Molecules which have larger molecular masses have ______ melting points.

• A. Larger
• B. Smaller
• C. Unchanged
• D. Unknown

Answer: (A)

Melt temperature hinges on the strength of the forces holding the solid together. For simple covalent molecules, the main attraction is London dispersion forces, which grow stronger as the molecule becomes larger (more electrons and a bigger surface area). Because these intermolecular forces are stronger in bigger molecules, more energy is needed to separate them into a liquid, so the melting point tends to be higher. Therefore, molecules with larger molecular masses have larger melting points. Keep in mind that shape and structure (like branching) can modify this trend, but the general idea is that bigger masses mean stronger dispersion forces and higher melting points.