actinium ac·tin·i·um (āk-tĭn'ē-əm)
A radioactive element found in uranium ores. Its longest lived isotope is Ac 227 with a half-life of 21.6years. Atomic number 89; melting point 1,050°C; boiling point (estimated) 3,200°C; specific gravity (calculated) 10.07; valence 3.
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