UPSC Combined Geo-Scientist, Mains · 2025 · Mains Paper I
Q4(c). Magic numbers; stability of ²⁰⁸₈₂Pb
Question
What are magic numbers? ²⁰⁸₈₂Pb is very stable. Explain why.
Answer in brief
Magic numbers are 2, 8, 20, 28, 50, 82, 126: nuclei with these numbers of protons or neutrons have closed shells; ²⁰⁸Pb has Z = 82 and N = 126, so it is doubly magic.
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Coversmagic numbersnuclear shell modeldoubly magiclead-208nuclear stability
All parts of Question 4
- 4(a) Wilkinson’s catalyst; hydrogenation cycle15 marks
- 4(b) 18-electron rule: [(η⁶-C₆H₆)Cr(CO)₃] and [(CO)₂Rh(μ-Cl)₂Rh(CO)₂]10 marks
- 4(c) Magic numbers; stability of ²⁰⁸₈₂Pb5 marks