Why don't ionic compounds melt easily as molecular compounds?
Solid ionic substances are composed of crystal lattice structures. These structures are held together by a strong force of attraction between positive and negative ions. In order for a solid ionic substance to melt, enough energy must be applied to disrupt the strong attractive forces between the positive and negative ions.
The atoms in solid covalent substances are held together by relatively weak forces of attraction. In order for a covalent substance to melt, less energy is needed to disrupt the weaker attraction between the atoms.
Increasing the Temperature, increases the kinetic energy of the particles in a substance and disrupts the attractive forces between the particles. Since ionic substances have stronger attractive forces between their particles, a higher melting point is needed to disrupt those forces and enable the substance melt.