noun
- A radioactive element of the actinide series; found in uranium ores.
- synonyms: Ac, atomic number 89
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| APA | WordNet. (2010). actinium. Retrieved September 21, 2026, from http://smartdefine.org/actinium/definitions/1247598 |
| Chicago | WordNet. 2010. "actinium" http://smartdefine.org/actinium/definitions/1247598 (accessed September 21, 2026). |
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| MLA | WordNet. "actinium" 23 October 2010. Web. 21 September 2026. <http://smartdefine.org/actinium/definitions/1247598> |