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 atoms1.5 : 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:

Kr 36 93

U 92235

+ n01

Ba 56140

+ n01

+ 3

• the uranium atom becomes unstable and splits into two lighter atoms nuclear fission

• fission of uranium also produces more neutrons

+ LOTS ofenergy

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

• 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

Nuclear Fusion

Nuclear Fusion powers the sun

H11

4 He24

+

• 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