Why might a substance with a weak molecular structure have lower melting points than ionic compounds?

A substance with a weak molecular structure exhibits lower melting points compared to ionic compounds primarily because it requires less energy to overcome the weak intermolecular forces that hold the molecules together.

In molecular substances with weak structures, the forces, such as van der Waals interactions or hydrogen bonds, are relatively weak compared to the strong electrostatic forces of attraction that exist between the ions in ionic compounds. To melt a solid, the forces holding the particles in place must be overcome. Since molecular substances have weaker interactions, less energy is needed to disrupt those forces, leading to a lower melting point.

In contrast, ionic compounds are characterized by a regular lattice structure formed by strong ionic bonds between positively and negatively charged ions. This necessitates a substantial amount of energy to break these bonds to change the state from solid to liquid, resulting in higher melting points.

Thus, the ability to overcome these weaker intermolecular forces in weak molecular structures is the key to understanding the lower melting points when compared to the robust ionic bonds in ionic compounds.