actinium


actinium
/ak tin"ee euhm/, n. Chem.
a radioactive silver-white metallic element that glows blue in the dark, resembling the rare earths in chemical behavior and valence. Symbol: Ac; at. no.: 89; at. wt.: 227.
[1900-05; ACTIN- + -IUM]

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 (Ac), radioactive chemical element, in Group IIIb of the periodic table, atomic number 89. Actinium was discovered (1899) by André-Louis Debierne in pitchblende residues left after Pierre and Marie Curie had extracted radium and was also discovered (1902) independently by Friedrich Otto Giesel. A ton of pitchblende ore contains about 0.15 mg of actinium. The rare, silvery-white metal is highly radioactive, glowing blue in the dark.

      The most common isotope of actinium is actinium-227; the others, natural and artificial, are too short-lived to accumulate in macroscopic quantity. Actinium-227, which is one of the decay products of uranium-235, has a 21.6-year half-life and in turn decays almost entirely to thorium-227, but about 1 percent decays to francium-223. This whole disintegration chain with its branches is called the actinium (actinium series) decay series.

      Actinium, the ions of which in solution are colourless, exhibits an oxidation state of +3, closely resembling the rare-earth lanthanoid elements in its chemical properties, and is the prototype of a second rare-earth-like series, the actinoid elements.

atomic number
89
stablest isotope
227
oxidation state
+3
electronic config.
[Rn]6d17s2

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Universalium. 2010.

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