The actinide elements (atomic numbers 89 through 103) involve the filling of 5f orbitals. All actinides are radioactive, but only uranium and the lighter actinides have half-lives long enough to be present in Earth's environment. The heavier actinides are produced by nuclear reactions and some have very short half-lives. The actinides also undergo a radius contraction as do the lanthanides with an increasing atomic number . They are characterized by variable oxidation numbers, but the importance of the +3 state and the similarities to lanthanides increase for heavier elements. American chemist Glenn Seaborg is credited with revising the Periodic Table so actinides were placed under lanthanides.

SEE ALSO Actinium ; Americium ; Berkelium ; Einsteinium ; Fermium ; Lawrencium ; Mendelevium ; Neptunium ; Nobelium ; Plutonium ; Protactinium ; Rutherfordium ; Seaborg, Glenn Theodore .

Herbert B. Silber


Cotton, Simon (1991). Lanthanides and Actinides. New York: Oxford University Press.

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