Astatine is a halogen found below iodine in Group 7. A student states: 'Astatine must be a better oxidising agent than iodine and will displace iodine from a solution of potassium iodide.' Evaluate the student's statement.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
Astatine is a worse oxidising agent than iodine.
1. Oxidising ability decreases down Group 7 because the outermost electrons are further from the nucleus.
2. Greater distance means increased shielding and a weaker effective nuclear charge.
3. Consequently astatine attracts or gains an electron less readily than iodine.
4. Therefore astatine would not displace iodine from a potassium iodide solution.
5. The student’s conclusion is therefore incorrect.
1. Oxidising ability decreases down Group 7 because the outermost electrons are further from the nucleus.
2. Greater distance means increased shielding and a weaker effective nuclear charge.
3. Consequently astatine attracts or gains an electron less readily than iodine.
4. Therefore astatine would not displace iodine from a potassium iodide solution.
5. The student’s conclusion is therefore incorrect.
Examiner tips
- Use the term ‘oxidising agent’ and ‘displace’ explicitly.
- Show the trend of decreasing oxidising power down the group.
- Explain the role of shielding and effective nuclear charge.
- State the final verdict clearly.
Common mistakes
- Assuming astatine is more reactive because it is heavier.
- Forgetting to mention shielding/effective nuclear charge.
- Claiming astatine would displace iodine without justification.
Mark scheme (5 marks)
- Astatine is a worse (less powerful) oxidising agent than iodine, not a better one
- Oxidising ability / reactivity decreases going down Group 7
- Because the outer shell / outermost electrons are further from the nucleus in astatine than in iodine
- So astatine attracts / gains an electron less easily than iodine (increased shielding / less effective nuclear attraction)
- Therefore astatine would NOT displace iodine from potassium iodide solution — the student's conclusion is incorrect
Key terms in this question
Related
- All AQA A-Level Chemistry (7405) revision notes →
- How to answer a "Evaluate" question →
- Decode the mark scheme abbreviations →
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