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A chemical element, Ac, atomic number 89, and atomic weight 227.0. Actinium was discovered by A. Debierne in 1899. Milligram quantities of the element
are available by irradiation of radium in a nuclear reactor. Actinium-227 is a betaemitting element whose half-life is 22 years. Six other radioisotopes with half-lives ranging from 10 days to less than 1 minute have been identified.
The relationship of actinium to the element lanthanum, the prototype rare earth, is striking. In every case, the actnium compound can be prepared by the method used to form the corresponding lanthanum compound with which it is isomorphous in the solid, anhydrous state.