Actinium

radium ← actinium → thorium La ↑ Ac ↓ —

Atomic properties Atomic number 89 Electron configuration [Rn] 6d1 7s2 Physical properties Melting point 1050 °C (1350 K) Boiling point 3300 °C (3600 K) Density 10.07 g cm−3 Chemical properties Electronegativity 1.1 (Pauling) Ionization energies 1st 5.3807(3) eV 159.16(3) kJ mol−1 2nd 12.06 eV 1164 kJ mol−1 Atomic radii Covalent radius 215 pm Metallic radius 188 pm Ionic radius 126 pm (Ac3+, Oh) Thermodynamic properties 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).

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: however, actinium-227 (t_½ = 21.772(3) a) is formed as a decay product of uranium-235 (t_½ = 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. Credit is usually given to French chemist André-Louis Debierne, who described a new source of radiation from pitchblende in 1899, with further details in 1900. He named the new element actinium, 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; 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, with further details in 1903. Giesel called his new element emanium, from "emanation". 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, t_½ = 3.96(1) s) and the "induced radioactivity" (lead-211, t_½ = 36.1(2) min, deposited on surfaces by decay of the ²¹⁹Rn). 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. Both Rutherford and Debierne 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. 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.

Occurance and preparation

Actinium-227 (t_½ = 21.772(3) a) is a decay product of uranium-235 (t_½ = 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, equivalent to a total of about 14000 tonnes of actinium at any one time. Actinium-228 (t_½ = 6.15(2) h) also occurs naturally, as a decay product of thorium-232 (t_½ = 14.05(6) × 10⁹ a): taken over the whole of the Earth's crust, its abundance (6 × 10⁻¹³ ppm) 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 (t_½ = 10.0(1) d) have also been found in uranium refinary wastes and in Brazilian monazite: 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.

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.

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