Strong inside, weaker between
A small molecular substance is made of separate molecules. Each molecule has strong covalent bonds inside it. There are much weaker intermolecular forces between the molecules. “Intermolecular” means between molecules.
Lesson 2.5 · AQA Combined Science
Small molecules and their properties
What happens when it boils?
When a molecular substance melts or boils, energy overcomes the forces between molecules. The strong covalent bonds within each molecule stay intact. That is why many small molecular substances have low melting and boiling points.
Compare molecule sizes
For similar small molecular substances, larger molecules generally have stronger intermolecular forces and higher melting or boiling points. More energy is needed to separate the molecules. This is not because the covalent bonds are weaker in a smaller molecule.
Why no electrical current?
Small molecular substances do not usually conduct electricity because they have no mobile ions or delocalised electrons. Neutral molecules moving around cannot carry an electrical current. This describes the molecular substance itself; some substances form ions when they dissolve.
Remember
Melting and boiling overcome forces between molecules. Covalent bonds inside the molecules remain intact.
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