Cambridge O Level Chemistry · Syllabus 5070 · Atoms, Elements and Compounds
Intermolecular forces
What is Intermolecular forces?
The weak forces of attraction that act between one molecule and the next in a simple molecular substance. They are much weaker than the covalent bonds within a molecule, so only a small amount of energy is needed to overcome them, which is why simple molecular substances have low melting and boiling points. When such a substance melts or boils, these forces are overcome while the covalent bonds inside each molecule remain intact.
This definition is part of the Atoms, Elements and Compounds chapter in Cambridge O Level Chemistry.
Questions students ask about Intermolecular forces
Why do simple molecular substances have low melting points if covalent bonds are strong?
Because melting or boiling a simple molecular substance does not break any covalent bonds. The strong covalent bonds are inside each molecule; between the molecules there are only weak intermolecular forces, and it is these weak forces that are overcome when the substance melts or boils, which needs little energy. Steam is still made of \(\mathrm{H_2O}\) molecules with their covalent bonds intact. Giant covalent substances such as diamond, graphite and silicon(IV) oxide are different: melting them means breaking the covalent bonds themselves, so their melting points are very high.