14 concentration differences observed between the lanthantdes in different marine phases. In general, the actinides tend towards covalent-hybrid bonding L sia, Hence, complex To raition should be especially typical of the hiaher oxidation states of these elements. A characteristic property of transuranium elements is their ability to exist in several oxidation states. The oxidation states of the elements uranium through americium are shown below, after Keller (op.cit.). The acti- nides of atomic number greater than americum exist predominantly in the +3 State. In this diagram, the most stable state in an aqueous solution free of complexing agents is underlined. Atomic number 89 909) 92 93 94 95 Element Ac Th U Np Pu Am 3 3 3 3 3 3 4 4 4 4 4 5 5 5 5 5 6 6 6 6 7 7 Oxidation state 4 Pa The general chemical behavior of the compounds or ions of elements, uranium through americium, when in the same oxidation state, is similar eycert for their oxidation-reduction potentials (Cotton and Wilkinson, op. C1t.). Because of the differences in the electronic environments of succeed- ‘wc actinides, elements in different transition groups which have the same tvoe cf bonding electrons as a particular actinide may be more similar chemicelly to that actinide than are neighbor actinides. In a pure aqueous solution, the principal cationic forms of the +3, +4, Ars-.1 and the +6 states are we ait MO and MO."”, respectively. In the stectest (aqueous) case, plutonfum has the unique chemical property of being