The difference in mass of a nucleus and its constituent nucleons is called the __________.
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Mass defect
The mass of a nucleus is always a little less than the total mass of its separate protons and neutrons. This difference is the mass defect:
\( \displaystyle \Delta M = \left[Z m_p + (A - Z)\,m_n\right] - M \)
The missing mass appears as the binding energy \( E_b = \Delta M\,c^{2} \).
Option (1) Packing fraction — Incorrect. The packing fraction is a different quantity; it compares the nuclear mass with the mass number, not with the masses of the constituent nucleons.
Option (2) Mass defect — Correct. The difference between the total mass of the nucleons and the mass of the nucleus is the mass defect.
Option (3) Binding energy — Incorrect. The binding energy is an energy, \( \Delta M\,c^{2} \), the energy equivalent of the mass defect, not the mass difference itself.
Option (4) Binding energy per nucleon — Incorrect. The binding energy per nucleon is \( E_b/A \), again an energy, used to compare the stability of nuclei.
NCERT (2026-27):
Summary – “… 7. The nuclear mass M is always less than the total mass, Σm, of its constituents. The difference in mass of a nucleus and its constituents is called the mass defect,
\( \displaystyle \Delta M = (Z\,m_p + (A - Z)\,m_n) - M \)
Using Einstein’s mass energy relation, we express this mass difference in terms of energy as
\( \displaystyle \Delta E_b = \Delta M\,c^{2} \)
The energy ΔEb represents the binding energy of the nucleus.”