JEE Main 2025 · 23 January · evening · Single correct · +4 / −1

Given below are two statements. One is labelled as Assertion (A) and the other is labelled as Reason (R).
Assertion (A) : The binding energy per nucleon is found to be practically independent of the atomic number A , for nuclei with mass numbers between 30 and 170 .
Reason (R) : Nuclear force is long range. In the light of the above statements, choose the correct answer from the options given below :

  1. (a) (A) is true but (R) is false
  2. (b) Both (A) and (R) are true and (R) is the correct explanation of (A)
  3. (c) (A) is false but (R) is true
  4. (d) Both (A) and (R) are true but (R) is NOT the correct explanation of (A)
Answer and solution

Answer (a) (A) is true but (R) is false

Solution

Physics sir · 4 steps

The plateau in the binding energy curve is real, but it comes from nuclear force being short range, not long.

  1. Check the Assertion. On the binding energy curve, B/A stays close to 8 MeV per nucleon for mass numbers from about 30 to 170. So the Assertion is true. The question writes atomic number, but it means mass number A.
  2. Check the Reason. Nuclear force acts only over about 2 to 3 fm, roughly the distance between neighbouring nucleons. Beyond that it dies out quickly. So it is short range, and the Reason is false.
  3. Why the plateau. A nucleon deep inside a large nucleus is pulled only by its few close neighbours, not by every nucleon in the nucleus. Adding more nucleons does not add more pull on each one. This is called saturation, and it keeps B/A nearly constant.
  4. So. (A) is true and (R) is false. The answer is (a).

The idea to keep. Nuclear force is short range and saturates: each nucleon feels only its near neighbours. That is exactly why B/A levels off near 8 MeV for medium-mass nuclei.

Answer from the NTA final answer key. Solution by PrepWiser.

Study Nuclear Physics

Asked about 110 times in JEE Main since 2020. This question is from 2025.