Actinium

(Ac), radioactive chemical element, in Group IIIb of the periodic table, atomic number 89. Actinium was discovered (1899) by Andr�-Louis Debierne in pitchblende residues left after Pierre and Marie Curie had extracted radium and was also discovered (1902) independently by Friedrich Otto Giesel. A ton of pitchblende ore contains about 0.15 mg of actinium. The rare, silvery-white metal