Distinguish homolytic and heterolytic bond fission
Track the bonding electron pair to distinguish radical formation from ion formation.
AS Level; Cambridge International Chemistry 9701, 2025–2027 syllabus. Use the published functional-group scope and nomenclature limits; draw formula and mechanism evidence with correct connectivity, electron movement and stereochemistry. Written practice is self-checked
Before you begin
- Use covalent structures, valency, molecular formulas, lone pairs, and single, double and triple bonds.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What forms when a bond breaks homolytically?
What you will learn
- Describe homolytic fission
- Describe heterolytic fission
- Identify radicals and ions from the electron split
Homolytic fission splits the pair evenly
In homolytic fission, a covalent bond breaks and each atom takes one electron from the shared pair. The products are radicals, each with an unpaired electron.
A single-headed arrow is often used for movement of one electron in radical mechanisms. This differs from the two-electron curly arrow used to show an electron pair.
Heterolytic fission gives both electrons to one atom
In heterolytic fission, both bonding electrons go to the same atom. The products are ions: one positive and one negative, with charges determined by the electron assignment.
A full curly arrow shows an electron pair moving from the bond to the atom that receives both electrons. Check that total charge is conserved.
Pause and explain
Where do both bonding electrons go in heterolytic fission?
Worked example
Compare the products of homolytic and heterolytic fission of a generic A–B bond.
Show the worked solution
- Homolytic: one electron goes to A and one to B.
- This forms A• and B• radicals.
- Heterolytic: both electrons go to one atom, forming A⁺ and B⁻ or the reverse.
Answer Homolytic fission forms two radicals; heterolytic fission forms oppositely charged ions by assigning both bonding electrons to one atom.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Show the products of homolytic cleavage of Cl–Cl.
Give me a hint
Give one electron to each chlorine atom.
Compare my reasoning
- Break the Cl–Cl bond.
- Each atom receives one electron.
- Two chlorine radicals form.
Cl₂ → 2Cl•.
Look for these in your work
- Bond identified
- Electron split even
- Radicals shown
Describe heterolytic fission of H–Cl with both electrons assigned to chlorine.
Give me a hint
Chlorine is more electronegative and takes the bonding pair.
Compare my reasoning
- The H–Cl bond breaks.
- Both bonding electrons go to Cl.
- Products are H⁺ and Cl⁻.
H–Cl → H⁺ + Cl⁻.
Look for these in your work
- Pair destination stated
- Ion charges correct
- Charge conserved
A mechanism arrow shows one electron moving from a bond to each atom. Classify the fission and products.
Give me a hint
The number of electrons received by each atom determines the type.
Compare my reasoning
- Each atom receives one electron.
- This is homolytic fission.
- The products are radicals, not ions.
Homolytic fission forms two radicals.
Look for these in your work
- Electron allocation read
- Fission named
- Product type stated
Common mistakes
- Using two-headed curly arrows for single-electron movement
- Calling an uncharged odd-electron species an ion
- Forgetting charge conservation in heterolysis
What can you explain now?
What forms when a bond breaks homolytically?
Compare with the explanation
Two radicals
Each atom receives one electron from the shared pair.
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Make it stick
What does a two-headed curly arrow represent compared with a single-headed arrow?
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