- Actinium
 - Includes data tables and history of the chemical element. - Chemical Elements.com: Actinium
- Contains basic information, atomic structure, and isotopes.
- EnvironmentalChemistry.com: Actinium
- Offers an overview, name in other languages, atomic structure, chemical properties, physical properties, and regulation information.
- Wikipedia: Actinium
- Hyperlinked overview of the element discussing notable characteristics, applications, history, occurrence, isotopes, precautions, and more.
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