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Before we take up the discussion on radius ratio rule and its effete on the structure of ionic solids, it will be appropriate to understand the term of ionic. The radius of an isolated ion is difficult to define. Theoretically the radius of an ion extends to infinity. So the absolute radius of an ion is not possible to measure. The radii are calculated form the observed distance between two nearest neighbors.

When a neutral atom gains an electron to become an anion(-ve ion), its radius increases. This is expected due to the increase in the number of electrons which the same nuclear change has to hold. The force of attraction on the electron decreases and so the radius increases. On the other hand, when a neutral atom loses an electron to form a cation, the radius decreases. because the nuclear charge is now acting on a fewer number of electrons. It is also expended that the size of the ions decreases in an is electronic series with increase in nuclear charge.

The evaluation of ionic radii is different form that of atomic radii. Since two similar atoms do not combine to form an ionic bond, so the measured distance between an ion pair cannot be halved to get ionic radii as can be done in case of atomic radii. It is difficult to decide the contribution due to cation and anion to the intrinsic distance.

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