Actinium

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radium ← actinium → thorium
La
↑
Ac
↓
—
Atomic properties
Atomic number 89
Electron configuration [Rn] 6d1 7s2
Physical properties[1][note 1]
Melting point 1050 °C (1350 K)
Boiling point 3300 °C (3600 K)
Density 10.07 g cm−3
Chemical properties[2][note 1]
Electronegativity 1.1 (Pauling)
Ionization energies[3][4]
1st 5.3807(3) eV
159.16(3) kJ mol−1
2nd 12.06 eV
1164 kJ mol−1
Atomic radii[1][5][6]
Covalent radius 215 pm
Metallic radius 188 pm
Ionic radius 126 pm (Ac3+, Oh)
Thermodynamic properties[1][7]
Enthalpy change of atomization 406 kJ mol−1
Enthalpy change of fusion 10.5 kJ mol−1
Enthalpy change of vaporization 293 kJ mol−1
Miscellaneous
CAS number 7440-34-8
Where appropriate, and unless otherwise stated, data are given for 100 kPa (1 bar) and 298.15 K (25 °C).
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Actinium (symbol: Ac) is a chemical element, one of the transition metals and also an actinoid. All isotopes of actinium are unstable, with half-lives of less than 22 years:[8] however, actinium-227 (t½ = 21.772(3) a) is formed as a decay product of uranium-235 (t½ = 704(1) × 106 a) and so small amounts of actinium are present in all samples of natural uranium.

Discovery

The story of the discovery of actinium has generated a certain amount of controversy.[1][9][10] Credit is usually given to French chemist André-Louis Debierne, who described a new source of radiation from pitchblende in 1899,[11] with further details in 1900.[12] He named the new element actinium,[12] from the Greek ἀκτίς, aktis (genitive: ἀκτίνος, aktinos) meaning "ray". However, Debierne's descriptions of his concentrates are not entirely consistent with what is now known about is now known about the chemistry of actinium;[9][10] nor, it should be said, were they entirely consistent among themselves.

The German chemist Friedrich Giesel was also studying the composition of pitchblende, and described a new source of radiation in 1902,[13] with further details in 1903.[14] Giesel called his new element emanium, from "emanation".[15] Crucially for his claim to discovery, at least as viewed with hindsight, Giesel noted that the chemistry of emanium was very similar to that of lanthanum and cerium.

Debierne had certainly prepared an actinium-containing concentrate by 1904, when he published the half-life of the "emanation" (radon-219, t½ = 3.96(1) s) and the "induced radioactivity" (lead-211, t½ = 36.1(2) min, deposited on surfaces by decay of the 219Rn).[16] In the meantime, Giesel had sent a sample of his emanium concentrate to the laboratory of Ernest Rutherford in Montreal, where Harriet Brooks determined the half-lives of the "emanation" and of the "induced radioactivity": Brooks's results from Giesel's sample were the same as those found by Debierne in Paris for actinium.[17] Both Rutherford[17] and Debierne[18] proclaimed that emanium was the same as actinium, and Debierne's name stuck for the new element.

Neither Debierne nor Giesel would isolate a pure actinium compound from pitchblende: their concentrates contained minute amounts of actinium in a non-radioactive carrier, and it has been questioned whether Debierne's early concentrates (from 1899 and 1900) contained actinium at all.[9][10] Indeed, nobody would be able to prepare carrier-free actinium preparations from pitchblende, as the concentration of actinium is just too low compared to the chemically-similar lanthanum. Before the advent of techniques to artificially produce radioelements by neutron bombardment, the most concentrated preparation of actinium was 0.5 mCi (about 7 µg) in 0.1 mg of lanthanum oxide.[1][19]

Occurance and preparation

Actinium-227 (t½ = 21.772(3) a) is a decay product of uranium-235 (t½ = 704(1) × 106 a), and so occurs naturally in all uranium ores. Its natural occurance in the Earth's crust is calculated to be 5.7 × 10−10 ppm,[note 2] equivalent to a total of about 14000 tonnes of actinium at any one time.[1] Actinium-228 (t½ = 6.15(2) h) also occurs naturally, as a decay product of thorium-232 (t½ = 14.05(6) × 109 a): taken over the whole of the Earth's crust, its abundance (6 × 10−13 ppm)[note 2] is much lower than 227Ac. Nevertheless, the ratio n(227Ac)/n(228Ac) in samples of natural actinium is entirely dependent on whether the original source was uranium-rich or thorium-rich, and this prevents a standard atomic weight being calculated. Trace amounts of actinium-225 (t½ = 10.0(1) d) have also been found in uranium refinary wastes and in Brazilian monazite:[22] this arises from the decay of 237Np and 233U produced by the capture of neutrons from spontaneous fission or (α,n) reactions by 238U and 232Th respectively.[1]

The only isotope available in macroscopic quantities is 227Ac. It is unfeasible to separate and purify this from uranium ores because of the large quantities of rare earths that are present, and so the isotope is prepared artificially by irradiation of radium-226 with thermal neutrons.

22688Ra(n,γ)22788Ra → 22789Ac + β−

The yield of the reaction is limited because 227Ac is much better at absorbing thermal neutrons than 226Ra (capture cross-sections: 762 and 20 barn respectively).

22789Ac(n,γ)22889Ac → 22890Th + β−

Hence the actinium must be separated from both unconverted 226Ra and by-product 228Th.

Notes and references

Notes

  1. ↑ 1.0 1.1 Many of the properties of actinium are only known through estimation and/or extrapolation. Several estimates of the melting point and thermodynamic properties have been made; the boiling point was estimated on the basis of vapour pressure measurements; the electronegativity was estimated on the basis of periodic trends. The density and ionic radii were determined by X-ray crystallography.
  2. ↑ 2.0 2.1 The amount ratios of the isotopes of actinium to their parent nuclides is equal to the ratio of the half-lives. Crustal abundances are calculated on the basis that the abundances of uranium[20] and thorium[21] are 2.7 ppm and 12 ppm respectively.

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Kirby, H. W.; Morss, L. R. Actinium. In The Chemistry of the Actinide and Transactinide Elements, 3rd ed.; Morss, Lester R.; Edelstein, Norman M.; Fuger, Jean, Eds.; Springer: Dordrecht, the Netherlands, 2006; Vol. 1, Chapter 2, pp 18–51. doi:10.1007/1-4020-3598-5_2, <http://radchem.nevada.edu/classes/rdch710/files/actinium.pdf>.
  2. ↑ Pauling, Linus The Nature of the Chemical Bond, 3rd ed.; Ithaca, NY, 1960; pp 88–95. ISBN 0-8014-0333-2.
  3. ↑ Backe, H.; Dretzke, A.; Eberhardt, K.; Fritzsche, S.; Kube, G.; Gwinner, G.; Haire, R. G.; Huber, G., et al. First Determination of the Ionization Potential of Actinium and First Observation of Optical Transitions in Fermium. J. Nucl. Sci. Technol. 2002, 86–89.
  4. ↑ Moore, Charlotte E. Ionization potentials and ionization limits derived from the analyses of optical spectra. Natl. Stand. Ref. Data Ser., (U.S. Natl. Bur. Stand.) 1970, 34, 1–22, <http://www.nist.gov/data/nsrds/NSRDS-NBS34.pdf>.
  5. ↑ Cordero, Beatriz; Gómez, Verónica; Platero-Prats, Ana E.; Revés, Marc; Echeverría, Jorge; Cremades, Eduard; Barragán, Flavia; Alvarez, Santiago Covalent radii revisited. Dalton Trans. 2008 (5), 2832–38. DOI: 10.1039/b801115j.
  6. ↑ Shannon, R. D. Revised effective ionic radii and systematic studies of interatomic distances in halids and chalcogenides. Acta Crystallogr. A 1976, 32 (5), 751–67. DOI: 10.1107/S0567739476001551.
  7. ↑ Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; pp 1102–10. ISBN 0-08-022057-6.
  8. ↑ Audi, G.; Bersillon, O.; Blachot, J.; Wapstra, A. H. The NUBASE evaluation of nuclear and decay properties. Nucl. Phys. A 2003, 729, 3–128. doi:10.1016/j.nuclphysa.2003.11.001, <http://amdc.in2p3.fr/nubase/Nubase2003.pdf>.
  9. ↑ 9.0 9.1 9.2 Kirby, H. W. The Discovery of Actinium. Isis 1971, 62 (3), 290–308. DOI: 10.1086/350760.
  10. ↑ 10.0 10.1 10.2 Adloff, J. P. The centenary of a controversial discovery: actinium. Radiochim. Acta 2000, 88, 123–28. DOI: 10.1524/ract.2000.88.3-4.123.
  11. ↑ Debierne, A. Sur un nouvelle matière radio-active. C. R. Hebd. Seances Acad. Sci. 1899, 129, 593–95, <http://gallica.bnf.fr/ark:/12148/bpt6k3085b/f593.image.langEN>.
  12. ↑ 12.0 12.1 Debierne, A. Sur un nouvelle matière radio-actif – l'actinium. C. R. Hebd. Seances Acad. Sci. 1900, 130, 906–8, <http://gallica.bnf.fr/ark:/12148/bpt6k3086n/f906.image.langEN>.
  13. ↑ Giesel, F. Ueber Radium und radioactive Stoffe. Ber. Dtsch. Chem. Ges. 1902, 35, 3608–11. DOI: 10.1002/cber.190203503187.
  14. ↑ Giesel, F. Ueber den Emanationskörper aus Pechblende und über Radium. Ber. Dtsch. Chem. Ges. 1903, 36 (1), 342–47. DOI: 10.1002/cber.19030360177.
  15. ↑ Giesel, F. Ueber den Emanationskörper (Emanium). Ber. Dtsch. Chem. Ges. 1904, 37 (2), 1696–99. DOI: 10.1002/cber.19040370280.
  16. ↑ Debierne, A. Sur l'émanation de l'actinium. C. R. Hebd. Seances Acad. Sci. 1904, 138, 411–14, <http://gallica.bnf.fr/ark:/12148/bpt6k3092p/f435.image.langEN>.
  17. ↑ 17.0 17.1 Rutherford, E. The Succession of Changes in Radioactive Bodies. Philos. Trans. R. Soc., A 1905, 204, 169–219, <http://gallica.bnf.fr/ark:/12148/bpt6k56009j/f185.image.langEN>.
  18. ↑ Debierne, A. Sur l'actinium. C. R. Hebd. Seances Acad. Sci. 1904, 139, 538–40, <http://gallica.bnf.fr/ark:/12148/bpt6k30930/f538.image.langEN>.
  19. ↑ Lecoin, M.; Perey, M.; Riou, M.; Teillac, J. Sur les rayonnements β et γ de l'actinium et de l'actinium K. J. Phys. Radium 1950, 11 (5), 227–34. DOI: 10.1051/jphysrad:01950001105022700.
  20. ↑ Taylor, S. R. Abundance of chemical elements in the continental crust: a new table. Geochim. Cosmochim. Acta 1964, 28 (8), 1273–85. DOI: 10.1016/0016-7037(64)90129-2.
  21. ↑ Mason, Brian Principles of Geochemistry; Wiley: New York, 1952.
  22. ↑ Peppard, D. F.; Mason, G. W.; Gray, P. R.; Mech, J. F. Occurrence of the (4n + 1) Series in Nature. J. Am. Chem. Soc. 1952, 74, 6081–84. DOI: 10.1021/ja01143a074.

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