The element with atomic number 89 is actinium (Ac). Actinium is a radioactive element in the actinide series of the periodic table. It is a silvery-white, lustrous metal that is highly reactive and can be found in trace amounts in uranium ores. Actinium was discovered in 1899 by Friedrich Oskar Giesel, and its name is derived from the Greek word "aktis," meaning beam or ray, due to its radioactivity.