Properties Of Substances With Covalent Bonding (AQA GCSE Combined Science: Synergy: Physical Sciences): Revision Note

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

  • This is because the strength of the intermolecular forces increases

  • So, more energy is needed to overcome them 

Electrical conductivity of small molecules

• Substances that consist of small molecules are poor conductors of electricity (even when molten)

• This is 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

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 substance's melting / boiling point 

• Due to the intermolecular forces being weak:

  • Little energy is required to overcome them

  • Substances made up of small molecules have low melting and boiling points 

• Polymers have much larger molecules

  • So, the intermolecular forces between polymer chains are stronger

  • They require more energy to overcome, which makes the melting and boiling points higher

  • This is why polymers are solids at room temperature

Properties of giant covalent structures

• Some covalently bonded substances have giant covalent structures

  • These structures can be known as giant covalent lattices

• Examples include diamond, graphite and silicon dioxide

• These substances are solids with very high melting points because:

  • All of the atoms are linked to other atoms by strong covalent bonds

  • These bonds require lots of energy to overcome

• Diamond and graphite are both made entirely from carbon atoms

• Silicon dioxide (silica) is made from silicon and oxygen atoms