The missing mass that holds nuclei together

Science · Matter & Materials · ages 17-18

Name ______________________ Date ____________

The binding energy per nucleon curve peaks very near iron-56.
Circle one: True False
What is the mass defect of a nucleus?
- Whole nucleus minus separate nucleons
- Protons minus neutrons
- Separate nucleons minus whole nucleus
Which nucleus sits near the top of the binding energy curve?
- Hydrogen-2
- Iron-56
- Uranium-238
Helium-4 holds 28.3 MeV of binding energy across 4 nucleons. Give the energy per nucleon in MeV.
Answer: ______________
Proton 938.28 plus neutron 939.57 minus bound deuteron 1875.61. Give the mass defect in MeV/c squared.
Answer: ______________
A heavy nucleus splits into fragments sitting higher on the curve. What happens to energy?
- Energy is released
- Energy is absorbed
- Nothing changes at all
Helium-4 manages about 7.07 MeV per nucleon while deuterium manages about 1.12. What does breaking helium apart demand?
- Energy is released
- Energy must be put in
- Mass appears from nowhere
Mia wants energy from fusion. Which pair should she fuse?
- Two iron nuclei
- Two heavy nuclei
- Two light nuclei below iron

Answer key

For grown-ups. Fold this page away before handing over the rest.

True · Iron-56 sits at the top, the most tightly bound nucleus in nature.
Separate nucleons minus whole nucleus · The free parts weigh more, and the difference is the defect.
Iron-56 · Iron-56 crowns the curve, which is why fusion in stars ends there.
7.075 · Divide the total by the headcount: 28.3 over 4 is 7.075 MeV.
2.24 · Add the free parts and subtract the whole: 1877.85 minus 1875.61 is 2.24.
Energy is released · Climbing the curve frees the difference, so the split pays out energy.
Energy must be put in · The split moves from tightly bound to loosely bound, so it soaks up energy instead of paying.
Two light nuclei below iron · Only joins below the peak climb upward, and only climbing releases energy.