Actinium

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Not to be confused with Actin or Actium.

Actinium, 89Ac
Actinium
Pronunciation/ækˈtɪniəm/ ​(ak-TIN-ee-əm)
Appearancesilvery-white, glowing with an eerie blue light;[1] sometimes with a golden cast[2]
Mass number[227]
Actinium in the periodic table
Hydrogen Helium
Lithium Beryllium Boron Carbon Nitrogen Oxygen Fluorine Neon
Sodium Magnesium Aluminium Silicon Phosphorus Sulfur Chlorine Argon
Potassium Calcium Scandium Titanium Vanadium Chromium Manganese Iron Cobalt Nickel Copper Zinc Gallium Germanium Arsenic Selenium Bromine Krypton
Rubidium Strontium Yttrium Zirconium Niobium Molybdenum Technetium Ruthenium Rhodium Palladium Silver Cadmium Indium Tin Antimony Tellurium Iodine Xenon
Caesium Barium Lanthanum Cerium Praseodymium Neodymium Promethium Samarium Europium Gadolinium Terbium Dysprosium Holmium Erbium Thulium Ytterbium Lutetium Hafnium Tantalum Tungsten Rhenium Osmium Iridium Platinum Gold Mercury (element) Thallium Lead Bismuth Polonium Astatine Radon
Francium Radium Actinium Thorium Protactinium Uranium Neptunium Plutonium Americium Curium Berkelium Californium Einsteinium Fermium Mendelevium Nobelium Lawrencium Rutherfordium Dubnium Seaborgium Bohrium Hassium Meitnerium Darmstadtium Roentgenium Copernicium Nihonium Flerovium Moscovium Livermorium Tennessine Oganesson
La
↑
Ac
↓
(Uqt)
radium ← actinium → thorium
Discovery and first isolation
Friedrich Oskar Giesel (1902, 1903)
Named byAndré-Louis Debierne (1899)
Isotopes of actinium
Main isotopes[4] Decay
abun­dance half-life (t1/2) mode pro­duct
225Ac trace 9.919 d α
221Fr
CD
211Bi
226Ac synth 29.37 h
β−
226Th
ε 226Ra
α
222Fr
227Ac trace 21.772 y β−
227Th
α
223Fr
 Category: Actinium
| references

Actinium is a

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 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 226Ra 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. In 1899, Debierne described the substance as similar to titanium[5] and (in 1900) as similar to thorium.[6] Friedrich Oskar Giesel found in 1902[7] a substance similar to lanthanum and called it "emanium" in 1904.[8] After a comparison of the substances' half-lives determined by Debierne,[9] 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.[10][11]

Articles published in the 1970s

laboratory equipment. This has led some authors to advocate that Giesel alone should be credited with the discovery.[2] A less confrontational vision of scientific discovery is proposed by Adloff.[13] 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.[13] 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.[12]

The name actinium originates from the

acetyl, acetate[15] and sometimes acetaldehyde.[16]

Properties

Actinium is a soft, silvery-white,

Solvent extraction and ion chromatography are commonly used for the separation.[21]

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[22] 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[23] (although it had been proposed as early as 1892 by British chemist Henry Bassett).[24]

Actinium reacts rapidly with oxygen and moisture in air forming a white coating of

actinium oxide that impedes further oxidation.[17] As with most lanthanides and actinides, actinium exists in the oxidation state +3, and the Ac3+ ions are colorless in solutions.[25] 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.[18] 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.[26][27][28] Ac3+ is the largest of all known tripositive ions and its first coordination sphere contains approximately 10.9 ± 0.5 water molecules.[29]

Chemical compounds

Main article: Actinium compounds

Due to actinium's intense radioactivity, only a limited number of actinium compounds are known. These include:

Ac2O3, AcPO4 and Ac(NO3)3. They all contain actinium in the oxidation state +3.[25][30] In particular, the lattice constants of the analogous lanthanum and actinium compounds differ by only a few percent.[30]

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

Main article: Actinium(III) oxide

Actinium oxide (Ac2O3) 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.[30]

Halides

oxyfluoride 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.[30][36]

AcF3 + 2 NH3 + H2O → AcOF + 2 NH4F

Actinium trichloride is obtained by reacting actinium hydroxide or

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.[30]

Reaction of aluminium bromide and actinium oxide yields actinium tribromide:

Ac2O3 + 2 AlBr3 → 2 AcBr3 + Al2O3

and treating it with ammonium hydroxide at 500 °C results in the oxybromide AcOBr.[30]

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 LaH2 hydride. The source of hydrogen in the reaction was uncertain.[37]

Mixing

actinium oxide at 1000 °C.[30]

Isotopes

Main article: Isotopes of actinium

Naturally occurring actinium is principally composed of two radioactive

meta states.[38] The most significant isotopes for chemistry are 225Ac, 227Ac, and 228Ac.[2]

Purified 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%);

u (203
Ac) to 236 u (236
Ac).[38]

Isotope Production Decay Half-life
221Ac 232Th(d,9n)→225Pa(α)→221Ac α 52 ms
222Ac 232Th(d,8n)→226Pa(α)→222Ac α 5.0 s
223Ac 232Th(d,7n)→227Pa(α)→223Ac α 2.1 min
224Ac 232Th(d,6n)→228Pa(α)→224Ac α 2.78 hours
225Ac 232Th(n,γ)→233Th(β−)→233Pa(β−)→233U(α)→229Th(α)→225Ra(β−)→225Ac α 10 days
226Ac 226Ra(d,2n)→226Ac α, β−
electron capture 		29.37 hours
227Ac 235U(α)→231Th(β−)→231Pa(α)→227Ac α, β− 21.77 years
228Ac 232Th(α)→228Ra(β−)→228Ac β− 6.15 hours
229Ac 228Ra(n,γ)→229Ra(β−)→229Ac β− 62.7 min
230Ac 232Th(d,α)→230Ac β− 122 s
231Ac 232Th(γ,p)→231Ac β− 7.5 min
232Ac 232Th(n,p)→232Ac β− 119 s

Occurrence and synthesis

Uraninite ores have elevated concentrations of actinium.

Actinium is found only in traces in

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.[41] Instead, actinium is prepared, in milligram amounts, by the neutron irradiation of 226Ra in a nuclear reactor.[40][42]

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

eluant.[43]

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.[44] 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.[45]

Actinium metal has been prepared by the reduction of actinium fluoride with lithium 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.[14][17]

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.[14][28] 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.[46] 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:

The 227AcBe neutron sources can be applied in a

neutron radiography, tomography and other radiochemical investigations.[49]

Chemical structure of the DOTA carrier for 225Ac in radiation therapy

225Ac is applied in medicine to produce

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.[51][52][53]

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.[54][55]

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.[56]

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.[57] 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.[58]

See also

Notes

References

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Bibliography

External links

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