If we use X-ray diffraction to find the edge length of a NaCI-like unit cell and if we know the radius of one ion, we can determine the radius of the other ion in the compound.

KCl is isomorphous with NaCl, with a cell edge of 628 pm. We can determine the radius of the potassium ion from the cell dimension and the radius of the chloride ion, 181 pm.

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