Which of the following best describes the concept of mass defect?

• A. The mass defect is the total mass of the electrons in an atom.
• B. The mass defect is the difference in mass when an atom loses an electron.
• C. The mass defect is the additional mass gained by an atom when neutrons are added.
• D. The mass defect is the mass lost when protons and neutrons combine to form an atomic nucleus.

Answer: (D)