Actinium, named from the Greek aktinos (ray) is a rare, extremely radioactive metal that glows in the dark (the photo shown above is of \(Ac_2O_3\)). It was discovered by Debierne in 1899 and again in 1902 by Giesel. Eventual priority was given to Debierne's work.
Samples of actinium quickly decay to thorium and francium. The longest-lived isotope (Ac-227) has a half-life of 21.8 years. The element is obtained as an impurity in pitchblende, an ore mined for its uranium content. One-tenth of one gram of actinium can be recovered from 1 ton of pitchblende!
Stephen R. Marsden (ChemTopics)