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

Analysis

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

Concept Explanation

To understand this, we must distinguish between two types of forces:

1. Intramolecular forces: These are the strong covalent bonds within the molecule (the sharing of electron pairs). These bonds 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 substance melts or boils, we are not breaking the strong covalent bonds inside the molecule; rather, we are providing enough energy to overcome the weak intermolecular forces holding the molecules together in a solid or liquid state. Because these forces are weak, it takes very little heat energy to separate the molecules, resulting in low melting and boiling points.

Step-by-Step Solution

• Step 1: Evaluate Option B and C. Having a three-dimensional structure or a definite shape describes the geometry of a 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. As noted, these bonds are very strong. If we had to break these bonds to melt a substance, covalent molecules 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 pull them apart is minimal. This directly explains the low melting and boiling points.

Conclusion

Exam Tip: Always remember the "Inter vs. Intra" rule. Intramolecular bonds (covalent/ionic) are strong and hold the structure together; Intermolecular forces (Van der Waals/Hydrogen bonds) are weak and determine physical properties like boiling point, melting point, and volatility.