Group 7 Halogens – Properties, Reactions, Colours | O Level 5070 & IGCSE 0620
Complete Cambridge Syllabus Guide with Past Paper Questions & Expert Solutions
📋 Complete Group 7 Halogens Mastery Guide
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What Are Group 7 Halogens? Cambridge Definition
Cambridge O Level 5070 & IGCSE 0620 Exam Definition:
"Group 7 Halogens are non-metal elements in Group 7 of the periodic table: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I). They have seven electrons in their outer shell, form diatomic molecules (F₂, Cl₂, Br₂, I₂), and are highly reactive."
Fluorine (F)
Atomic Number: 9
State: Pale yellow gas
Reactivity: Most reactive
Chlorine (Cl)
Atomic Number: 17
State: Greenish-yellow gas
Reactivity: Very reactive
Bromine (Br)
Atomic Number: 35
State: Red-brown liquid
Reactivity: Less reactive
Iodine (I)
Atomic Number: 53
State: Dark grey solid
Reactivity: Least reactive
🔗 Essential Foundation Knowledge:
Master these first: Cambridge Periodic Table Guide, Group 1 Alkali Metals, and Electronic Configuration.
Physical Properties of Group 7 Halogens
| Property | Trend Down Group 7 | O Level 5070 Focus | IGCSE 0620 Focus |
|---|---|---|---|
| State at Room Temp | Gas → Liquid → Solid | Must know states & colors | Practical observations |
| Color | Pale yellow → Green → Red-brown → Dark grey | Identification questions | Color change predictions |
| Melting/Boiling Point | Increases down group | Trend explanation required | Intermolecular forces |
| Density | Increases down group | Basic trend knowledge | MCQ applications |
🎨 Halogen Color Guide - Must Memorize for Exams!
Fluorine: Pale yellow gas
Exam Tip: Too reactive to handle in school labs
Chlorine: Greenish-yellow gas
Exam Tip: Pungent smell, used in water treatment
Bromine: Red-brown liquid
Exam Tip: Only liquid non-metal at room temp
Iodine: Dark grey solid / Purple vapor
Exam Tip: Sublimes on heating (solid → gas)
🎯 O Level Chemistry 5070 - Halogens Focus
Syllabus Requirements:
- Describe the trends in physical properties down Group 7
- Explain the trend in reactivity down the group
- Describe displacement reactions between halogens and halide ions
- Identify halogens by their colors
Download: O Level 5070 Full Syllabus | 5070 Past Papers
🎯 IGCSE Chemistry 0620 - Halogens Focus
Syllabus Requirements:
- Identify trends in Group 7 physical properties
- Predict the properties of unknown halogens
- Describe and explain displacement reactions
- Understand halogen uses in everyday life
Download: IGCSE 0620 Full Syllabus | 0620 Past Papers
Chemical Properties & Reactivity
🎯 Seven Valence Electrons
All Group 7 elements have seven electrons in their outer shell
They need one more electron to achieve stable noble gas configuration
Form X⁻ ions (F⁻, Cl⁻, Br⁻, I⁻)
⚡ High Electronegativity
Strong tendency to gain electrons
Electronegativity decreases down the group
F (4.0) > Cl (3.0) > Br (2.8) > I (2.5)
🔥 Form Diatomic Molecules
Exist as X₂ molecules in elemental form
Weak London forces between molecules
F₂, Cl₂, Br₂, I₂
Reactivity Trend - Down Group 7
Reactivity DECREASES Down the Group
Fluorine → Chlorine → Bromine → Iodine
More Reactive → Less Reactive
Why Reactivity Decreases (Exam Answer Framework):
- Atomic size increases down the group
- Outer electrons are further from nucleus
- More shielding from inner electron shells
- Weaker attraction for incoming electrons
- Harder to gain an electron (lower electron affinity)
📊 Group 1 vs Group 7 Reactivity Comparison
| Aspect | Group 1 (Alkali Metals) | Group 7 (Halogens) |
|---|---|---|
| Reactivity Trend | Increases down group | Decreases down group |
| Reason | Easier to lose electrons | Harder to gain electrons |
| Ions Formed | Positive ions (M⁺) | Negative ions (X⁻) |
| Atomic Size Effect | Helps reactivity | Hinders reactivity |
Displacement Reactions - Key Exam Topic!
⚖️ Displacement Principle
"A more reactive halogen will displace a less reactive halogen from its aqueous compound."
More reactive halogen + Metal halide salt → Less reactive halogen + New metal halide salt
Key Displacement Reactions (Must Know):
Chlorine + Potassium Bromide
Cl₂(aq) + 2KBr(aq) → 2KCl(aq) + Br₂(aq)
Observation: Colorless solution turns orange (bromine formed)
✅ Chlorine is more reactive than bromine, so it displaces bromine
Chlorine + Potassium Iodide
Cl₂(aq) + 2KI(aq) → 2KCl(aq) + I₂(aq)
Observation: Colorless solution turns brown (iodine formed)
✅ Chlorine is more reactive than iodine, so it displaces iodine
Bromine + Potassium Iodide
Br₂(aq) + 2KI(aq) → 2KBr(aq) + I₂(aq)
Observation: Colorless solution turns brown (iodine formed)
✅ Bromine is more reactive than iodine, so it displaces iodine
Iodine + Potassium Chloride
I₂(aq) + 2KCl(aq) → No reaction
Observation: No color change (solution remains colorless)
❌ Iodine is less reactive than chlorine, so no displacement occurs
Past Paper Questions & Model Answers
📝 Question 1: Explain why chlorine is more reactive than iodine (4 marks)
Model Answer (Full Marks):
Chlorine has a smaller atomic radius than iodine. The outer shell in chlorine is closer to the nucleus and experiences less shielding from inner electrons. This means chlorine has a stronger attraction for incoming electrons, making it easier to gain an electron and more reactive than iodine.
📝 Question 2: Describe the displacement reaction between chlorine and potassium iodide (3 marks)
Model Answer (Full Marks):
When chlorine gas is bubbled through potassium iodide solution, the solution turns from colorless to brown. Chlorine displaces iodine from potassium iodide because chlorine is more reactive. The equation is: Cl₂ + 2KI → 2KCl + I₂
📝 Question 3: State the trend in boiling points down Group 7 and explain why (3 marks)
Model Answer (Full Marks):
Boiling points increase down Group 7. This is because the molecules get larger with more electrons, leading to stronger London dispersion forces between molecules. More energy is required to overcome these stronger intermolecular forces.
🎓 Solved Past Papers - Group 7 Questions
See exactly how Group 7 questions appear in actual Cambridge exams:
IGCSE Chemistry 0620 Solved - Halogens Focus
O Level Chemistry 5070 Solved - Displacement Reactions
📚 Download More Resources: Complete Past Papers Collection | Group 7 Summary Notes PDF | Displacement Reactions Guide
Exam Techniques & Common Mistakes
✅ Trend Explanation Framework
When explaining halogen trends, always mention:
- Atomic size/radius increases down group
- Distance from nucleus increases
- Shielding effect increases
- Electron attraction ability decreases
⚠️ Common Mistakes to Avoid
- Confusing Group 7 trend with Group 1 trend
- Forgetting state symbols in equations
- Mixing up halogen colors
- Incorrect displacement predictions
- Not explaining WHY trends occur
🎯 Memory Aid - Reactivity Series
F > Cl > Br > I (Fluorine most reactive)
"Funny Cats Bring Insects"
More reactive halogens displace less reactive ones from their compounds
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Prof. Janjowa has specialized in teaching Group 7 Halogens with unique memory techniques that have helped thousands of students achieve A* grades in O Level 5070 and IGCSE 0620.
Specialized Teaching Methods for Group 7:
- Color-coded memory techniques for halogen properties
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- Common mistake prevention strategies
- Past paper pattern recognition techniques
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✅ Complete Syllabus Coverage
O Level 5070 & IGCSE 0620 Full Syllabus
✅ Group 7 Mastery Module
Specialized halogens techniques & displacement reactions
✅ Past Paper Solutions
Step-by-step exam question walkthroughs
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