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ac·tin·i·um AHD[ăk-tĭnʹē-əm] D.J.[ækˈtɪniːəm]K.K.[ækˈtɪniəm]n.Symbol Ac （名词）符号 Ac - A radioactive element found in uranium ores, used in equilibrium with its decay products as a source of alpha rays. Its longest lived isotope is Ac 227 with a half-life of 21.7 years. Atomic number 89; melting point 1,050°C; boiling point (estimated) 3,200°C; specific gravity (calculated) 10.07; valence 3. See table at element 锕：一种在铀矿中发现的放射性元素，作为阿尔法射线能源增入点用于与其衰变产物的平衡过程中，其最长的活同位素是半衰期为21.7年的锕227。原子序数为89；熔点为1,050摄氏度；（估计的）沸点为3,200摄氏度；（计算出的）比重为10.07；原子价为3 参见 element
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