The seventeen elements which make up the rare earth family consist of two non-4f-electron members, Sc and Y, and the fifteen 4f electron containing lanthanide elements. In general the physical properties, which do not directly involve the 4f electrons, vary in a smooth and regular manner, especially for the lanthanide group. These include: the metallic radii, electronegativities, melting points, and crystal structure sequence of the metallic elements. By utilizing this sytematic behavior, we have learned a great deal about solid solution formation and the crystal chemistry of intermetallic compounds of, not only the rare earth elements, but also the other elements in the periodic table. Furthermore, the application of systematics has demonstrated 4f electron hybridization with the valence electrons in metallic and non-metallic systems, and also lead to a method for predicting heats (free energies) of formation of rare earth compounds.

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