Appeared in: 2025 (2 sets)
Assertion (A) : The binding energy per nucleon is practically constant for mass number in the range ($30 < A < 170$).
Reason (R) : Nuclear forces between the nucleons for mass numbers in the range ($30 < A < 170$) are not short-range.
✓ Verified CBSE Marking Scheme
Official Evaluation Steps & EL10 Insight
Question Analysis: Assertion-Reasoning type question based on Nuclear Physics (NCERT Class 12, Chapter 13).
Step 1 (Governing Law & Concept Definition): [0.5 Mark]
- Assertion (A): The binding energy per nucleon ($BE/A$) is approximately constant (about $8 \text{ MeV}$) for nuclei with mass number $30 < A < 170$. This is because nuclear forces are short-ranged and a nucleon only interacts with its nearest neighbors, leading to the saturation property of nuclear forces.
- Reason (R): Nuclear forces are fundamentally short-range forces. They act only over distances of the order of a few femtometers ($1 \text{ fm} = 10^{-15} \text{ m}$). The statement in the Reason claims they are "not short-range," which is factually incorrect.
Step 2 (Step-by-Step Logical Deduction): [0.25 Mark]
- Evaluate Assertion (A): The statement is True. The saturation of nuclear forces ensures that the binding energy per nucleon remains nearly constant in the specified range.
- Evaluate Reason (R): The statement is False. Nuclear forces are essentially short-range forces, not long-range.
Step 3 (Final Concluded Answer): [0.25 Mark]
Correct Option: Assertion (A) is true, but Reason (R) is false.
💡 EL10 Concept Insight: Think of a nucleon like a person in a crowded room; they can only shake hands with the people standing right next to them, not the ones across the hall. Because they only "feel" their immediate neighbors, the energy per person stays the same no matter how big the crowd gets, proving that nuclear forces are very short-ranged!