Mass Defect

NCERT defines the mass defect as the difference between the total mass of the free constituent nucleons and the actual mass of the bound nucleus, $\Delta m=[Z{m}_p+(A-Z)m_n]-M_{nucleus}$. This missing mass has been converted into the binding energy that holds the nucleus together. For helium-4, two protons ($2\times 1.00728$ u) plus two neutrons ($2\times 1.00867$ u) sum to about $4.0319$ u, while the bound helium nucleus mass is about $4.0015$ u, giving $\Delta m\approx 4.0319-4.0015=0.0304$ u. The value 4.0026 u is wrong because that is close to the neutral atomic mass of helium, not the defect. The value 0.0304 kg is wrong because it attaches the wrong unit; the defect is in atomic mass units, not kilograms. The value 0.930 u is wrong because it mistakes the near-unity per-nucleon conversion factor for the defect. As stated in NCERT Class 12, Chapter 13 (Nuclei), a positive mass defect is precisely what makes a nucleus stable, since the vanished mass reappears as the energy binding the nucleons together. The larger the defect per nucleon, the more tightly bound and stable the nucleus, which is why the mass defect is central to comparing nuclear stability. A plausibility check: a defect of about 0.03 u times 931.5 MeV per u gives roughly 28 MeV of binding, matching helium's known total binding energy and confirming the computed defect is of the right size.