David Lewis Clark

The synthesis and characterization of transuranium elements played an important and in the history of the periodic table. Prior to their discovery, elements up to uranium were thought to be 6d elements, and in the late 1930s, they were placed in the periodic table as such. The discovery of neptunium and plutonium and the determination of their chemical properties suggested that valence electrons were entering 5f orbitals and represented the emergence of a new series in the periodic table. The original characterization of americium and curium failed, until it was realized that they may behave more like uranium and plutonium than transition elements. Glenn Seaborg introduced the actinide concept and proposed rearranging the periodic table to create a new 5f actinide series akin to the 4f lanthanide series. Due to imperfect screening of the 5f electrons, and the subsequent changes in the energetics of both the 5f and 6d orbitals as one progresses along the series, the actinides experience an actinide contraction and display a fascinating set of periodic trends. Moving from left to right, the valence 5f electrons contract and lose their ability to form chemical bonds. The crossover from bonding (itinerant) to ionic (magnetic) behavior gives rise to many exotic and interesting chemical and physical properties and has challenged modern approaches to electronic structure both in theory and experiment. The multiplicity of oxidation states, coupled with the hydrolysis behavior of the aqueous ions, makes the chemical behavior of protactinium through americium among the most complex of the elements in the periodic table.