Actinium

Actinium is a soft, silvery, intensely radioactive metal that glows pale blue in the dark as its radiation excites the surrounding air. André-Louis Debierne discovered it in 1899 in uranium ore residues left by Marie and Pierre Curie, and named it from the Greek aktis, ray. It gives its name to the actinides, the row of elements from actinium to lawrencium.

Actinium behaves chemically much like lanthanum. It is so scarce in ores that it is usually made in nuclear reactors or particle accelerators. Its most promising use is medical: actinium-225, with a half-life of about ten days, is attached to molecules that home in on cancer cells and destroys them with short-range alpha radiation.