|Atomic mass||(229) amu|
Actinium, discovered in 1899, is the second rarest of the elements. It is a heavy, silvery-white metal that is very radioactive. It has a half-life of 22 years. Samples decay quickly to thorium and francium, making it difficult to determine precise physical properties.
Actinium is obtained as an "impurity" in the pitchblende that is mined for its uranium content. In a ton of pitchblende, one can expect to squeeze about 1/10 of a gram of actinium.
Shown in the image to the right is a minute quantity (approximately 40 mg) of actinium oxide sealed in a radiation-resistant quartz tube.