The first actinide metals to be prepared were those of the three
members of the actinide series present in nature in macro amounts,
namely, thorium (Th), protactinium (Pa), and uranium (U). Until the
discovery of neptunium (Np) and plutonium (Pu) and the subsequent
manufacture of milligram amounts of these metals during the hectic
World War I1 years (i.e., the early 1940s), no other actinide element was
known. The demand for Pu metal for military purposes resulted in rapid
development of preparative methods and considerable study of the
chemical and physical properties of the other actinide metals in order to
obtain basic knowledge of these unusual metallic elements.