The relative size of atoms can also be studied by measuring the radii of their ions.

The first ionic radii were obtained by studying the structure of LiI, which contains a relatively small positive ion and a relatively large negative ion. The analysis of the structure of LiI was based on the following assumptions.

• The relatively small Li⁺ ions pack in the holes between the much larger I^- ions, as shown in the figure below.
• The relatively large I^- ions touch one another.
• The Li⁺ ions touch the I^- ions.

If these assumptions are valid, the radius of the I^- ion can be estimated by measuring the distance between the nuclei of adjacent iodide ions. The radius of the Li⁺ ion can then be estimated by subtracting the radius of the I^- ion from the distance between the nuclei of adjacent Li⁺ and I^- ions.

Unfortunately only two of the three assumptions that were made for LiI are correct. The Li⁺ ions in this crystal do not quite touch the I^- ions. As a result, this experiment overestimated the size of the Li⁺ ion. Repeating this analysis with a large number of ionic compounds, however, has made it possible to obtain a set of more accurate ionic radii.

Information provided by: http://chemed.chem.purdue.edu