Ionic Radii

The experimentally calculated anion-cation distances in highly ionic solids have to be interpreted on the assumption that each and every ion has a nearly fixed radius. As like, the difference in cation-anion distance between the halides KX and NaX is close to 36 pm irrespective of the anion X, and it is natural to attribute this to the difference in radii between K+ and Na+. To separate the observed distances into the sum of two ionic radii is, Difficult to do in satisfactory way. One procedure is to look for the optimal value in the electron density distribution between neighboring ions, but apart from the experimental problems involved such measurements do not really support the assumption of constant radius.