Properties of simple molecular substances

The physical properties of simple molecular substances can be explained by thinking about their structure and bonding.

Low melting points and boiling points

Simple molecular substances generally have low melting points and boiling points and are often liquids or gases at room temperature.

Melting and boiling

Melting and boiling are state changes.

Energy is transferred to a substance to melt or boil it. This energy is needed to overcome the bonds or forces of attraction 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 needed, the higher the melting point or boiling point.

Explanation

There are intermolecular forces between simple molecules. These 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. Very little energy is needed to overcome the intermolecular forces, so simple molecular substances usually 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.

Conduction of electricity

A substance can conduct electricity if:

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

Simple molecules have no overall charge, or charged particles that can separate, so simple molecular substances 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 the strong covalent bonds.
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