If we use x-ray diffraction to find a, the edge length of a CsCl-like unit cell, we can determine ionic sizes.

CsI is isomorphous with CsCl, with a cell edge of 457 pm. We can determine the radius of the iodide ion from the cell dimensions.

d=1.732 x a = 792pm

With d and the published radius of the Cs⁺ ion, 177 pm, we can estimate the radius of the I ^- ion.

Radius of I ^- ion = 219 pm

Use the links on the right to illustrate these concepts. The links above display definitions.