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Actinium is the first element of the actinide series. It is a soft, silvery-white metal that is found naturally in uranium ores, but is now synthesized readily from the uranium in atomic reactors.

Actinium's name stems from the Greek "aktino" meaning "ray." The element's intense radioactivity excites the surrounding air molecules, thereby allowing actinium to "glow" in the dark.

Biologically, actinium is considered to be an extremely hazardous radioactive poison, approximately 150 times more radioactive than radium. In the body, actinium tends to accumulate in the skeleton.

Only two applications of this element are significant. Actinium serves as both a source of thermoelectric power and a source of neutrons.