Any of the series of fifteen metallic elements from actinium (atomic number 89) to lawrencium (atomic number 103) in the periodic table. They are all radioactive, the heavier members being extremely unstable and not of natural occurrence.
- Like the lanthanide elements, the actinides have very similar chemical properties.
- The members of the first of these blocks are called the lanthanide elements, and those in the second are the actinides.
- The elements that make up this family are also known as the actinides, after the first member of the family.
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