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

The melting and boiling points of a substance depend on the strength of the forces that must be overcome to change its state. For simple covalent molecules, there are two types of forces to consider:

1. Intramolecular forces (covalent bonds within the molecule): These are strong but are not broken during melting or boiling.
2. Intermolecular forces (forces between molecules): These are the forces that must be overcome during phase changes. In simple covalent molecules, these are typically weak van der Waals forces or dipole-dipole interactions.

Because the intermolecular forces are weak, relatively little energy is needed to separate the molecules from one another. This is why simple covalent substances such as water, methane, and carbon dioxide have low melting and boiling points compared to ionic or metallic substances.

The other options do not explain the low melting and boiling points:

• Having definite shapes describes molecular geometry but does not determine the strength of forces between molecules.
• Being three-dimensional structures is a feature of giant covalent substances (like diamond), which actually have very high melting points.
• Possessing shared electron pairs is what makes them covalent, but the shared pairs (covalent bonds) are not broken during melting or boiling, so they do not account for the low melting/boiling points.