1. 7.2 Nuclear Fission and Fusion

2. Nuclear Fission Why are some elements radioactive? There is an optimal ratio of neutrons to protons 1 : 1 for smaller atoms 1.5 : 1 for larger atoms excessive number of neutrons  nucleus becomes unstable  decays to a more stable state with the release of energy

3. Nuclear Fission Lise Meitner (1878-1968)Otto Hahn (1879-1968)

4. Nuclear Fission Meitner and Hahn bombarded uranium with neutrons: Kr 36 93 U 92 235 + n 0 1 Ba 56 140 + n 0 1 + 3 the uranium atom becomes unstable and splits into two lighter atoms  nuclear fission fission of uranium also produces more neutrons + LOTS of energy

5. Nuclear Fission

7. Chain Reaction

8. Requires a minimum amount of fissionable material  critical mass

9. Chain Reaction Generation of a critical mass forms the basis of fission bombs

10. Mass Defect Mass cannot be created or destroyed Energy cannot be created or destroyed But: mass can be converted into energy and energy can be converted into mass E = m c 2 Albert Einstein (1879-1955) speed of light

11. Nuclear Fusion Nuclear Fusion powers the sun H 1 1 4 He 2 4 +  Fusion of hydrogen to form helium releases more energy than fission of uranium Hydrogen-, thermonuclear- or fusion bomb Fusion reactors (produce less dangerous radioactive isotopes) + energy