Actinium is a silvery-white, radioactive metal with the symbol Ac and atomic number 89. It was discovered in 1899 by the German chemist Friedrich Oskar Giesel. Actinium is part of the actinide series and is found in trace amounts in uranium and thorium ores. This element is notable for its use in radiation therapy for cancer treatment due to its ability to emit alpha particles. Actinium-227, a common isotope, has a half-life of 21.77 years and is used in targeted alpha therapy. Its radioactivity makes it both useful and hazardous, requiring careful handling.