Covalent bonding — AQA GCSE Chemistry
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Covalent bonding explained
The covalent bonds within a molecule are strong because the shared electron pair is attracted to both nuclei.
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Breaking one mole of these bonds needs a large amount of energy, so the bonds do not break during melting or boiling. For example, when water boils, the strong O–H covalent bonds stay intact; the weak forces between separate water molecules are overcome instead. This explains why simple molecular substances have low melting and boiling points even though their covalent bonds are strong. Always distinguish between the strong bonds inside a molecule and the weak intermolecular forces between molecules.
Your focus
- Describe why covalent bonds are strong in terms of attraction between the shared pair and both nuclei.
- Distinguish between strong covalent bonds within molecules and weak forces between molecules.
- Use the strength of covalent bonds to explain why simple molecular substances have low melting and boiling points.
Covalent bonding exam tips
Marking Points
- Covalent bonds are strong because the shared pair is attracted to both nuclei.
- A large amount of energy is needed to break covalent bonds, so they remain intact during physical changes such as melting and boiling.
- When a simple molecular substance boils, weak forces between molecules are overcome, not the strong covalent bonds within each molecule.
- Strong covalent bonds explain why molecules stay together, while weak intermolecular forces explain low melting and boiling points.
Examiner Tips
- 💡Use the phrase 'strong covalent bonds between atoms' and 'weak forces between molecules' to make the distinction clear.
- 💡When explaining a low boiling point, refer to energy needed to overcome intermolecular forces, not to break covalent bonds.
- 💡Link the strength of a covalent bond to the attraction between the shared pair and both nuclei.
Common Mistakes
- Saying that covalent bonds break when a simple molecular substance boils; correct this by stating that intermolecular forces break instead.
- Confusing strong covalent bonds with weak intermolecular forces; correct this by naming each type of attraction separately.
- Claiming that all attractions in a simple molecular substance are weak; correct this by stating that the bonds within each molecule are strong.