Actinium, a radioactive chemical element that occurs in nature in trace amounts as a result of the radioactive decay of uranium and thorium. Actinium, a heavy, silver-white metal, is usually prepared artificially by bombarding radium with neutrons in a nuclear reactor. It is the first element of the actinide series. The longest-lived isotope of actinium has a half-life period of about 22 years.

Actinium was discovered in 1899 by the French chemist Andr Debierne. Its name is derived from a Greek word meaning ray in recognition of its radioactivity.

Symbol: Ac. Atomic number: 89. Atomic weight: 227. Specific gravity: 10.07. Melting point: 1922 F. (1050 C). Boiling point: about 5400 F. (3000 C). Three actinium isotopes, Ac225, Ac227, and Ac228, occur in nature, and eight others have been produced artificially. Actinium belongs to the actinide series of the Periodic Table and has a valence of +3.