The constancy of the binding energy in the range \( 30 \lt A \lt 170 \) in binding energy curve, is a consequence of the given fact :
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Nuclear forces are short ranged
The nuclear force acts only over a few femtometres, so a nucleon interacts only with its nearest neighbours, not with every nucleon in the nucleus. Once a nucleus is large enough for each nucleon to have its full set of neighbours (about \( A \gt 30 \)), adding more nucleons does not increase the binding of any one nucleon. This is the saturation of the nuclear force, and it keeps the binding energy per nucleon nearly constant (about 8 MeV) for \( 30 \lt A \lt 170 \).
Option (1) Nuclear forces are charge independent — Incorrect. Charge independence (the same force between n-n, p-p and n-p) is true, but it does not explain why the binding per nucleon stops increasing with A.
Option (2) Nuclear forces are short ranged — Correct. The short range of the force makes each nucleon feel only its neighbours, which gives the nearly constant binding energy per nucleon.
Option (3) Nuclear forces are the strongest forces — Incorrect. The nuclear force is the strongest force, but a strong force of long range would make the binding per nucleon keep increasing with A.
Option (4) Nuclear forces are the attractive in nature — Incorrect. The force is attractive (at these distances), but attraction alone, without the short range, would not give a constant binding per nucleon.
NCERT (2026-27):
Nuclear Binding Energy – “… (ii) The constancy of the binding energy in the range 30 < A < 170 is a consequence of the fact that the nuclear force is short-ranged. Consider a particular nucleon inside a sufficiently large nucleus. It will be under the influence of only some of its neighbours, which come within the range of the nuclear force.”