actinium

actinium

a radioactive element of the actinide series, occurring as a decay product of uranium. It is used as an alpha-particle source and in neutron production. Symbol: Ac; atomic no.: 89; half-life of most stable isotope,²²⁷Ac: 21.6 years; relative density: 10.07; melting pt.: 1051°C; boiling pt.: 3200 ± 300°C

Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005

actinium

[ak′tin·ē·əm]

(chemistry)

A radioactive element, symbol Ac, atomic number 89; its longest-lived isotope is ²²⁷Ac with a half-life of 21.7 years; the element is trivalent; chief use is, in equilibrium with its decay products, as a source of alpha rays.

McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.

Actinium

 

Ac, a radioactive chemical element in the Group III of the Mendeleev periodic table, atomic number 89. No stable isotopes.

Actinium was discovered in 1899 by the French chemist A. Debierne while he was studying uranium ore tailings. The ten known radioactive isotopes of actinium have mass numbers ranging from 221 to 230. The longest-lived actinium isotope, ²²⁷Ac (half-life T_½=21.8 years), emits beta particles (98.8 percent) and alpha particles (1.2 percent). The isotopes ²²⁷Ac and ²²⁸Ac (T_½=6.13 h; also known as mesothorium II, MsThll) occur naturally in uraniferous and thoriferous ores as members of the naturally occurring radioactive families. The surface layer of the earth’s crust (thickness 1.6 km) contains 11,300 tons of ²²⁷Ac, but the actinium content in the earth’s crust is very small compared to that of other elements (6.10^-10 percent by mass).

The element actinium is a silvery white metal with a face-centered cubic lattice (T_m=1050° ± 50°C, T_b probably about 3300°C); it glows faintly in the dark because of its high radioactivity. In moist air it becomes coated with a white film of oxide which prevents further oxidation of the metal. Actinium is trivalent in chemical compounds. Almost all actinium salts are white and are colorless in solution. Most of them (except AcP0₄) are isomorphic with the corresponding lanthanum compounds. Actinium forms the same insoluble compounds as does La (hydroxide, phosphate, oxalate, carbonate, fluosilicate). Actinium hydroxide Ac(OH)₃ is more basic than lanthanum hydroxide La(OH)₃. Because of the extraordinary similarities between the chemical properties of actinium and lanthanum, isolation of actinium in pure form from its natural hosts (containing La and other rare earths) involves great difficulties, and therefore milligram amounts of actinium (²²⁷Ac) are obtained artificially through neutron bombardment of radium ²²⁶Ra. Since probability of ²²⁷Ac decay with alpha emission is low, and the energy of beta particles emitted by this isotope is very small (46 keV), any emission accompanying the radioactive decay of ²²⁷Ac went undetected for a long time; until 1935 it was assumed that radioactive decay of ²²⁷Ac occurred without emission. Modern instruments are capable of identifying such soft beta radiation, but it is still quite difficult to make quantitative measurements; therefore, the behavior of ²²⁷Ac is usually monitored in terms of the activity of its daughters, in experiments involving trace amounts of the radionuclide. Mixed with beryllium, ²²⁷Ac is used in the preparation of neutron sources, in which neutrons are formed by irradiating ⁹Be nuclei with alpha particles emitted by ²²⁷Ac daughters.

REFERENCES

Seaborg, G., and J. Katz. Khimiia aktinidnykh elementov. Moscow, 1960. (Translated from English.)
Bagnall, K. Khimiia redkikh radioaktivnykh elementov, poloniiaktinii. Moscow, 1960. (Translated from English.)

S. S. BERDONOSOV

The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.