Advanced Topic 102 of 1102 Atomic Structure & Periodicity
Mass Defect
The difference between the mass of a nucleus and the sum of its separate nucleons.
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In-depth explanation of Mass Defect
The mass defect is the difference between the mass of a nucleus and the sum of the masses of its separate protons and neutrons. The missing mass is converted into the binding energy that holds the nucleus together. It is related to energy by E = delta m c^2, and 1 u of mass defect corresponds to 931.5 MeV. Mass defect is the reason nuclear reactions can release energy. It is measured with precise mass spectrometers.
Going deeper: Mass Defect
Key ideas: Delta m = (Z x m_p + N x m_n + electrons if atomic masses) - M_nucleus, always positive. Example: 4He, 2 x 1.00728 + 2 x 1.00866 - 4.00151 = 0.0304 u, which is 28.3 MeV. Use atomic masses with hydrogen atom mass 1.00783 u so electron masses cancel out.
Common mistakes and connections: Students mix nuclear and atomic masses, causing electrons to be double counted, or forget to convert u to MeV (x 931.5). Mass is not destroyed: it is released as energy per E = m c^2, though chemical changes have negligible values (about 10^-10 relative). It is the basis for calculating energy in nuclear reactions.
Visualization of Mass Defect
Free nucleons weigh 4.0319 u but the helium-4 nucleus is 4.0015 u; the missing 0.0304 u equals 28.3 MeV by E = mc^2. The interactive lesson shows this as a live animation or chart.
Worked example and use case
For helium-4, using atomic masses of 1.007825 u for hydrogen and 1.008665 u for the neutron, the mass defect is 0.0304 u, which corresponds to about 28.3 MeV.
Frequently asked questions about Mass Defect
What is Mass Defect?
The difference between the mass of a nucleus and the sum of its separate nucleons.
Is Mass Defect a beginner, intermediate or advanced topic?
Mass Defect is a advanced topic in Atomic Structure & Periodicity. It builds on beginner and intermediate topics, so review those first if needed.
What should I learn before Mass Defect?
Study these earlier topics in Atomic Structure & Periodicity first: Hydrogen Position, Quark Structure of Nucleons, Nuclear Binding Energy.
What comes after Mass Defect?
The next topics in Atomic Structure & Periodicity are Radioactivity and Alpha Decay.
Can I practise Mass Defect with quizzes?
Yes. The interactive lesson includes a self-check question on Mass Defect, and the Atomic Structure & Periodicity quiz draws on every topic in this area.
Where Mass Defect fits in Atomic Structure & Periodicity
Mass Defect is topic 102 of 115 in Atomic Structure & Periodicity.