The stability of the following alkali metal chlorides follows the order

Stability of a compound depends upon its enthalpy of formation $\Delta Hf$ . The more negative value of $\Delta Hf$ shows more stability of a compound. Thus KCl is more stable then LiCl,

$\Delta H_f$ for $LiCl$ $=-408kj/mol$

$\Delta H_f$ for $KCl$ $= -436kj/mol$

$∆$H_f for CsCl = -443 kj/mol