actinoid or actinide any member of the series of 15 metallic elements with proton numbers 89 (actinium) to 103 (lawrencium) inclusive that occur together in group 3 and period 7 of the IUPAC periodic table; sometimes the term is restricted to the 14 elements following actinium. Actinoid is now the preferred name. All actinoids are radioactive, and those of proton number 93 or greater are artificial. They are all electropositive, and their chemical properties are similar, due usually to the filling of an inner electron subshell (5f) progressively across the series. Like the lanthanoids, they thus represent a series of inner transition elements.

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