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crystal-field theory A theory describing the behaviour of elements with partly filled d- or f-orbitals. In geochemistry it is particularly concerned with the crystal-field (ligand-field) effects on the first transition metal ions, i.e. Sc, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu. If the anions surrounding a cation are considered as point charges then the electrostatic interactions between them will vary according to: (a) ionic distance; (b) the strength of the charge; and (c) the coordination number (how anions are distributed around the cation). Some of the five d-orbitals will have maximum electron density nearer to the anion than others. Interactions between cation and anion will affect the energies of the d-orbitals and cause a separation of energy levels between orbitals (crystal-field splitting).