Ionic Radii:
     Na⁺, 97 pm
     Cl ^-, 181 pm
Radius Ratio: = 97/181 = 0.54

The radius ratio is such that we expect the sodium ions to occupy octahedral holes in an array of chloride ions.

The stoichiometry (one Na⁺ ion per each Cl ^- ion) requires that all of the octahedral holes be occupied. In fact, the NaCl structure consists of a close packed array of Cl ^- ions with Na⁺ ions filling all of the octahedral holes.

Click on the image to the right to view the NaCl structure. The links in the text above go to definitions.