Physics 11–12 · Year 12
Mass defect and binding energy: the deuteron from measured masses
Module 8: From the Universe to the Atom (Properties of the Nucleus)
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The idea
A nucleus weighs less than its separated nucleons, and the missing mass times c squared is the energy that holds it together and the energy released when it forms.
What you need
- Particle Data Group 2024 masses: proton 938.272 088 16 MeV, neutron 939.565 420 52 MeV, deuteron 1875.612 942 57 MeV, atomic mass unit 931.494 102 42 MeV
How to do it
- Add the proton and neutron masses and subtract the deuteron mass to find the mass defect in MeV/c^2.
- Convert to atomic mass units and to kilograms.
- Divide by the number of nucleons for the binding energy per nucleon.
- Compare the energy released when a proton captures a neutron with the energy of a visible photon and with the energy released per atom in a chemical reaction.
What you should see
The deuteron's mass defect is 2.224 566 MeV/c^2, or 0.002 388 u, or 3.966e-30 kg: binding energy 2.2246 MeV, 1.112 MeV per nucleon. That single capture releases about a million times the energy of a visible photon (2 to 3 eV) and the mass change is 0.12 per cent of the deuteron's mass.
What changes
This activity lists no variables to change, measure and keep the same.
Common misconceptions
Each of these ideas is wrong, and the activity is a chance to test it.
- Binding energy is energy stored inside the nucleus; it is energy that had to leave for the nucleus to form.
- Mass is conserved in nuclear reactions; mass-energy is conserved and mass alone is not.
Safety card
Hazards
No hazard is listed.
Controls
No control is listed.
Note
Screen activity only.
Curriculum references
The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.
- Physics Stage 6 Syllabus (2017), current: Year 11 until the end of 2026, Year 12 until Term 3 2027PH12-15PH11/12-6
- Physics 11-12 Syllabus (2025), not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027, first HSC examination 2028PY-12-04PY-12WS-06
- Australian Curriculum v9No Australian Curriculum v9 code is listed.
Sources
The pages the author read to write this activity.
- www.nsw.gov.au/sites/default/files/noindex/2025-03/physics-stage-6-syllabus-2017.docx
- education.nsw.gov.au/content/dam/main-education/teaching-and-learning/curriculum/key-learning-areas/science/s-6/physics/12Physics_-module-8-guide.docx
- pdg.lbl.gov/2024/reviews/rpp2024-rev-phys-constants.pdf
- curriculum.nsw.edu.au/learning-areas/science/physics-11-12-2025/outcomes