Case: Read the passage and answer the question.
An atom contains a dense mass at its centre, called the nucleus. The nucleus is positively charged and is made up of neutrons and protons. Therefore, it may be expected that the mass of the nucleus is equal to the total mass of its individual protons and neutrons. However, the nuclear mass M is found to be always less than this.
What is this difference in masses called?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Mass defect
Option (1) Nuclear force — Incorrect. The nuclear force is the strong, short-range attraction that holds the nucleons together. It is a force between nucleons, so it cannot be the name of a difference in mass.
Option (2) Mass-energy equivalence — Incorrect. This is Einstein’s relation \( E=mc^{2} \). It gives the meaning of the missing mass, but it is not the name of the difference itself.
Option (3) Mass defect — Correct. The difference \( \Delta M=[Zm_p+(A-Z)m_n]-M \) between the total mass of the separate protons and neutrons and the mass \( M \) of the nucleus is called the mass defect.
Option (4) Binding energy — Incorrect. Binding energy is the energy equivalent of the mass defect, \( E_b=\Delta M\,c^{2} \). It is an energy. The mass difference itself is the mass defect.
NCERT (2026-27):
Nuclear Binding Energy – “… The difference in mass of a nucleus and its constituents, ΔM, is called the mass defect, and is given by
\( \Delta M=[Zm_p+(A-Z)m_n]-M \qquad (13.7) \)”