In a mass defect calculation, what is the expression used to compute the mass defect?

In a mass defect calculation, what is the expression used to compute the mass defect?

• A. Mass of the atom minus the sum of the masses of its protons, neutrons, and electrons
• B. Sum of the masses of the atoms alone
• C. Sum of the masses of protons and neutrons
• D. Mass of the electrons plus the mass of the nucleus

Answer: (A)

Mass defect is the difference between the sum of the rest masses of a system’s separate constituents and the actual mass of the bound system. For an atom, that means taking the atomic mass and subtracting the total rest mass of its components: protons, neutrons, and electrons (Δm = M_atom − [Z m_p + N m_n + Z m_e]). This positive difference corresponds to the binding energy released when the nucleus forms, via E = Δm c^2. The other ideas don’t measure how much mass is “lost” to binding energy—summing just atoms, or omitting electrons, or adding masses back together—so the stated expression best captures the mass defect.

Mass defect is the difference between the sum of the rest masses of a system’s separate constituents and the actual mass of the bound system. For an atom, that means taking the atomic mass and subtracting the total rest mass of its components: protons, neutrons, and electrons (Δm = M_atom − [Z m_p + N m_n + Z m_e]). This positive difference corresponds to the binding energy released when the nucleus forms, via E = Δm c^2. The other ideas don’t measure how much mass is “lost” to binding energy—summing just atoms, or omitting electrons, or adding masses back together—so the stated expression best captures the mass defect.