Intermolecular forces

Intermolecular forces are the forces of attraction between separate molecules. They are much weaker than the covalent bonds within a molecule, and they must be overcome for a molecular substance to melt or boil. Malay: daya antara molekul · Chinese: 分子间作用力.

A simple molecular substance, such as water, iodine or carbon dioxide, is made of small molecules. Inside each molecule the atoms are held by strong covalent bonds, but between one molecule and the next there are only weak intermolecular forces. When such a substance melts or boils, it is only these weak forces between the molecules that break, the strong covalent bonds inside the molecules stay intact. Because so little energy is needed to overcome them, simple molecular substances have low melting and boiling points and many are liquids or gases at room temperature.

Example. When ice melts and then water boils, the molecules move apart but each H₂O molecule stays whole, because only the intermolecular forces are broken, not the O–H covalent bonds. The hydrogen bond is a particularly strong type of intermolecular force, which is why water boils at a higher temperature than a molecule of its size otherwise would.

Confusion to avoid. Do not confuse the weak intermolecular forces between molecules with the strong covalent bonds inside a molecule. A common exam error is to say that boiling a molecular substance breaks its covalent bonds, it does not. The size of the molecule matters too: larger molecules generally have stronger intermolecular forces and therefore higher melting and boiling points than smaller ones.

Intermolecular forces explain why simple molecular substances differ so sharply from giant ionic compounds in this chapter, a comparison of properties tested often in SPM Chemistry.