A soft, silvery-whiteradioactive metal, actinium reacts rapidly with oxygen and moisture in air forming a white coating of actinium oxide that prevents further oxidation. As with mostlanthanides and manyactinides, actinium assumesoxidation state +3 in nearly all its chemical compounds. Actinium is found only in traces inuranium andthorium ores as theisotope227Ac, which decays with ahalf-life of 21.772 years, predominantly emittingbeta and sometimesalpha particles, and228Ac, which is beta active with a half-life of 6.15 hours. Onetonne of naturaluranium in ore contains about 0.2 milligrams of actinium-227, and one tonne ofthorium contains about 5 nanograms of actinium-228. The close similarity of physical and chemical properties of actinium andlanthanum makes separation of actinium from the ore impractical. Instead, the element is prepared, in milligram amounts, by the neutron irradiation of226Ra in anuclear 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 forradiation therapy.
André-Louis Debierne, a French chemist, announced the discovery of a new element in 1899. He separated it frompitchblende residues left byMarie andPierre Curie after they had extractedradium.[5] In 1899, Debierne described the substance as similar totitanium[6] and (in 1900) as similar tothorium.[7]Friedrich Oskar Giesel found in 1902[8] a substance similar tolanthanum and called it "emanium" in 1904.[9] After a comparison of the substances' half-lives determined by Debierne,[10]Harriet Brooks in 1904, andOtto Hahn andOtto 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.[11][12]
Articles published in the 1970s[13] and later[5] 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 identifiedprotactinium, which would not be discovered for another fourteen years, only to have it disappear due to its hydrolysis and adsorption onto hislaboratory 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.[5] 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.[5] 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.[13]
The name actinium originates from theAncient Greekaktis, aktinos (ακτίς, ακτίνος), meaning beam or ray.[14] Its symbol Ac is also used in abbreviations of other compounds that have nothing to do with actinium, such asacetyl,acetate[15] and sometimesacetaldehyde.[16]
Actinium is a soft, silvery-white,[17][18]radioactive, metallic element. Its estimatedshear modulus is similar to that oflead.[19] 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.[20] Actinium has similar chemical properties tolanthanum and other lanthanides, and therefore these elements are difficult to separate when extracting from uranium ores.Solvent extraction andion chromatography are commonly used for the separation.[21]
Actinium reacts rapidly with oxygen and moisture in air forming a white coating ofactinium oxide that impedes further oxidation.[17] As with most lanthanides and actinides, actinium exists in theoxidation 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 thenoble gasradon.[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]
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 andAc(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]
Herea,b andc are lattice constants, No is space group number andZ is the number offormula units perunit cell. Density was not measured directly but calculated from the lattice parameters.
Actinium oxide (Ac2O3) can be obtained by heating the hydroxide at 500 °C (932 °F) or theoxalate at 1,100 °C (2,010 °F), in vacuum. Its crystal lattice isisotypic with the oxides of most trivalent rare-earth metals.[30]
Actinium trifluoride can be produced either in solution or in solid reaction. The former reaction is carried out at room temperature, by addinghydrofluoric acid to a solution containing actinium ions. In the latter method, actinium metal is treated with hydrogen fluoride vapors at 700 °C (1,292 °F) in an all-platinum setup. Treating actinium trifluoride withammonium hydroxide at 900–1,000 °C (1,650–1,830 °F) yieldsoxyfluoride AcOF. Whereas lanthanum oxyfluoride can be easily obtained by burning lanthanum trifluoride in air at 800 °C (1,470 °F) 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 oroxalate withcarbon tetrachloride vapors at temperatures above 960 °C (1,760 °F). Similarly to the oxyfluoride, actiniumoxychloride can be prepared by hydrolyzing actinium trichloride withammonium hydroxide at 1,000 °C (1,830 °F). However, in contrast to the oxyfluoride, the oxychloride could well be synthesized by igniting a solution of actinium trichloride inhydrochloric acid withammonia.[30]
Actinium hydride was obtained by reduction of actinium trichloride with potassium at 300 °C (572 °F), and its structure was deduced by analogy with the corresponding LaH2 hydride. The source of hydrogen in the reaction was uncertain.[37]
Naturally occurring actinium is principally composed of two radioactiveisotopes;227 Ac (from the radioactive family of235 U) and228 Ac (a granddaughter of232 Th).227 Ac decays mainly as abeta emitter with a very small energy, but in 1.38% of cases it emits analpha particle, so it can readily be identified throughalpha spectrometry.[2] Thirty-threeradioisotopes have been identified, the most stable being227 Ac with ahalf-life of 21.772 years,225 Ac with a half-life of 10.0 days and226 Ac with a half-life of 29.37 hours. All remainingradioactive 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 is217 Ac (half-life of 69 nanoseconds) which decays throughalpha decay. Actinium also has two knownmeta states.[38] The most significant isotopes for chemistry are225Ac,227Ac, and228Ac.[2]
Purified227 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%);[38] the successive decay products are part of theactinium series. Owing to the low available amounts, low energy of its beta particles (maximum 44.8 keV) and low intensity of alpha radiation,227 Ac is difficult to detect directly by its emission and it is therefore traced via its decay products.[25] The isotopes of actinium range inatomic weight from 203 u (203 Ac) to 236 u (236 Ac).[38]
Uraninite ores have elevated concentrations of actinium.
Actinium is found only in traces inuranium ores – one tonne of uranium in ore contains about 0.2 milligrams of227Ac[39][40] – and inthorium ores, which contain about 5 nanograms of228Ac per one tonne of thorium. The actiniumisotope227Ac is a transient member of theuranium-actinium seriesdecay chain, which begins with the parent isotope235U (or239Pu) and ends with the stable lead isotope207Pb. The isotope228Ac is a transient member of thethorium series decay chain, which begins with the parent isotope232Th and ends with the stable lead isotope208Pb. Another actinium isotope (225Ac) is transiently present in theneptunium seriesdecay chain, beginning with237Np (or233U) and ending with thallium (205Tl) and near-stable bismuth (209Bi); even though allprimordial237Np has decayed away, it is continuously produced by neutron knock-out reactions on natural238U.
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 of227Ac 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 of226Ra in anuclear reactor.[40][42]
The reaction yield is about 2% of the radium weight.227Ac can further capture neutrons resulting in small amounts of228Ac. 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 withthenoyltrifluoroacetone-benzene solution from an aqueous solution of the radiation products, and the selectivity to a certain element is achieved by adjusting thepH (to about 6.0 for actinium).[39] An alternative procedure is anion exchange with an appropriateresin innitric 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 aseluant.[43]
225Ac was first produced artificially at theInstitute for Transuranium Elements (ITU) in Germany using acyclotron and atSt George Hospital in Sydney using alinac 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 MeVdeuterium ions. This reaction also yields226Ac which however decays with a half-life of 29 hours and thus does not contaminate225Ac.[45]
Actinium metal has been prepared by the reduction of actinium fluoride withlithium vapor in vacuum at a temperature between 1,100 and 1,300 °C (2,010 and 2,370 °F). Higher temperatures resulted in evaporation of the product and lower ones lead to an incomplete transformation. Lithium was chosen among otheralkali metals because its fluoride is most volatile.[14][17]
Owing to its scarcity, high price and radioactivity,227Ac currently has no significant industrial use, but225Ac 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 ofradioisotope thermoelectric generators, for example in spacecraft. The oxide of227Ac pressed withberyllium is also an efficientneutron 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:
The227AcBe neutron sources can be applied in aneutron probe – a standard device for measuring the quantity of water present in soil, as well as moisture/density for quality control in highway construction.[47][48] Such probes are also used in well logging applications, inneutron radiography, tomography and other radiochemical investigations.[49]
Chemical structure of theDOTA carrier for225Ac in radiation therapy
225Ac is applied in medicine to produce213Bi in a reusable generator[43] or can be used alone as an agent forradiation therapy, in particular targeted alpha therapy (TAT). This isotope has a half-life of 10 days, making it much more suitable for radiation therapy than213Bi (half-life 46 minutes).[28] Additionally,225Ac decays to nontoxic209Bi rather than toxiclead, which is the final product in the decay chains of several other candidate isotopes, namely227Th,228Th, and230U.[28] Not only225Ac itself, but also its daughters, emit alpha particles which kill cancer cells in the body. The major difficulty with application of225Ac 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 from225Ac, the radiation from the actinium and its daughters might induce new mutations. To solve this problem,225Ac was bound to achelating agent, such ascitrate,ethylenediaminetetraacetic acid (EDTA) ordiethylene 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 (1,4,7,10,13,16-hexaazacyclohexadecane-N,N′,N″,N‴,N‴′,N‴″-hexaacetic acid)[50] orDOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) coupled totrastuzumab, amonoclonal antibody that interferes with theHER2/neureceptor. The latter delivery combination was tested on mice and proved to be effective againstleukemia,lymphoma,breast,ovarian,neuroblastoma andprostate cancers.[51][52][53]
The medium half-life of227Ac (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 dispersed235U. Its decay product,231Pa, gradually precipitates to the bottom, so that its concentration first increases with depth and then stays nearly constant.231Pa decays to227Ac; however, the concentration of the latter isotope does not follow the231Pa depth profile, but instead increases toward the sea bottom. This occurs because of the mixing processes which raise some additional227Ac from the sea bottom. Thus analysis of both231Pa and227Ac 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 nearroom-temperature superconductor as they have Tc significantly higher than H3S, possibly near 250 K.[56]
227Ac is highly radioactive and experiments with it are carried out in a specially designed laboratory equipped with a tightglove 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 by227Ac are necessary.[58]
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