Actinium is a
chemical element with the
symbol Ac and
atomic number 89. It was first isolated 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. Actinium gave the name to the
actinide series, a group of 15 similar elements between actinium and
lawrencium in the
periodic table. Together with
polonium,
radium
Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rat ...
, and
radon
Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains th ...
, actinium was one of the first
non-primordial radioactive elements to be isolated.
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
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
227Ac, which decays with a
half-life
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ...
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
Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between l ...
makes separation of actinium from the ore impractical. Instead, the element is prepared, in milligram amounts, by the neutron irradiation of 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
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
Radium is a chemical element with the symbol Ra and atomic number 88. It is the sixth element in group 2 of the periodic table, also known as the alkaline earth metals. Pure radium is silvery-white, but it readily reacts with nitrogen (rat ...
. 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
Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between l ...
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
Otto Sackur (28 September 1880 in Breslau, Germany – 17 December 1914 in Berlin, Germany) was a German physical chemist.
He is known for the development of the Sackur–Tetrode equation, which he developed independently of Hugo Tetrode. ...
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
A laboratory (; ; colloquially lab) is a facility that provides controlled conditions in which scientific or technological research, experiments, and measurement may be performed. Laboratory services are provided in a variety of settings: physicia ...
. 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
Actinium is a soft, silvery-white,
[''Actinium'', in Encyclopædia Britannica, 15th edition, 1995, p. 70] 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
Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between l ...
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
Lanthanum is a chemical element with the symbol La and atomic number 57. It is a soft, ductile, silvery-white metal that tarnishes slowly when exposed to air. It is the eponym of the lanthanide series, a group of 15 similar elements between l ...
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
Actinium(III) oxide is a chemical compound containing the rare radioactive element actinium. It has the formula Ac2O3. It is similar to its corresponding lanthanum compound, lanthanum(III) oxide, and contains actinium in the oxidation state +3. that impedes further oxidation.
As with most lanthanides and actinides, actinium exists in the
oxidation state +3, and the Ac
3+ ions are colorless in solutions.
The oxidation state +3 originates from the
nd
17s
2 electronic configuration of actinium, with three valence electrons that are easily donated to give the stable closed-shell structure of the
noble gas radon
Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains th ...
.
Although the 5f orbitals are unoccupied in an actinium atom, it can be used as a valence orbital in actinium complexes and hence actinium is generally regarded by chemists working on it as the first 5f element.
Ac
3+ is the largest of all known tripositive ions and its first coordination sphere contains approximately 10.9 ± 0.5 water molecules.
Chemical compounds
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. Except for AcPO
4, they are all similar to the corresponding lanthanum compounds. 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.
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
Actinium oxide
Actinium(III) oxide is a chemical compound containing the rare radioactive element actinium. It has the formula Ac2O3. It is similar to its corresponding lanthanum compound, lanthanum(III) oxide, and contains actinium in the oxidation state +3. (Ac
2O
3) can be obtained by heating the hydroxide at 500 °C or the
oxalate at 1100 °C, in vacuum. Its crystal lattice is
isotypic with the oxides of most trivalent rare-earth metals.
Halides
Actinium trifluoride
Actinium(III) fluoride (AcF3) is an inorganic compound, a salt of actinium and fluorine.
Synthesis
Actinium fluoride can be prepared in solution or by a solid-state reaction. In the first method, actinium hydroxide is treated with hydrofluoric ...
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 700 °C in an all-platinum setup. Treating actinium trifluoride with
ammonium hydroxide at 900–1000 °C yields
oxyfluoride
In chemistry, molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula , where X = fluorine (F), chlor ...
AcOF. Whereas lanthanum oxyfluoride can be easily obtained by burning lanthanum trifluoride in air at 800 °C for an hour, similar treatment of actinium trifluoride yields no AcOF and only results in melting of the initial product.
[Meyer, pp. 87–88]
:AcF
3 + 2 NH
3 + H
2O → AcOF + 2 NH
4F
Actinium trichloride is obtained by reacting actinium hydroxide or
oxalate with
carbon tetrachloride vapors at temperatures above 960 °C. Similar to oxyfluoride, actinium
oxychloride
In chemistry, molecular oxohalides (oxyhalides) are a group of chemical compounds in which both oxygen and halogen atoms are attached to another chemical element A in a single molecule. They have the general formula , where X = fluorine (F), chlor ...
can be prepared by hydrolyzing actinium trichloride with
ammonium hydroxide at 1000 °C. 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:
:Ac
2O
3 + 2 AlBr
3 → 2 AcBr
3 + Al
2O
3
and treating it with ammonium hydroxide at 500 °C results in the oxybromide AcOBr.
Other compounds
Actinium hydride was obtained by reduction of actinium trichloride with potassium at 300 °C, and its structure was deduced by analogy with the corresponding LaH
2 hydride. The source of hydrogen in the reaction was uncertain.
Mixing
monosodium phosphate (NaH
2PO
4) with a solution of actinium in hydrochloric acid yields white-colored actinium phosphate hemihydrate (AcPO
4·0.5H
2O), and heating actinium oxalate with
hydrogen sulfide vapors at 1400 °C for a few minutes results in a black actinium sulfide Ac
2S
3. It may possibly be produced by acting with a mixture of
hydrogen sulfide and
carbon disulfide on
actinium oxide
Actinium(III) oxide is a chemical compound containing the rare radioactive element actinium. It has the formula Ac2O3. It is similar to its corresponding lanthanum compound, lanthanum(III) oxide, and contains actinium in the oxidation state +3. at 1000 °C.
Isotopes
Naturally occurring actinium is composed of two radioactive
isotope
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
s; (from the radioactive family of ) and (a granddaughter of ). 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-six
radioisotopes have been identified, the most stable being with a
half-life
Half-life (symbol ) is the time required for a quantity (of substance) to reduce to half of its initial value. The term is commonly used in nuclear physics to describe how quickly unstable atoms undergo radioactive decay or how long stable ...
of 21.772 years,
with a half-life of 10.0 days and 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 (half-life of 69 nanoseconds) which decays through
alpha decay. Actinium also has two known
meta states. The most significant isotopes for chemistry are
225Ac,
227Ac, and
228Ac.
Purified 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, is difficult to detect directly by its emission and it is therefore traced via its decay products.
[Actinium](_blank)
Great Soviet Encyclopedia (in Russian) The isotopes of actinium range in
atomic weight from 205
u () to 236 u ().
Occurrence and synthesis
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
Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers ( mass num ...
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
Primordial may refer to:
* Primordial era, an era after the Big Bang. See Chronology of the universe
* Primordial sea (a.k.a. primordial ocean, ooze or soup). See Abiogenesis
* Primordial nuclide, nuclides, a few radioactive, that formed before t ...
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, and complete separation was never achieved.
Instead, actinium is prepared, in milligram amounts, by the neutron irradiation of in a
nuclear reactor.
:
^_Ra + ^_n -> ^_Ra -> beta^-42.2 \ \ce] ^_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
Lithium (from el, λίθος, lithos, lit=stone) is a chemical element with the symbol Li and atomic number 3. It is a soft, silvery-white alkali metal. Under standard conditions, it is the least dense metal and the least dense soli ...
vapor in vacuum at a temperature between 1100 and 1300 °C. 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.
[Hammond, C. R. ''The Elements'' in ]
Applications
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.
[Russell, Alan M. and Lee, Kok Loong (2005]
''Structure-property relations in nonferrous metals''
Wiley. , pp. 470–471 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:
:
^_Be + ^_He -> ^_C + ^_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 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 stable but 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
Trastuzumab, sold under the brand name Herceptin among others, is a monoclonal antibody used to treat breast cancer and stomach cancer. It is specifically used for cancer that is HER2 receptor positive. It may be used by itself or together wi ...
, a
monoclonal antibody that interferes with the
HER2/neu receptor
Receptor may refer to:
*Sensory receptor, in physiology, any structure which, on receiving environmental stimuli, produces an informative nerve impulse
*Receptor (biochemistry), in biochemistry, a protein molecule that receives and responds to a n ...
. The latter delivery combination was tested on mice and proved to be effective against
leukemia
Leukemia ( also spelled leukaemia and pronounced ) is a group of blood cancers that usually begin in the bone marrow and result in high numbers of abnormal blood cells. These blood cells are not fully developed and are called ''blasts'' or ...
,
lymphoma
Lymphoma is a group of blood and lymph tumors that develop from lymphocytes (a type of white blood cell). In current usage the name usually refers to just the cancerous versions rather than all such tumours. Signs and symptoms may include en ...
,
breast,
ovarian,
neuroblastoma and
prostate cancers.
The medium half-life of
227Ac (21.77 years) makes it 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 AcH
x hydrides (in this case with very high pressure) are a candidate for a near
room-temperature superconductor as they have T
c significantly higher than H3S, possibly near 250 K.
Precautions
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
*
Actinium series
Notes
References
Bibliography
* Meyer, Gerd and Morss, Lester R. (1991
''Synthesis of lanthanide and actinide compounds'' Springer.
External links
at ''
The Periodic Table of Videos'' (University of Nottingham)
NLM Hazardous Substances Databank – Actinium, RadioactiveActiniumin
{{Good article
Chemical elements
Chemical elements with face-centered cubic structure
Actinides