Ions in crystal lattices of purely ionic compounds are spherical; however, if the positive ion is small and/or highly charged, it will distort the electron cloud of the negative ion, an effect summarised in Fajans' rules. This polarization of the negative ion leads to a build-up of extra charge density between the two nuclei, that is, to partial covalency. Larger negative ions are more easily polarized, but the effect is usually important only when positive ions with charges of 3+ (e.g., Al) are involved. However, 2+ ions (Be) or even 1+ (Li) show some polarizing power because their sizes are so small (e.g., LiI is ionic but has some covalent bonding present).  Note that this is not the ionic polarization effect that refers to displacement of ions in the lattice due to the application of an electric field.