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