Actinium

Actinium, chemical element, symbol Ac; for physical constants see Periodic Table. Actinium was discovered by André Debierne in 1899. It is radioactive and occurs naturally in minute amounts in uranium minerals. Actinium is synthesized by irradiating radium with neutrons. A reactive metal, it is obtained by reducing its fluoride with lithium vapor. It is about 150 times as active as radium and valuable in the production of neutrons. There are 26 known isotopes. Actinium- 227, a beta emitter and a powerful source of alpha rays, has the longest half-life (21.77 years). Chemically, actinium is similar to lanthanum. It is the first of the actinides, a series of homologous elements analogous to the lanthanide transition series.