A substance with small molecules has strong covalent bonds that hold the atoms together in its molecules. There are weak forces between molecules and their neighbours. The structure and bonding of substances with small molecules explains their properties.
Melting and boiling are changes of state.
The more energy that is needed, the higher the melting point or boiling point.
There are intermolecular forces between simple molecules. Intermolecular forces are much weaker than the strong covalent bonds in molecules. When simple molecular substances melt or boil, it is these weak intermolecular forces that are overcome. The covalent bonds are not broken. Relatively little energy is needed to overcome the intermolecular forces, so simple molecular substances have low melting and boiling points.
|Substance||Melting point||Boiling point||State at 20°C|
The intermolecular forces between water molecules are stronger than those between oxygen molecules.
In general, the bigger the molecule, the stronger the intermolecular forces, so the higher the melting and boiling points.
A substance can conduct electricity if:
Small molecules have no overall electric charge, so they cannot conduct electricity, even when liquid or dissolved in water.
Polymers are made up of very large molecules. The intermolecular forces between polymer molecules are strong compared to the intermolecular forces between small molecules. This means that polymers melt at higher temperatures than substances with small molecules. They are solids at room temperature.