Crystal field theory is one of several chemical bonding models and one that is applicable solely to the transition metal and lanthanide elements. The theory, which utilizes thermodynamic data obtained from absorption bands in the visible and near-infrared regions of the electromagnetic spectrum, has met with widespread applications and successful interpretations of diverse physical and chemical properties of elements of the first transition series. These elements comprise scandium, titanium, vanadium, chromium, manganese, iron, cobalt, nickel and copper. The position of the first transition series in the periodic table is shown in fig. 1.1. Transition elements constitute almost forty weight percent, or eighteen atom percent, of the Earth (Appendix 1) and occur in most minerals in the Crust, Mantle and Core. As a result, there are many aspects of transition metal geochemistry that are amenable to interpretation by crystal field theory.

Cosmic abundance data for the transition elements (Appendix 1) show that each metal of the first transition series is several orders of magnitude more abundant than all of the metals of the second (Y–Ag) and third (Hf–Au) transition series, as well as the rare earth or lanthanide series (La–Lu). Iron, Fe, is by far the predominant transition element, followed by Ni, Cr and Mn. Crustal abundance data also show high concentrations of Fe relative to the other first-series transition elements on the Earth's surface.