IB Chemistry HLTopic 6 — Electron SharingPaper 1 & 2Core skill~9 min read
Homolytic Fission
A covalent bond is two shared electrons. When the bond breaks, those two electrons have to go somewhere — and there are only two options. Either one atom takes both, or they split one each. That single choice decides whether you end up with ions or with radicals.
📚 What you need to know
Homolytic fission splits a bond evenly: one electron to each atom, giving two radicals.
Heterolytic fission splits it unevenly: one atom takes both electrons, giving two ions.
A single-headed (fish-hook) curly arrow shows one electron moving. A double-headed arrow shows a pair.
Curly arrows must start at an electron-rich place and finish where the electrons land.
Bond breaking is endothermic, so energy must be supplied.
Thermolytic fission uses heat for weaker bonds; photolytic fission uses UV light for stronger ones.
Homolytic fission of a halogen is the initiation step of a radical chain reaction.
Two ways to break the same bond
The arrowheads are the giveaway. Two half-arrows pulling in opposite directions means homolytic; one full arrow going to a single atom means heterolytic.
The names tell you the answer if you unpack them. Homo means “same” — both atoms end up with the same share, one electron each. Hetero means “different” — one atom takes two and the other takes none. Say that in an exam and you have already earned the mark.
Curly arrow rules
🧩 Drawing a fish-hook arrow correctly
Use half-headed arrows for radicals. A full arrowhead means a pair moved, which would be wrong.
Start from the bond itself — the middle of the line between the two atoms is where the electrons are.
Draw one arrow per electron. Breaking one bond homolytically needs two arrows.
Finish on the atom that keeps that electron.
Check the products carry dots. If you have drawn two fish-hooks, you must end up with two radicals.
Homolytic fission of chlorine
Cl−Cl → Cl• + •Cl
Watch the charges: homolytic fission produces no ions at all. Two neutral atoms each with one electron. If your products have plus or minus signs, you have drawn heterolytic fission by mistake.
Supplying the energy
Breaking a bond is always endothermic — you have to put energy in. How much depends on the bond enthalpy, and that decides which method will work.
UV photons carry far more energy than the gentle warming that heat provides, which is why sunlight can snap a Cl−Cl bond that a warm room cannot.
Worked examples
WORKED EXAMPLE
Describe the homolytic fission of a Br−Br bond, including the curly arrows and the products.
Step 1: Identify where the electrons startThe shared pair sits in the middle of the Br−Br bond.Step 2: Draw two single-headed arrowsOne from the bond to the left bromine, one from the bond to the right bromine.Step 3: Write the productsBr−Br → Br• + •BrTwo bromine radicals, each with one unpaired electron and no chargetwo arrows, two radicals — the numbers always match
WORKED EXAMPLE
Give the products of homolytic and heterolytic fission of an H−Cl bond, and explain which is more likely.
Step 1: Homolytic — one electron eachH−Cl → H• + •ClStep 2: Heterolytic — chlorine takes bothH−Cl → H+ + Cl−Step 3: CompareChlorine is much more electronegative than hydrogen, so the pair is already pulled towards it.Heterolytic is favoured, because the bond is polarhomolytic is favoured when a bond is non-polar, like Cl−Cl
WORKED EXAMPLE
Using bond enthalpies (Cl−Cl 242 kJ mol−1, O−O 144 kJ mol−1), explain why chlorine needs UV light but a peroxide can be split by warming.
Step 1: Compare the two bonds242 − 144 = 98 kJ mol−1 more needed for Cl−ClStep 2: Match the energy source to the needHeating gives molecules a modest spread of energies; UV photons deliver a much larger quantity in one go.Step 3: Draw the conclusionThe weaker O−O bond breaks thermolytically; the stronger Cl−Cl bond needs photolytic fissionquote the actual numbers — “one is stronger” on its own is not enough
💡 Exam tip
Draw half-headed arrows for radical mechanisms. A full head loses the mark even if the products are right.
Start every arrow at the bond, not at an atom, when a bond is being broken.
Products of homolytic fission are neutral radicals. No charges anywhere.
Say bond breaking is endothermic if asked why energy is needed.
Use “photolytic” and “UV” together, and “thermolytic” and “heat” together. Mixing them up is an easy avoidable error.
Link bond enthalpy to the choice of method — stronger bond, more energy, so UV rather than heat.
⚠ Common mix-up
Drawing one arrow for homolytic fission. Two electrons move separately, so you need two arrows.
Giving ions as the products of homolytic fission. Homolytic gives radicals, never ions.
Confusing homo and hetero. Homo means both atoms get the same share.
Starting the arrow at an atom instead of at the bond when breaking it.
Saying UV “gives energy to the alkane”. In initiation the UV breaks the halogen bond, which is the weaker one.
Forgetting the dot on the products. Without it you have not shown radicals were formed.
Up next: Halogenation of Alkanes — homolytic fission is only step one. Put those radicals next to an unreactive alkane and a chain reaction takes off.
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