Definition of actinide in US English:



  • 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.

    • ‘Local magnetism occurs in rare-earth metals (such as gadolinium) and the actinides (such as neptunium) due to the incomplete filling of electrons in the inner atomic shells.’
    • ‘Second, he emphasizes the economic value of actinides and fission products in the spent fuel but does not acknowledge either economic or proliferation concerns.’
    • ‘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.’
    • ‘Like the lanthanide elements, the actinides have very similar chemical properties.’


1940s: from actinium + -ide, on the pattern of lanthanide.