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6.3.1Additional lessons

Radicals — IB Chemistry SL Revision Notes & Topical Questions

5 min read 6 parts Lesson 94 of 98

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Part 1 of 6What Is a Radical

6.3.1 Radicals

Radicals are very reactive species that take part in many chain reactions.

Definition
  • A radical is an atom or group of atoms containing an unpaired electron.
Notation
  • The unpaired electron is shown as a dot, for example Cl•, •CH₃ and •OH.
Not charged
  • Radicals formed from neutral molecules are uncharged, and should not be confused with ions.
Lone pair or unpaired
  • A lone pair is two paired electrons, whereas a radical has a single electron in an orbital.

Part 2 of 6Examples of Radicals

Examples of Radicals
Chlorine radical
  • Cl• forms when the Cl–Cl bond breaks, and is involved in the breakdown of ozone.
Bromine radical
  • Br• forms from Br₂ in the presence of UV light.
Methyl radical
  • •CH₃ is formed when a hydrogen atom is removed from methane.
Hydroxyl radical
  • •OH is highly reactive and important in atmospheric chemistry.
Nitrogen monoxide
  • NO• is a stable-looking molecule that has an unpaired electron.

Part 3 of 6Why Radicals Are Reactive

1
The unpaired electron is in a high-energy state, so radicals have a high enthalpy.
2
Radicals try to form a stable electron pair by taking an electron from another species.
3
When a radical reacts with a molecule, a new radical is often produced, so reactions continue as a chain.
4
Two radicals can join to make a stable molecule, which ends the chain.
SpeciesRadical?Reason
Cl•YesOne unpaired electron
Cl⁻NoHas an octet, all paired
•CH₃YesCarbon has 7 electrons
CH₄NoAll electrons are paired

Part 4 of 6Where Radicals Matter

Where Radicals Matter
Ozone depletion
  • Cl• radicals from the breakdown of CFCs in the stratosphere catalyse the destruction of ozone.
Combustion
  • Radical chain reactions drive flames and explosions, because they multiply quickly.
Polymerisation
  • Radical addition is used to make plastics such as poly(ethene) under high pressure.
Antioxidants
  • Some molecules in foods stop radical chain reactions, so they prevent spoiling.

Part 5 of 6Worked Example

Question State which of these species are radicals: Cl•, Cl⁻, •CH₃, CH₄, •OH.

  1. A radical must have an unpaired electron.
  2. Cl•, •CH₃ and •OH each have a single unpaired electron.
  3. Cl⁻ has a complete octet of paired electrons and CH₄ has only bonding pairs.

Answer Cl•, •CH₃ and •OH are radicals.

Part 6 of 6Exam Checklist & Tips

6.3.1 – Key Points for the Exam

  • Radical A species with an unpaired electron.
  • Unpaired electron A single electron occupying an orbital by itself.
  • Chain reaction A reaction in which each step generates a species that starts the next.
  • Reactive Tending to undergo chemical change readily.

Common Mistakes

  • Writing the dot on the wrong atom in a radical.
  • Treating the chloride ion as a radical because it has a charge.
  • Saying radicals have a full outer shell.
  • Mixing up a lone pair with an unpaired electron.

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