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Transcript
Nuclear Reactions A X Z A = number of protons + number of neutrons Z = number of protons A – Z = number of neutrons Number of neutrons = Mass Number – Atomic Number There are many types of uranium: 235 238 U 92 U 92 A 235 A 238 Z 92 Z 92 Number of protons 92 Number of protons 92 Number of neutrons 143 Number of neutrons 146 Isotopes of any particular element contain the same number of protons, but different numbers of neutrons. Role of neutrons • The purpose of neutrons in an atom is to balance out the repulsion of protons in the nucleus. – It is like putting items in between opposing magnets. Radioactive Decay Radioactive decay is due to instability and results in the emission of either: • an alpha particle (a), • a beta particle (b), • or a gamma ray(g). Alpha Decay The element releases an alpha particle which is identical to that of a helium nucleus. (Two protons and two neutrons). This can be stopped by a thin piece of paper. The new element has 2 fewer neutrons and 2 fewer protons. Alpha Decay A X Z A-4 4 Y He + Z-2 2 unstable atom alpha particle more stable atom Alpha Decay 222 226 Ra 88 Rn 86 4 He 2 Alpha Decay A A-4 4 226 222 4 X Z Ra 88 Y + Z-2 Rn + 86 He 2 He 2 Alpha Decay 222 Rn 86 222 Rn 86 A 4 Y He + Z 2 218 Po + 84 4 He 2 Alpha Decay A 230 4 234 230 4 X Z U 92 Th He + 90 2 Th He + 90 2 Alpha Decay 230 Th 90 230 Th 90 A 4 226 4 Y He + Z 2 Ra He + 88 2 Alpha Decay A 214 4 218 214 4 X Z Po 84 Pb He + 82 2 Pb He + 82 2 Beta Decay A beta particle is a fast moving electron which is emitted from the nucleus of an atom undergoing radioactive decay. Can be stopped by 3 mm of aluminium foil or 10 mm of wood. Beta decay occurs when a neutron changes into a proton and an electron. Beta Decay As a result of beta decay, the nucleus has one less neutron, but one extra proton. The atomic number, Z, increases by 1 and the mass number, A, stays the same. Beta Decay 218 218 Po 84 At 85 b -1 0 Beta Decay A X Z 218 Po 84 A b -1 218 b -1 Y + Z+1 Rn + 85 0 0 Beta Decay 234 A b -1 234 234 b -1 Th 90 Th 90 Y + Z Pa + 91 0 0 Beta Decay A 210 b -1 210 210 b -1 X Z Tl 81 Pb + 82 Pb + 82 0 0 Beta Decay 210 A b -1 210 210 b -1 Bi 83 Bi 83 Y + Z Po + 84 0 0 Beta Decay A 214 b -1 214 214 b -1 X Z Pb 82 Bi + 83 Bi + 83 0 0 Gamma Decay Gamma rays are not charged particles like a and b particles. Gamma rays are electromagnetic radiation with high frequency which are stopped by several cm of lead. When atoms decay by emitting a or b particles to form a new atom, the nuclei of the new atom formed may still have too much energy to be completely stable. This excess energy is emitted as gamma rays (gamma ray photons have energies of ~ 1 x 10-12 J). Nuclear Fission This occurs when a larger nucleus divides to create a smaller nucleus. Alpha decay is a simple version of this. When it divides, it creates a large amount of energy, and often times releases neutrons, which if surrounded by enough fissionable material can lead to a chain reaction. By using control rods to slow the process down, it powers nuclear power plants. Nuclear Fission Nuclear Fusion This occurs when two smaller nuclei combine together to form a single larger nuclei. This produces far more energy than a fission reaction, and also does not have a dangerous by-product. However we currently don’t have the means to use it as a reliable energy source, as we barely get more energy out, than we put in. Nuclear Fusion