The general aN index is established for mols. containing heteroatoms, rings, and multiple bonds. The general aN index is able to describe mols. with minute differences in structure and can also reveal the properties of mols. This theory is successfully applied to the case of neutral phosphorus extractants. Both the mol. polarity and steric effect are characterized by the general aN index. The relationships between these properties and the distribution ratios for extracting Y, Ce, U, and Th are also shown by the general aN index.

Key words: URANIUM, THORIUM, EXTRACTIVE AGENT, EXPONENT, CERIUM, STERIC EFFECT, MOLECULAR ORBITAL THEORY, YTTRIUM, TOPOLOGY, OXYGEN COMPOUNDS