Actinium, ak TIHN ee uhm, a chemical element, is an extremely rare, silvery-white, radioactive metal that glows in the dark. A radioactive element decays (breaks down) by sending out particles from its nucleus. In this process, it changes into another element. Natural actinium is itself a product of radioactivity. It is created by the decay of an isotope of uranium known as uranium 235. An element's isotopes have the same number of protons but different numbers of neutrons in their nucleus.