actinium [Gr.,=like a ray], radioactive chemical element; symbol Ac; at. no. 89; at. wt. 227.0278; m.p. about 1,050°C; b.p. 3,200°C±300°C; sp. gr. 10.07; valence +3. Actinium is a silver-white metal with a cubic crystalline structure. It is found with uranium minerals in pitchblende . The pure metal can be prepared by reducing its fluoride with lithium vapor at about 1,200°C. Actinium-227, the most stable isotope, has a half-life of 21.6 years. The other seven isotopes of actinium have very short half-lives ranging from 10 days to less than 1 min. Actinium is in... Read more