Ionic Radius
The effective radius of an ion within a crystal lattice, deduced from measured interionic distances.
Unit
pm
In depth
Cations are always smaller than their parent atoms because electron loss reduces repulsion and often empties a whole shell, while anions are larger because added electrons increase repulsion at constant nuclear charge. Within an isoelectronic series, radius falls steadily as nuclear charge rises.
Examples in the real world
Na is 186 pm but Na⁺ only 102 pm; Cl is 99 pm but Cl⁻ expands to 181 pm.