Concept preview

What you will be able to do

1. 01Explain mass defect and binding energy
2. 02Describe nuclear fission, the chain reaction and how a reactor works
3. 03Describe nuclear fusion and compare it with fission

Weigh a nucleus and something astonishing shows up: it has less mass than its protons and neutrons would have separately. This missing mass is the mass defect, and by $E = mc^2$ it corresponds to the binding energy — the energy released when the nucleus formed, and the energy you'd need to pull it apart again. A nucleus is bound because its mass was converted to binding energy.