13 the valence electronic environments of actinides, and transition element groups which have been studied more thoroughly. Chemical bonding is related we shialdina of boncing (d or f) orbital electrons by s and p orbitals. Arong the groups of transition elements, the degree of shielding follows the order: Lanthanides (4f mites Actinides (Sf orbitals) » main group transition elements (3,4 or 5d orbitals) This spatial difference in electronic structure results in chemical properties of the actinide elements which are expected to be intermediate between the characteristic behavior of the lanthanides and main group transition elements (Cotton and Wilkinson, 1966). The main aroup transition elements, by nature of having their valence electrons at the periphery of atom, interact strongly with ligands. These interactions result in large variations in the chemical properties of succeeding main qroup transition elements which are well known. In contrast to the variations exhibited in succeeding "d” group transition elements, the cherical behavior of the trivalent lanthanide elements in the marine environrent is quite similar. This behavior is well illustrated by the relatively tow concentration differences (when each element concentration is normalized te its natural abundance in shale) observed for the trivalent lanthanides in sea water and mineralogical phases of marine sediments (Goldberg et al.,1963; Piner, 1975). Unlike the main group transition elements, the lanthanides form restly fonic compounds and exhibit chemical properties which are largely deterwined by the size of the nrs jon. The size of each of the 14 succeeding Varthanides steadily decreases with increasing atomic number (the lanthanide Cemtrection) and these elements thus form increasingly stronger metal-ligand coetleres with decreasing size; which may partly explain the small