Properties of Small Molecules (AQA GCSE Chemistry)

Melting and boiling point of small molecules

• Substances that consist of small molecules are gases, liquid or solids with relatively low melting and boiling points 

• They have strong covalent bonds between the atoms, but only weak forces between molecules
  

  • Examples include carbon dioxide and methane

• They have relatively low melting and boiling points because: 

  • There are weak intermolecular forces between the molecules

  • These forces require little energy to overcome 

• As the molecules increase in size, the melting and boiling points generally increase because the strength of these intermolecular forces increases and so more energy is needed to break them 

Electrical conductivity of small molecules

• Substances that consist of small molecules are poor conductors of electricity (even when molten) because there are no free ions or electrons to move and carry the charge

• Most covalent compounds do not conduct at all in the solid state and are thus insulators

• Common insulators include the plastic coating around household electrical wiring, rubber and wood

The plastic coating around electrical wires is made from covalent molecules that do not allow a flow of charge

Intermolecular Forces v. covalent bonds

• Substances consisting of small molecules have two types of bond:  

  • Covalent bonds- found between the atoms 

  • Intermolecular forces- found between the molecules

• Covalent bonds are very strong in comparison to intermolecular forces 

• It is the intermolecular forces that are broken at a substances melting / boiling point 

  • So due to the intermolecular forces being weak, little energy is required to overcome them and substances made up of small molecules have low melting and boiling points 

Diagram to show the difference between covalent and intermolecular bonds

Covalent bonds between the hydrogen and oxygen atoms are strong, whereas the intermolecular forces are weak