a radioactive element of the actinide series, occurring as a decay product of uranium. It is used as an alpha-particle source and in neutron production. Symbol: Ac; atomic no: 89; half-life of most stable isotope,227Ac: 21.6 years; relative density: 10.07; melting pt: 1051°C; boiling pt: 3200 ± 300°C
WordReference Random House Unabridged Dictionary of American English © 2025
ac•tin•i•um
(ak tin′ē əm),USA pronunciation n. [Chem.]
- Chemistrya radioactive silver-white metallic element that glows blue in the dark, resembling the rare earths in chemical behavior and valence. Symbol: Ac;
at. no.: 89;
at. wt.: 227.
- actin- + -ium 1900–05
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'actinium' also found in these entries (note: many are not synonyms or translations):
AC
- Ac
- actinide series
- actinium emanation
- actinium series
- actinon
- nobelium
- protactinium
- radioactinium