Methane (CH₄) is a covalently bonded molecule with a tetrahedral shape and bond angles of 109.5°. Carbon dioxide (CO₂) is also a covalently bonded molecule, but it has a linear shape with bond angles of 180°. Explain why methane has a tetrahedral shape and why carbon dioxide has a linear shape.

Edexcel A-Level Chemistry (9CH0) — 2.2 Covalent bonding and shapes · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Methane (CH₄) contains one carbon atom bonded to four hydrogen atoms. Carbon dioxide (CO₂) contains one carbon atom bonded to two oxygen atoms, each by a double bond. Neither molecule contains lone pairs on the central atom.

Model answer (5 marks)

Covalent bonds are formed by sharing a pair of electrons between two atoms.
In methane there are four bonding pairs around the central carbon atom.
These four electron pairs repel each other and arrange themselves as far apart as possible, giving a tetrahedral shape with bond angles of 109.5°.
In carbon dioxide there are only two bonding regions (two double bonds) around the central carbon atom.
These two electron pairs/bonding regions repel each other and arrange as far apart as possible, giving a linear shape with bond angles of 180°.

Examiner tips

  • Use the exact wording from the mark scheme – e.g. ‘covalent bonds are formed by sharing a pair of electrons’. Show the reasoning for each molecule separately. Keep the answer concise – 5 marks, so one sentence per point.
  • Avoid extra words such as ‘in this essay’ or ‘the reason is…’ – just state the facts.
  • Make sure to mention the number of bonding pairs for each molecule.

Common mistakes

  • Mixing up the shape of CO₂ (linear) with that of CH₄ (tetrahedral). Failing to state the number of bonding pairs for each molecule. Using vague terms like ‘sp³ hybridisation’ instead of the required VSEPR explanation.

Mark scheme (5 marks)

  1. Covalent bonds are formed by the sharing of a pair of electrons between two atoms
  2. In methane, there are four pairs of electrons (four bonding pairs) around the central carbon atom
  3. The four electron pairs in methane repel each other and arrange themselves as far apart as possible, giving a tetrahedral shape with bond angles of 109.5°
  4. In carbon dioxide, there are only two bonding regions / two double bonds around the central carbon atom
  5. The two electron pairs / bonding regions in carbon dioxide repel each other and arrange as far apart as possible, giving a linear shape with bond angles of 180°

Key terms in this question

tetrahedral · linear

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