Which statement describes mass defect in nuclei?

• A. The mass of the nucleus is less than the sum of the masses of its nucleons.
• B. The mass defect is converted into energy which binds the particles in the nucleus.
• C. Mass defect has no relation to energy.
• D. Mass defect equals the total rest mass energy of the nucleus.

Answer: (A)

Mass defect is the difference between the sum of the free nucleons’ masses and the actual mass of the bound nucleus. When nucleons come together, a small amount of mass is lost, and that lost mass is converted into the binding energy that holds the nucleus together (E = Δm c^2). So the nucleus has less mass than the sum of its parts, which is why this description is correct. The other statements either deny the energy connection, misstate the relationship to energy, or claim the defect equals the total rest-mass energy, which isn’t true—the defect only accounts for the binding energy.