Ionic compounds consists of + ve and –ve ions held together by electrostatic interactions in such a way that the distance between cations and anions is minimum. If each is considered as a hard sphere the optimum arrangement of ions for the most stable structure would be one that involves touching of the largest number of oppositely charged spheres without any crowding . Such a situation leads to maximum attraction between opposite charges and minimum repulsion between like charges. Such an arrangement of ions in a crystal is greatly influenced by the relative sizes of the ions, that is ratio of radii of the ions. In other words, radius ratio
 (  r⁺  ) determines how many ions of a given size can be held in contact with a single ion of smaller size.
    r^-

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