Properties of substances with small molecules

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.

Low melting points and boiling points

Most substances with simple molecules have low melting points and boiling points. Many are in the liquid or gas state at room temperature.

Melting and boiling

Melting and boiling are changes of state.

Energy must be transferred to a substance to make it melt or boil. This energy overcomes the attractive forces between the particles in the substance:

  • some forces of attraction are overcome during melting, allowing molecules to move over each other
  • more of the forces of attraction are overcome during boiling, allowing the molecules to move freely away from each other

The more energy that is needed, the higher the melting point or boiling point.

Explanation

There are intermolecular forces between small molecules. Intermolecular forces are much weaker than the strong covalent bonds in molecules. When small 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 small molecular substances have low melting and boiling points.

SubstanceMelting pointBoiling pointState at 20°C
O2-218°C-183°CGas
H2O0°C100°CLiquid

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.

Conduction of electricity

A substance can conduct electricity if:

  • it contains charged particles, and
  • these particles are free to move from place to place

Small molecules have no overall electric charge, so they cannot conduct electricity, even when liquid or dissolved in water.

curriculum-key-fact
When simple molecular substances melt or boil, their weak intermolecular forces are overcome, not their strong covalent bonds.
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