Abstract
We attempted to simplify and unify the former concepts describing the acid base character of oxides on the basis of their band structure. Duffy optical basicity, Smith acid scale and Bratsch electronegativity model can simply be linked by taking into account the electronegativity and the chemical hardness of the oxides: an acidic oxide has a strong electronegativity together with a strong chemical hardness while a basic oxide has a low electronegativity associated with a weak chemical hardness.