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This is actinium.
Symbol Ac
Atomic number 89
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.

See also

Periodic table of the elements