CBSE Class 12 Physics Nuclei PYQs Analysis (2021-2025)

Based on detailed analysis of last 5 years' papers. Perfect for 2026 Boards prep!

Key Trends

Weightage Breakdown: 1-2 MCQs/assertion-reason (1 mark), 1-2 short answers (2 marks), 1-2 numericals/derivations on binding energy & radioactivity (3-6 marks).
Repeated Topics: Atomic mass unit, mass defect & binding energy curve (BE/nucleon vs A), nuclear stability, radioactivity (alpha, beta, gamma decay), decay law (N = N₀ e^{-λt}), half-life & mean life, activity (becquerel/curie), radioactive series, nuclear fission & fusion, energy released (Q-value), nuclear reactor (moderator, control rods), fusion in stars.
Difficulty: Medium; binding energy per nucleon curve & radioactivity numericals (half-life, activity) are most repeated & scoring.
Changes: More numericals on Q-value in fission/fusion & half-life calculations in 2023–2025.

Most Important and Repeated Questions

Question Example	Type/Marks	Years Repeated	Notes
Define mass defect & binding energy. Explain binding energy per nucleon curve & its significance for nuclear stability & fusion/fission.	Derivation/Theory (3-4 marks)	2021, 2022, 2023, 2024, 2025	Repeated 5x; BE = [Z m_p + (A-Z) m_n - M] c²; Peak at Fe-56 → most stable; Fusion below Fe, fission above Fe release energy.
A radioactive sample has half-life T_{1/2}. Find fraction remaining after time t = n T_{1/2} or activity after time t.	Numerical (3 marks)	2021 Term 2, 2022, 2023, 2024	Repeated 4x; N/N₀ = (1/2)^n or e^{-λt}; λ = ln2 / T_{1/2}; Activity A = λ N.
Calculate Q-value of nuclear reaction (e.g., fission or fusion) given masses. Find energy released.	Numerical (3 marks)	2022, 2023, 2024, 2025	Repeated 4x; Q = (mass defect) c² = Δm × 931.5 MeV/u; Positive Q → exothermic.
Assertion: Binding energy per nucleon is maximum for iron. Reason: Iron is most stable nucleus.	Assertion-Reason (1 mark)	2023, 2024, 2025	Repeated 3x; Both true, reason explains.
Explain alpha, beta & gamma decay processes with equations & change in Z & A.	Short Answer (3 marks)	2021 Term 1, 2022, 2023, 2025	Repeated 4x; Alpha: Z-2, A-4; Beta⁻: Z+1, A same; Gamma: no change in Z/A.
Find number of nuclei left & activity after time t given initial activity & half-life.	Numerical (3 marks)	2022, 2024	Repeated 2x; N = N₀ (1/2)^{t/T}; A = A₀ (1/2)^{t/T}.
MCQ: The half-life of a radioactive element is T. After time 2T, fraction remaining is: (a) 1/2 (b) 1/4 (c) 1/8 (d) 1/16	MCQ (1 mark)	2021 Term 1, 2023, 2024	Repeated 3x; Answer (b) 1/4 (two half-lives).
Explain controlled nuclear fission in a reactor. Mention role of moderator, control rods & coolant.	Short Answer (3 marks)	2023, 2025	Repeated 2x; Chain reaction controlled by control rods (neutron absorber), moderator (slow neutrons), coolant (heat removal).
Case-based: Given decay chain or fusion/fission reaction, find energy released, half-life, or activity.	Case-Based (4 marks)	2023, 2025	Repeated 2x; Use Q-value or decay law.
Why fusion releases more energy per nucleon than fission for light nuclei?	Short Answer (2 marks)	2021 Term 2, 2024	Repeated 2x; Binding energy curve: light nuclei fuse → move towards Fe peak → release energy.

Best Format for Preparation and Answering

Study Format: Memorize binding energy curve & formulas (r_n, E_n, λ, T_{1/2}, Q-value). Practice numericals on half-life, activity & energy released daily. Make table for radioactive decay types & series.
Answer Format: Derivations: Use mass defect → BE = Δm c² → BE/nucleon. Numericals: Calculate ΔE or fraction remaining → show steps. Always mention units (MeV, years, Bq).
Tips: Binding energy per nucleon curve & radioactivity decay law numericals are highest repeat & marks. Learn difference fission vs fusion clearly. Time: 8–10 mins for 3–5 mark questions. Very scoring with correct calculations & units.

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