture/density for quality control in highway
construction. Such probes are also used in well logging applications,
in neutron radiography, tomography and other radiochemical
investigations.
225Ac is applied in medicine to produce
{{chem2|^{213}Bi|link=Bismuth-213}} in a reusable generator or can be
used alone as an agent for radiation therapy, in particular targeted
alpha therapy (TAT). This isotope has a half-life of 10 days, making
it much more suitable for radiation therapy than 213Bi (half-life 46
minutes). Additionally, 225Ac decays to nontoxic 209Bi rather than
toxic lead, which is the final product in the decay chains of several
other candidate isotopes, namely 227Th, 228Th, and 230U. Not only
225Ac itself, but also its daughters, emit alpha particles which kill
cancer cells in the body. The major difficulty with application of
225Ac was that intravenous injection of simple actinium complexes
resulted in their accumulation in the bones and liver for a period of
tens of years. As a result, after the cancer cells were quickly killed
by alpha particles from 225Ac, the radiation from the actinium and its
daughters might induce new mutations. To solve this problem, 225Ac was
bound to a chelating agent, such as citrate,
ethylenediaminetetraacetic acid (EDTA) or diethylene triamine
pentaacetic acid (DTPA). This reduced actinium accumulation in the
bones, but the excretion from the body remained slow. Much better
results were obtained with such chelating agents as HEHA () or DOTA ()
coupled to trastuzumab, a monoclonal antibody that interferes with the
HER2/neu receptor. The latter delivery combination was tested on mice
and proved to be effective against leukemia, lymphoma, breast,
ovarian, neuroblastoma and prostate cancers.
The medium half-life of 227Ac (21.77 years) makes it a very convenient
radioactive isotope in modeling the slow vertical mixing of oceanic
waters. The associated processes cannot be studied with the required
accuracy by direct measurements of current velocities (of the order 50
meters per year). However, evaluation of the concentration
depth-profiles for different isotopes allows estimating the mixing
rates. The physics behind this method is as follows: oceanic waters
contain homogeneously dispersed 235U. Its decay product, 231Pa,
gradually precipitates to the bottom, so that its concentration first
increases with depth and then stays nearly constant. 231Pa decays to
227Ac; however, the concentration of the latter isotope does not
follow the 231Pa depth profile, but instead increases toward the sea
bottom. This occurs because of the mixing processes which raise some
additional 227Ac from the sea bottom. Thus analysis of both 231Pa and
227Ac depth profiles allows researchers to model the mixing behavior.
There are theoretical predictions that AcHx hydrides (in this case
with very high pressure) are a candidate for a near room-temperature
superconductor as they have Tc significantly higher than H3S, possibly
near 250 K.
Precautions
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227Ac is highly radioactive and experiments with it are carried out in
a specially designed laboratory equipped with a tight glove box. When
actinium trichloride is administered intravenously to rats, about 33%
of actinium is deposited into the bones and 50% into the liver. Its
toxicity is comparable to, but slightly lower than, that of americium
and plutonium. For trace quantities, fume hoods with good aeration
suffice; for gram amounts, hot cells with shielding from the intense
gamma radiation emitted by 227Ac are necessary.
See also
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* Actinium series
External links
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* [
http://www.periodicvideos.com/videos/089.htm Actinium] at 'The
Periodic Table of Videos' (University of Nottingham)
*
[
http://toxnet.nlm.nih.gov/cgi-bin/sis/search/r?dbs+hsdb:@term+@na+@rel+actinium,+radioactive
NLM Hazardous Substances Databank - Actinium, Radioactive]
*
[
https://web.archive.org/web/20110825123745/http://radchem.nevada.edu/classes/rdch710/files/actinium.pdf
Actinium] in
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