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AS · Practice questions · Shapes of organic molecules, sigma and pi bonds

Count the bonds.

Six original Cambridge-style questions on the shapes of organic molecules and σ and π bonds.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Atoms bonded to carbon decide its shape.

01
Analysis
[3 marks]

State the shape and bond angle around each carbon in CH3CH=CH2.

  • CH3 carbon: tetrahedral, 109.5°. ✓
  • CH carbon: trigonal planar, 120°. ✓
  • CH2 carbon: trigonal planar, 120°. ✓
02
Analysis
[2 marks]

How many σ and π bonds are there in ethanal, CH3CHO?

  • Bonds: 4 C–H, 1 C–C, 1 C=O = 6 σ. ✓
  • 1 π (in C=O). ✓
03
Explain
[3 marks]

Describe how the π bond in ethene forms and explain why ethene is planar.

  • Each carbon is sp2 hybridised; three σ bonds lie in one plane at 120°. ✓
  • Each carbon has an unhybridised p orbital at right angles to this plane; they overlap sideways to form the π bond. ✓
  • This overlap requires the p orbitals to be parallel, so all six atoms are held in one plane. ✓
04
Analysis
[2 marks]

How many σ and π bonds are there in CH2=CH–CN?

  • Bonds: 3 C–H, C=C, C–C, C≡N = 6 σ. ✓
  • 1 π in C=C + 2 π in C≡N = 3 π. ✓
05
Explain
[2 marks]

Explain why but-2-ene can form two isomers but butane cannot.

  • Rotation about the C=C is restricted because twisting would break the π bond, so groups are fixed on the same or opposite sides. ✓
  • Butane has only σ bonds, which rotate freely, so all arrangements interconvert. ✓
06
Recall
[1 marks]

State the hybridisation of carbon in methane, ethene and ethyne.

  • sp3, sp2 and sp respectively. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.