Properties of simple molecular substances

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

There are intermolecular forces between simple molecules . Intermolecular forces are much weaker than the strong covalent bonds within the 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. They are often liquids or gases at room temperature.

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When simple molecular substances melt or boil, their weak intermolecular forces are overcome, not the strong covalent bonds.

Differences in melting and boiling points

Energy is transferred to a substance to melt or boil it. This energy is needed to overcome the intermolecular forces of attraction between the molecules.

  • some intermolecular forces are overcome during melting, allowing molecules to move over each other
  • more of the intermolecular forces 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.

For example, the longer the alkane molecule, the higher the boiling point.

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.

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