Ac is the chemical symbol for the element actinium, which is a soft, silvery-white radioactive metal. It is part of the actinide series in the periodic table and has the atomic number 89. Actinium is found in trace amounts in uranium and thorium ores and is primarily used in radiation therapy for cancer treatment due to its radioactive properties. Actinium was discovered in 1899 by Friedrich Oskar Giesel, a German chemist. It is highly radioactive and glows faintly in the dark due to its radioactivity. Actinium-227, one of its isotopes, has a half-life of about 21.8 years and is used in various scientific research applications.