Covalent Bonding
Get to grips with Covalent Bonding for GCSE Chemistry with this free worksheet and full mark scheme — Foundation and Higher exam-style questions with worked answers for AQA, Edexcel and OCR. Covalent bonding is a shared pair of electrons between non-metals — draw dot-and-cross diagrams for molecules such as H₂O, NH₃ and CH₄.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA, Edexcel and OCR specifications. Every worksheet comes with a full mark scheme.
Topic overview
Covalent bonding occurs when non-metal atoms share pairs of electrons. Each shared pair is a single covalent bond, and the shared electrons are attracted to both nuclei, which holds the atoms together.
Atoms share enough electrons to achieve a full outer shell. A hydrogen atom needs one more electron so forms one bond; oxygen needs two so forms two bonds; nitrogen forms three and carbon forms four.
Double bonds involve two shared pairs, as in oxygen O₂ and carbon dioxide CO₂. Covalent bonds themselves are very strong, but the forces between separate small molecules are weak, which is why many covalent substances have low boiling points despite their strong bonds.
Revision notes
How covalent bonds form
Non-metal atoms share pairs of electrons so that each achieves a full outer shell.
The shared pair is attracted to the nuclei of both atoms, and this attraction is the covalent bond. Dot and cross diagrams show which electrons come from which atom.
Number of bonds formed
An atom forms as many bonds as it needs electrons to fill its outer shell.
Hydrogen forms 1 bond, oxygen 2, nitrogen 3 and carbon 4. This lets you predict formulae: water is H₂O, ammonia is NH₃ and methane is CH₄.
Strong bonds, weak forces
The covalent bonds within a molecule are very strong.
But the intermolecular forces between separate molecules are weak. When a simple molecular substance melts or boils, it is these weak forces that are overcome, not the covalent bonds — which is why such substances have low melting and boiling points.
Key points
- Covalent bonding occurs between non-metals.
- Atoms share pairs of electrons.
- Each shared pair is one covalent bond.
- Atoms share enough electrons for a full outer shell.
- A double bond is two shared pairs.
- Covalent bonds are strong but intermolecular forces are weak.
Worked examples
Example 1
Explain how a covalent bond holds two atoms together. [2 marks]
Model answer
The two atoms share a pair of electrons (1 mark), and this shared pair is attracted to the nuclei of both atoms, holding them together (1 mark).
Example 2
Explain why carbon forms four covalent bonds. [2 marks]
Model answer
Carbon has four electrons in its outer shell (1 mark), so it needs four more to achieve a full outer shell of eight and therefore forms four bonds (1 mark).
Example 3
Explain why simple molecular substances have low boiling points despite having strong covalent bonds. [3 marks]
Model answer
The covalent bonds within each molecule are strong (1 mark), but the intermolecular forces between separate molecules are weak (1 mark). When the substance boils it is these weak intermolecular forces that are overcome, not the covalent bonds, so only a small amount of energy is needed (1 mark).
Common mistakes
Saying covalent bonds are broken when a substance boils.
The weak intermolecular forces are overcome, not the covalent bonds themselves.
Saying electrons are transferred.
They are shared in covalent bonding; transfer occurs in ionic bonding.
Predicting the wrong number of bonds.
Count how many electrons the atom needs to fill its outer shell.
Saying covalent bonding occurs between a metal and non-metal.
That is ionic bonding. Covalent bonding is between non-metals only.
Exam tips
- Say shared pair attracted to both nuclei when defining a covalent bond.
- Work out the number of bonds from the outer shell electrons.
- Always distinguish covalent bonds from intermolecular forces.
- Use dot and cross diagrams to show which electrons come from where.
Key terms
- Covalent bond
- A shared pair of electrons between two non-metal atoms.
- Double bond
- Two shared pairs of electrons between the same two atoms.
- Intermolecular force
- A weak force of attraction between separate molecules.
- Molecule
- A group of atoms held together by covalent bonds.
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Written and reviewed against the current AQA, Edexcel and OCR specifications. Spotted an error? Let us know.