Group 7 Halogens – Complete Notes + Past Paper Questions
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📋 Complete Group 7 Halogens Guide
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What Are Group 7 Halogens?
Cambridge Exam Definition:
"Group 7 Halogens are the elements in the seventeenth column of the periodic table: Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), and Astatine (At). They have seven electrons in their outer shell and are highly reactive non-metals."
Fluorine (F)
Atomic Number: 9
Pale yellow gas
Chlorine (Cl)
Atomic Number: 17
Greenish-yellow gas
Bromine (Br)
Atomic Number: 35
Red-brown liquid
Iodine (I)
Atomic Number: 53
Dark grey solid
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 | Cambridge Exam Importance |
|---|---|---|
| State at Room Temp | Gas → Liquid → Solid | High - Must know states and colors |
| Color | Pale yellow → Green → Red-brown → Dark grey | Very High - Frequently tested |
| Melting/Boiling Point | Increases down the group | Very High - Trend explanation required |
| Density | Increases down the group | Medium - Often in MCQ questions |
| Atomic Radius | Increases down the group | High - Linked to reactivity trend |
🎨 Halogen Color Guide - Must Memorize!
Fluorine: Pale yellow gas
Chlorine: Greenish-yellow gas
Bromine: Red-brown liquid
Iodine: Dark grey solid / Purple vapor
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
⚡ High Electronegativity
Strong tendency to gain electrons
Electronegativity decreases down the group
🔥 Form -1 Ions
Always form X⁻ ions in ionic compounds
Examples: F⁻, Cl⁻, Br⁻, I⁻
Reactivity Trend - Down Group 7
Reactivity DECREASES Down the Group
Fluorine → Chlorine → Bromine → Iodine
More Reactive → Less Reactive
Why Reactivity Decreases:
- 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
🎯 Exam Tip: Group 1 vs Group 7 Reactivity
Group 1 (Alkali Metals): Reactivity INCREASES down the group (easier to lose electrons)
Group 7 (Halogens): Reactivity DECREASES down the group (harder to gain electrons)
Displacement Reactions - Key Concept!
⚖️ Displacement Principle
"A more reactive halogen will displace a less reactive halogen from its compound."
Key Displacement Reactions:
Chlorine + Potassium Bromide
Cl₂ + 2KBr → 2KCl + Br₂
Observation: Colorless solution turns orange (bromine formed)
Chlorine is more reactive than bromine, so it displaces bromine
Chlorine + Potassium Iodide
Cl₂ + 2KI → 2KCl + I₂
Observation: Colorless solution turns brown (iodine formed)
Chlorine is more reactive than iodine, so it displaces iodine
Bromine + Potassium Iodide
Br₂ + 2KI → 2KBr + I₂
Observation: Colorless solution turns brown (iodine formed)
Bromine is more reactive than iodine, so it displaces iodine
Past Paper Questions & Model Answers
Question 1: Explain why chlorine is more reactive than iodine (4 marks)
Model Answer: 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: 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: 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
- Distance from nucleus
- Shielding effect
- Electron attraction ability
⚠️ 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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✅ Group 7 Mastery Module
Specialized halogens techniques & displacement reactions
✅ Past Paper Solutions
Step-by-step exam question walkthroughs
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