Actinium

Actinium is a soft, silvery-white radioactive metal that can glow with a pale blue light as its radiation ionizes the surrounding air. It occurs naturally only in tiny amounts in uranium and thorium ores, so it is usually made in nuclear reactors for research and medical applications. Certain actinium isotopes emit alpha particles that can be used in targeted radiation therapy, while the element's scarcity and radioactivity limit its uses.