Which of the following best explains why most solid covalent compounds (molecular solids) have low melting points?

• A. They are composed of ions that easily separate when heated.
• B. Their molecules are very large and cannot pack closely together.
• C. They are held together by weak intermolecular forces rather than strong ionic or covalent bonds.
• D. They contain free electrons that disrupt the crystal lattice.

Answer: (C)

Verified step by step guidance

1. Understand the nature of solid covalent compounds, also known as molecular solids, which consist of individual molecules held together in the solid state.
2. Recognize that within each molecule, atoms are bonded by strong covalent bonds, but between molecules, the forces holding them together are much weaker intermolecular forces (such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds).
3. Recall that melting a solid involves overcoming the forces holding the particles together in the solid phase, so the melting point depends on the strength of these forces.
4. Compare molecular solids to ionic solids: ionic solids are held together by strong electrostatic forces between ions, which require more energy (higher temperature) to break, resulting in higher melting points.
5. Conclude that molecular solids have low melting points because the weak intermolecular forces between molecules require relatively little energy to overcome, unlike strong ionic or covalent bonds.