7 2 Nuclear Fission and Fusion Nuclear Fission

7. 2 • Nuclear Fission and Fusion

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

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

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

Nuclear Fission

Chain Reaction

Chain Reaction • Requires a minimum amount of fissionable material critical mass

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

Mass Defect Albert Einstein (1879 -1955) • 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=mc 2 speed of light

Nuclear Fusion powers the sun 1 4 1 H 4 He 2 + g + energy • 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)