Studies on the Si-O distances and the geometrical features of the CrO6 octahedra in several chromium silicates reveal that the electronegativity of the Cr3+ ion depends on the geometrical feature of the CrO6 octahedron. Since the electron density and the screening constant of the Cr3+ ion in the regions directed toward the oxygens decrease with decrease of the repulsion between the 2p orbitals of the oxygen ions and the 3d orbitals in the t2g state of the Cr3+ ion, the effective nuclear charge and the electronegativity of the Cr3+ ion increase with decrease of the repulsion. It seems that the electronegativity of the Cr3+ ion based on the heat of formation corresponds to the value in a regular octahedron.
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