The mass defect \( (\Delta M) \) for a nucleus of mass number (A), atomic number (Z) and mass (M) is given by
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( \Delta M = [Z m_p + (A - Z) m_n] - M \)
The mass defect is the total mass of the separate nucleons minus the mass of the nucleus they form. A nucleus of atomic number \( Z \) and mass number \( A \) contains \( Z \) protons and \( A - Z \) neutrons.
\( \displaystyle \text{mass of nucleons} = Z m_p + (A - Z)\,m_n \)
\( \displaystyle \Delta M = [Z m_p + (A - Z)\,m_n] - M \)
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
\( \displaystyle \Delta M = [Z m_p + (A - Z) m_n] - M \qquad (13.7) \)”