Actinium is a chemical element; it has chemical 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 after 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 and many , 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 particle and sometimes , 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 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.
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.
The first element of the , actinium gave the set its name, much as lanthanum had done for the . 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 actinide concept, 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.
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 | whiteMeyer, p. 71 | 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 per unit cell. Density was not measured directly but calculated from the lattice parameters.
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:
and treating it with ammonium hydroxide at results in the oxybromide AcOBr.
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 .
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, Great Soviet Encyclopedia (in Russian) The isotopes of actinium range in atomic weight from () to ().
!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 |
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 in a nuclear reactor.
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 because its fluoride is most volatile.Hammond, C. R. The Elements in
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.Majumdar, D. K. (2004) Irrigation Water Management: Principles and Practice. p. 108Chandrasekharan, H. and Gupta, Navindu (2006) Fundamentals of Nuclear Science – Application in Agriculture. pp. 202 ff 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 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 Chelation agent, such as citrate, ethylenediaminetetraacetic acid (EDTA) or pentetic 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 cancer, Ovarian cancer, neuroblastoma and .
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.
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