Simple molecules

Covalently bonded substances fall into two main types:

  1. simple molecules
  2. 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.

Dot and cross model for carbon dioxide. There are two shared groups of electrons, each with two dots and two crosses.

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.

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.

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

A large number of water molecules, showing the strong covalent bonds between the oxygen and hydrogen atoms in each molecule, and the weak forces between the molecules themselves.