6.3.1Additional lessons
Radicals — IB Chemistry SL Revision Notes & Topical Questions
5 min read 6 parts Lesson 94 of 98
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6What Is a Radical
Radicals are very reactive species that take part in many chain reactions.
Definition
- Definition A radical is an atom or group of atoms containing an unpaired electron.
Notation
- Notation The unpaired electron is shown as a dot, for example Cl•, •CH₃ and •OH.
Not charged
- Not charged Radicals formed from neutral molecules are uncharged, and should not be confused with ions.
Lone pair or unpaired
- Lone pair or unpaired A lone pair is two paired electrons, whereas a radical has a single electron in an orbital.
Section 2 of 6Examples of Radicals
Examples of Radicals
Chlorine radical
- Chlorine radical Cl• forms when the Cl–Cl bond breaks, and is involved in the breakdown of ozone.
Bromine radical
- Bromine radical Br• forms from Br₂ in the presence of UV light.
Methyl radical
- Methyl radical •CH₃ is formed when a hydrogen atom is removed from methane.
Hydroxyl radical
- Hydroxyl radical •OH is highly reactive and important in atmospheric chemistry.
Nitrogen monoxide
- Nitrogen monoxide NO• is a stable-looking molecule that has an unpaired electron.
Section 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.
| Species | Radical? | Reason |
|---|---|---|
| Cl• | Yes | One unpaired electron |
| Cl⁻ | No | Has an octet, all paired |
| •CH₃ | Yes | Carbon has 7 electrons |
| CH₄ | No | All electrons are paired |
Section 4 of 6Where Radicals Matter
Where Radicals Matter
Ozone depletion
- Ozone depletion Cl• radicals from the breakdown of CFCs in the stratosphere catalyse the destruction of ozone.
Combustion
- Combustion Radical chain reactions drive flames and explosions, because they multiply quickly.
Polymerisation
- Polymerisation Radical addition is used to make plastics such as poly(ethene) under high pressure.
Antioxidants
- Antioxidants Some molecules in foods stop radical chain reactions, so they prevent spoiling.
Section 5 of 6Worked Example
Question State which of these species are radicals: Cl•, Cl⁻, •CH₃, CH₄, •OH.
- A radical must have an unpaired electron.
- Cl•, •CH₃ and •OH each have a single unpaired electron.
- Cl⁻ has a complete octet of paired electrons and CH₄ has only bonding pairs.
Answer Cl•, •CH₃ and •OH are radicals.
Section 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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