Fazan's rule predicts the solubility of ionic compounds based on the polarizability of the ions involved. According to Fazan's rule, the solubility of an ionic compound should increase with the increase in the size of the ions involved. This is because larger ions are more easily polarized, leading to a more efficient separation of the ions in the crystal lattice and therefore higher solubility.
However, the trend in the solubility of alkali metal halides (LiX, NaX, KX, RbX, CsX) down the group is the opposite of what Fazan's rule predicts. The solubility of alkali metal halides actually decreases down the group, even though the ionic character increases. This is due to the influence of hydration energy.
As the size of the cation increases down the group, the hydration energy (the energy released when a cation is hydrated by water molecules) decreases. This means that the larger cations are less strongly hydrated than the smaller ones. Since the solubility of ionic compounds is determined by the balance between lattice energy (the energy required to break the crystal lattice) and hydration energy, the decrease in hydration energy down the group results in a decrease in solubility.
In addition, the size of the anion also increases down the group, which can further decrease the solubility of the alkali metal halides. This is because the larger anions occupy more space in the crystal lattice, leading to a stronger attraction between the anions and cations and therefore a higher lattice energy.
Overall, the decrease in solubility of alkali metal halides down the group is due to a combination of factors, including the decrease in hydration energy of the cation and the increase in size of the anion.
