Chapter 20 Notes

Chapter 20 Notes - Chapter 20 Nuclear Chemistry The Nature...

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Chapter 20 Nuclear Chemistry
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Henri Becquerel (1896): • U salts emitted rays that “fog” a photographic plate. • U metal was a stronger emitter. Marie and Pierre Curie: • Isolated Po and Ra that did the same. • Marie Curie called the phenomenon radioactivity. Thomson and Rutherford: • Studied the radiation, and found two types: α and β . Villard: • Discovered γ radiation. The Nature of Radioactivity
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The Nature of Radioactivity Name Symbol Charge Mass (g) Pen. Power* alpha α He +2 6.65 x 10 -24 0.03 mm beta β e -1 9.11 x 10 -28 2 mm gamma γ γ 0 0 100 mm *Distance at which half the radiation has been stopped by water. 4 2 4 2 0 -1 0 -1 0 0
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(1902) “Radioactivity is the result of a natural change of a radioactive isotope of one element into an isotope of a different element”. Nuclear Reactions 226 88 Ra Rn + He 222 86 4 2 Radium-226 Radon-222 alpha particle Mass number ( A ) 226 = 222 + 4 Atomic number ( Z ) 88 = 86 + 2 Note: A and Z must always balance:
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Alpha – a nucleus ejects a helium nucleus: 238 92 U Th + He 234 90 4 2 Beta – a nucleus ejects an electron: 90 38 Sr Y + e 90 39 0 -1 How does a nucleus eject an e - ? A series of steps, but the net result is: 1 0 n p + e 1 1 0 -1 neutron proton electron
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Radioactive Series A decay product (daughter isotope) is often unstable. .. A radioactive series. Number of neutrons: N = A - Z
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Other Types of Radioactive Decay Positron emission Positron = positive electron ( e or β + ). Antimatter. 0 +1 Antimatter is annihilated by collision with matter: 43 21 Sc Ca + e 43 20 0 +1 β + + e - 2 γ
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Electron capture (EC) An inner-shell e - (K shell) is captured by the nucleus. Sometimes called K-capture. 7 4 Be + e 7 3 0 -1 Li Other Types of Radioactive Decay
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Radioactive iodine-131 is used to test thyroid function. It undergoes beta decay to form a new element. Write a balanced equation for the process. 131 54 Xe 131 53 I e + 0 -1 Nuclear Equations Add β (product): 131 53 I e + 0 -1 Element 54 Look up Z for I (Z = 53): I 131 53 Calculate the Z and A for the new isotope:
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Stability of Atomic Nuclei
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Stable nuclei have N ≥ Z . Nuclei with Z < 20: N / Z ≈ 1. Nuclei with Z > 20: N / Z gradually increases. 209 Bi ( Z = 83) is the heaviest stable nucleus. Even- Z isotopes are more common than odd. When Z is odd, an even- N isotope is more stable. 200 “even-even” 120 “odd-even” or “even-odd” Only 4 “odd-odd” isotopes known Stability of Atomic Nuclei
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Elements with Z < 83 Use a periodic table to determine whether an isotope is too heavy or too light and hence its mode of decay. Elements with
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Chapter 20 Notes - Chapter 20 Nuclear Chemistry The Nature...

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