Ionic compounds have high enthalpy of fusion and high enthalpy of vaporization. High heat is required to melt and vaporize ionic compounds because ionic compounds are formed between cation and anion and the oppositely charged ions are held by strong electrostatic force of attraction.
Ionic compounds form crystal lattice in which there is alternate arrangement of cation and anion in an array of three dimensional space. Ionic compounds do not form amorphous solids.
