lattice vs thermal stability!

1.
AgCl is mainly an ionic compound. In AgCl, chloride ions are polarized by silver ions, as the electronclouds of chloride ions are distorted. The polarization of chloride ions causes partially covalent character of the ionic bonds between silver and chloride ions. In other words, the bonds between silver and chloride ions are mainly ionic and partially covalent. Such partial covalent character increases the bond strength between silver and chloride ions, and thus more energy is released during the formation of the lattice. Therefore,the experimental lattice enthalpy of AgCl is more negative than the theoretically one.


2.
The lattice energy is mainly due to the attractive forces between the opposite ions. According to Coulomb's law, such attractive forces are inversely proportional to the square of the inter-ionic distance (the distance between neighbouring opposite ions). Sodium ion is smaller in size than potassium ion, because sodium ion has one less filled electronic shells than potassium ion. The smaller size of sodium ion leads to a shorter inter-ionic distance, and thus strong attractive forces between opposite ions. More energy is released during the formation of NaI lattice than that of KI. In other words, the lattice enthalpy of NaI is more negative than that of KI.