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Chapter 21 Nuclear Chemistry

CHEMISTRY The Central Science 9th Edition. Chapter 21 Nuclear Chemistry. 21.1: Radioactivity. Nuclear Equations Nucleons : particles in the nucleus: p + : proton n 0 : neutron Mass number : the number of p + + n 0 Atomic number : the number of p +

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Chapter 21 Nuclear Chemistry

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  1. CHEMISTRYThe Central Science 9th Edition Chapter 21Nuclear Chemistry Chapter 21

  2. 21.1: Radioactivity • Nuclear Equations • Nucleons: particles in the nucleus: • p+: proton • n0: neutron • Mass number: the number of p+ + n0 • Atomic number: the number of p+ • Isotopes: have the same number of p+ and different numbers of n0 • In nuclear equations, number of nucleons is conserved: • 23892U 23490Th + 42He Chapter 21

  3. Types of Radioactive Decay • There are three types of radiation: • -Radiation is the loss of 42He from the nucleus • -Radiation is the loss of an electron from the nucleus • -Radiation is the loss of high-energy photon from the nucleus • In nuclear chemistry, to ensure conservation of nucleons we write all particles with their atomic and mass numbers: • 42He and 42a represent -radiation Chapter 21

  4. Text, P. 833 Chapter 21

  5. Types of Radioactive Decay • Nucleons can undergo decay: • 10n 11p+ + 0-1e- (-emission) • 0-1e- + 01e+ 200g (positron annihilation) • 10p+10n + 01e+ (positron or +-emission) • 11p+ + 0-1e-10n(electron capture) • A positron is a particle with the same mass as an electron but a positive charge Chapter 21

  6. Text, P. 834

  7. 21.2: Patterns of Nuclear Stability • Neutron-to-Proton Ratio • The proton has high mass and high charge • The proton-proton repulsion is large • In the nucleus the protons are very close to each other • The cohesive forces in the nucleus are called strong nuclear forces • As more protons are added (the nucleus gets heavier) the proton-proton repulsion gets larger Chapter 21

  8. Neutron-to-Proton Ratio • The heavier the nucleus, the more neutrons are required for stability • The belt of stability deviates from a 1:1 neutron to proton ratio for high atomic mass • At Bi (83 protons) the belt of stability ends and all nuclei are unstable • Nuclei above the belt of stability undergo -emission • Nuclei below the belt of stability undergo +-emission • Nuclei with atomic numbers greater than 83 usually undergo -emission Chapter 21 Text, P. 835

  9. Radioactive Series • A nucleus usually undergoes more than one transition on its path to stability • The series of nuclear reactions that accompany this path is the radioactive series • Nuclei resulting from radioactive decay are called daughter nuclei Text, P. 837 Chapter 21

  10. Further Observations • Magic numbers are nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons • Nuclei with even numbers of protons and neutrons are more stable than nuclei with any odd nucleons • http://www.pbs.org/wgbh/nova/physics/stability-elements.html Chapter 21

  11. 21.3: Nuclear Transmutations • Using Charged Particles • Nuclear transmutations are the collision between nuclei. • For example, nuclear transmutations can occur using high velocity -particles: • 14N + 417O + 1p • This is written as 14N(α,p)17O Chapter 21

  12. Sample Problems # 8, 23, 24, 25, 26 Chapter 21

  13. 21.4: Rates of Radioactive Decay • 90Sr has a half-life of 28.8 yr • 90Sr decays as follows: • 9038Sr 9039Y + 0-1e • Each isotope has a characteristic half-life • Half-lives are not affected by temperature, pressure or chemical composition • Natural radioisotopes tend to have longer half-lives than synthetic radioisotopes Chapter 21

  14. Half-lives can range from fractions of a second to millions of years • Naturally occurring radioisotopes can be used to determine how old a sample is • This process is radioactive dating Chapter 21

  15. Calculations Based on Half Life • Radioactive decay is a first order process: • In radioactive decay the constant, k, is the decay constant • The rate of decay is called activity (disintegrations per unit time) • If N0 is the initial number of nuclei and Nt is the number of nuclei at time t, then Chapter 21

  16. Calculations Based on Half Life • With the definition of half-life • (the time taken for Nt = ½N0), we obtain: Chapter 21

  17. Sample Problems # 29, 30, 31, 32, 33, 38 Chapter 21

  18. 21.7: Nuclear Fission • Splitting of heavy nuclei is exothermic for large mass numbers • During fission, the incoming neutron must move slowly because it is absorbed by the nucleus • The heavy 235U nucleus can split into many different daughter nuclei: • 10n + 23892U 14256Ba + 9136Kr + 310n • (releases 3.5  10-11 J per 235U nucleus) Chapter 21

  19. For every 235U fission, 2.4 neutrons are produced • Each neutron produced can cause the fission of another 235U nucleus • The number of fissions and the energy increase rapidly • Eventually, a chain reaction forms • Without controls, an explosion results Chapter 21

  20. Nuclear Reactors • Use fission as a power source • Use a subcritical mass of 235U • (enrich 238U with about 3% 235U) • Enriched 235UO2 pellets are encased in Zr or stainless steel rods • Control rods are composed of Cd or B, which absorb neutrons Chapter 21

  21. Moderators are inserted to slow down the neutrons • Heat produced in the reactor core is removed by a cooling fluid to a steam generator and the steam drives an electric generator Chapter 21

  22. Chapter 21

  23. 21.8: Nuclear Fusion • Light nuclei can fuse to form heavier nuclei • Most reactions in the sun are fusion • Fusion products are not usually radioactive, so fusion is a good energy source • the hydrogen required for reaction can easily be supplied by seawater • However, high energies are required to overcome repulsion between nuclei before reaction can occur Chapter 21

  24. High energies are achieved by high temperatures: the reactions are thermonuclear • Fusion of tritium and deuterium requires about 40,000,000K: • 21H + 31H 42He + 10n • These temperatures can be achieved in a nuclear bomb or a tokamak • A tokamak is a magnetic bottle: strong magnetic fields contained a high temperature plasma so the plasma does not come into contact with the walls • No known material can survive the temperatures for fusion • To date, about 3,000,000 K has been achieved in a tokamak Chapter 21

  25. 21.9: Biological Effects of Radiation • The penetrating power of radiation is a function of mass • -radiation (zero mass) penetrates much further than -radiation, which penetrates much further than -radiation • Radiation absorbed by tissue causes excitation (nonionizing radiation) or ionization (ionizing radiation) • Ionizing radiation is much more harmful than nonionizing radiation Chapter 21

  26. Most ionizing radiation interacts with water in tissues to form H2O+ • The H2O+ ions react with water to produce H3O+and OH • OHhas one unpaired electron • It is called the hydroxy radical • Free radicals generally undergo chain reactions Chapter 21

  27. Sample Problems # 55 & 56 Chapter 21

  28. End of Chapter 21Nuclear Chemistry Chapter 21

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