What accounts for the low melting and boiling points of covalent molecules?

Analysis

The question asks for the fundamental reason why covalent molecular substances (like water, methane, or carbon dioxide) exhibit low melting and boiling points compared to ionic or metallic substances.

Concept Explanation

To understand this, we must distinguish between intramolecular forces and intermolecular forces:

1. Intramolecular forces: These are the strong covalent bonds within the molecule (the shared electron pairs holding the atoms together). These are very strong and require a lot of energy to break.
2. Intermolecular forces: These are the weak forces of attraction between separate molecules (such as Van der Waals forces or dipole-dipole interactions).

When a covalent substance melts or boils, you are not breaking the strong covalent bonds within the molecules; you are only overcoming the weak intermolecular forces that hold the molecules together in a solid or liquid state. Because these forces are weak, very little heat energy is required to pull the molecules apart, resulting in low melting and boiling points.

Step-by-Step Solution

• Step 1: Evaluate Option B and C. Having a definite shape or a 3D structure describes the geometry of the molecule, but it does not dictate the energy required to change its state.
• Step 2: Evaluate Option D. Shared electron pairs are what form the covalent bond itself. These are strong, so if we had to break these to melt a substance, covalent compounds would have extremely high melting points (like diamond or silica).
• Step 3: Evaluate Option A. Since the molecules are held together by weak intermolecular forces, the energy required to overcome these forces is minimal. This directly explains the low melting and boiling points.

Conclusion

Exam Tip: Always remember the "Inter vs. Intra" rule! If a question asks about melting/boiling points, it is almost always about the intermolecular forces (the "glue" between molecules), not the strong bonds inside the molecule.