The ionic radius is the radius of an atom in its ionized state. Ionic radii depend on two factors: the degree of ionization and the type and number of neighboring atoms (coordination). The radius decreases as electrons are lost and increases as electrons are gained. The length of an ionic bond reflects a balance between the electrostatic Coulomb attraction of the unlike charges and the mutual Born repulsion of the positively charged nuclei.
The effect of surrounding a cation with anions is to expand its radius. For example, a sodium ion (Na+) with four neighboring oxygen atoms has a radius of 99 picometers (pm), but when surrounded by 12 oxygen atoms its radius is 139 pm.
Cation radii are derived by measuring the bond length (d) and subtracting the anion radius (R-).