actinium
White, radioactive, metallic element, the first of the actinide series, atomic number 89, relative atomic mass 227; it is a weak emitter of high-energy alpha particles.
Actinium occurs with uranium and radium in pitchblende and other ores, and can be synthesized by bombarding radium with neutrons. The longest-lived isotope, Ac-227, has a half-life of 21.8 years (all the other isotopes have very short half-lives). Chemically, it is exclusively trivalent, resembling in its reactions the lanthanides and the other actinides. Actinium was discovered in 1899 by the French chemist André Debierne (1874–1949).
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