Ionic size crystal radii of ions

A set of empirical ionic radii can be derived from the direct measurement of internuclear distances in crystal structures. The additivity of ionic radii is substantiated by the near constancy of the differences in internuclear distances Ar between the alkali metal halides, as shown in Table 4.2.1. 

By assuming a value for the radius of each single ion (e.g., 140 pm for O2- and 133 pm for F ), a set of self-consistent ionic radii can be generated. There exist several compilations of ionic radii, and some selected representative values for ions with coordination number 6 are listed in Table 4.2.2. 

Some comments on the ionic radii collected in Table 4.2.2 are as follows  

Internuclear distance between contacting neighbors = ration + fanion- 

a hard sphere model is assumed for the ionic crystal, with ions of opposite charge touching one another in the crystal lattice. Such an approximation means that the assignment of individual radii is somewhat arbitrary. The values listed in Table 4.2.2 are a set of data which assume that the radius of 02 is 140 pm and that of F is 133 pm. 

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