Higher the stability of a substance, higher will be its standard molar enthalpy of formation value, |∆fH∘| Among the halides, F− is smallest and I− is largest in size. So, according to Fajan's rule, NaF is mostly ionic in nature. NaI<NaBr<NaCl<NaF ⟶ Ionic nature → Thermal stability So, NaF has highest value of |∆fH∘|.