Roughness indicator, physical meaning

Over the years numerous scales of percent ionic character have been proposed for both simple molecules and solids. They may depend on some physical property, such as dipole moments or refractive indices, or be extracted from some theoretical analysis. The various scales usually agree on a rough ordering but the actual numbers vary widely. It seems to be impossible to define exactly what is meant by percent ionic character . Therefore, the numbers in the various scales have meaning only in terms of the model from which they were derived. Each scale is useful for some experimental or theoretical property, but transfer to other properties is hazardous. 

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