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Covalently bonded substances fall into two main types:

• simple covalent molecules
• giant covalent structures

Simple molecules contain only a few atoms held together by covalent bonds. An example is carbon dioxide (CO₂), the molecules of which contain one atom of carbon bonded with two atoms of oxygen.

However, although the covalent bonds holding the atoms together in a simple molecule are strong, the intermolecular forces between simple molecules are weak.

Properties of simple molecular substances

• Low melting and boiling points – this is because little energy is needed to break the weak intermolecular forces.
• Do notconductelectricity – this is because they do not have any free electrons or an overall electric charge in any state of matter.

Hydrogen, ammonia, methane and pure water are also simple molecules. All have strong covalent bonds between their atoms, but much weaker intermolecular forces between molecules.

When one of these substances melts or boils, it is these weak intermolecular forces that break, not the strong covalent bonds. At room temperature, simple molecular substances are gases, or liquids or solids with low melting and boiling points. They are also soft, again due to the weak intermolecular forces that can be broken easily.

The slideshow shows how the weak intermolecular forces between water molecules are broken during boiling or melting.