不考虑晶格？

这种程度只需要答到The bond strength of the metal are weaker than that of the ionic compounds. 就够了。。。如果你要深究，就可以说 The enthalpy is a function of possible realizations, in other words, the more possible configurations you have, the higher enthalpy you will get. In this case,the ionic compounds are constrained by their coulomb interactions so that it suffers higher enthalpy(mp). Vice versa, the metallic compounds are randomly distributed charges which are quite flexible, one should know their enthalpy relatively lower.
Hence, mpmetal is smaller than mpion.