Editing Ionic bonding (section)

==Polarization power effects==
[[Ion]]s in [[crystal lattice]]s of purely ionic compounds are [[sphere|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 (electrostatics)|polarization]] of the negative ion leads to a build-up of extra charge density between the two [[atomic nucleus|nuclei]], that is, to partial covalency. Larger negative ions are more easily polarized, but the effect is usually important only when positive ions with [[electrical charge|charges]] of 3+ (e.g., Al<sup>3+</sup>) are involved. However, 2+ ions (Be<sup>2+</sup>) or even 1+ (Li<sup>+</sup>) 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 the displacement of ions in the lattice due to the application of an electric field.