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Chapter 25

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Chapter 25

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  1. Chapter 25 Nuclear Chemistry

  2. Nuclear Radiation • Nuclear chem • the study of the structure of atomic nuclei and the changes they undergo. • No e- / orbitals • No e- sharing or transferring • No cpds formed • No bondings

  3. The Discovery of Radioactivity • In 1895, Roentgen found that invisible rays were emitted when e- bombarded the surface of certain materials. • they caused photographic plates to darken. • named these invisible high-energy emissions X rays.

  4. The Discovery of Radioactivity • At that time, French physicist Becquerel was studying minerals that • emit light after being exposed to sunlight (phosphorescence). • Building on Roentgen’s work, Becquerel wanted to determine whether phosphorescent minerals also emitted X rays.

  5. The Discovery of Radioactivity Becquerel accidentally discovered that phosphorescent U salts – even when not exposed to light – produced spontaneous emissions that darkened photographic plates.

  6. The Discovery of Radioactivity Marie Curie (1867–1934) and her husband Pierre (1859–1906) took Becquerel’s mineral sample and isolated the components emitting the rays.

  7. The Discovery of Radioactivity • Conclusion: • the darkening of the photographic plates was due to rays emitted specifically from the U atoms in the mineral sample. • named the process by which materials give off such rays radioactivity; • the rays and particles emitted by a radioactive source are called radiation.

  8. Types of Radiation • Isotopes • are atoms of the same element that have different #s of n0. • Radioisotopes • Isotopes of atoms with unstable nuclei ready to emit radiations. • unstable nucleiis due to big diffce in the # of • p+and n0. e.g.U has 92 p+and over 140 n0 • Mg has 12 p+and 12 n0(stable)

  9. Quick write What kind of atoms are radioactive?

  10. Types of Radiation • radioactive decay • A process that unstable nuclei emit radiation to attain more stable atomic configns. • During radioactive decay, unstable atoms lose energy by emitting 1 of several types of radiation.

  11. Types of Radiation 3 common types of radiation • alpha (α) • beta (β) • gamma (γ)

  12. 2 -1 0 1837

  13. Quick Write What are α particles? How many p+, n0 and e- does each particle carry?

  14. Radioactive Decay Radioactive Decay • unstable nuclei loses energy by emitting radiation: ‘particles’ or ‘energy (non-particles)’. • become lighter • Natural (not human initiated; can’t stop or slow down) • not requiring any energy input. • Spontaneous

  15. Quick Write What are the areas that α decay and β decay have in common? (4 bullet pts)

  16. Radioactive Decay

  17. Half-life Half-life • Time required for one half of the nuclei of a radioisotope sample to decay. • e.g. U → Th + α-particle • Each radioisotope has a characteristic t 1/2

  18. Half-life Half-life • Time required for one half of the nuclei of a radioisotope sample to decay. • e.g. U → Th + α-particle 238U → 234Th + 4He • Each radioisotope has a characteristic t 1/2 92 90 2

  19. Half-life (t ½)

  20. Effect of an Electric Field (1) • What radiations are deflected toward the -ve plate? Why? • What radiations are deflected toward the +velyplate? Why?

  21. Quick Write What is radioactive decay? (5 bullet points)

  22. Quick Write Describe and explain the paths of α, β, and γ radiation under the influence of electric field?

  23. Effect of an Electric Field (2) • The +vely charged α particles are deflected towards the -ve plate. • The –vely charged β particles are deflected towards the +ve plate. • the neutral γ radiation travels in a straight line.

  24. Quick Write What are β particles? Do they carry any charges?

  25. Quick Write What is γ radiation? What charge does it carry?

  26. Effect of an Electric Field (2) • β particles are deflected towards the +ve plate. • β particles undergo greater deflection because →less mass.

  27. Effect of an Electric Field (3) • γ ray, (no electrical charge), are not deflected.

  28. Types of Radiation- α radiation • An α particle has the same composition as a He nucleus—2 p+and 2 n — • 2+ due to the presence of the 2 p+.

  29. Types of Radiation- α radiation • α radiation—a stream of αparticles. • Ra-226, (88 p+and 138 n0), undergoes α decay by emitting an α particle.

  30. Types of Radiation- α radiation • After the decay, the resulting atom has an atomic # of 86, a mass # of 222. • The new radiosiotope is Rn-222.

  31. Types of Radiation • The particles involved are balanced. • i.e. the sum of the mass #s (superscripts) = the sum of the atomic #s (subscripts) on each side of the arrow.

  32. Types of Radiation • Because of their mass and charge, α particles are relatively slow-moving compared with other … • Thus, α particles are not very penetrating—a single sheet of paper stops.

  33. Types of Radiation—β Radiation • A β particle is a very-fast moving e- that has been emitted from a n0of an unstable nucleus. • β particles are represented by the symbol . The ‘0’ superscript indicates the insignificant mass of an e- in comparison with the mass of a nucleus.

  34. Types of Radiation—β Radiation • The –1 subscript denotes the -ve charge of the particle. • β radiation consists of a stream of fast-moving e-.

  35. Types of Radiation An example of the β decay process is the decay of I-131 into Xe-131 by β-particle emission.

  36. Types of Radiation-- β Radiation • The mass # of the product nucleus is the same as that of the original nucleus (both 131), but its atomic # has increased by 1 (54 instead of 53).

  37. Types of Radiation-- β Radiation • This change in atomic #, and thus, change in identity, occurs because the e- emitted during the β decay has been removed from a n0, leaving behind a p+.

  38. Types of Radiation—β Radiation β radiation: fast-moving e- formed by decomposition of a n0in an atom. Quick write: why the mass # remain 14 while there is an additional p+?

  39. Types of Radiation • Because β particles are both lightweight and fast moving, they have greater penetrating power than α particles. • A thin metal foil is required to stop β particles.

  40. Types of Radiation • γ rays are high-energy (short wavelength) electromagnetic radiation. • They are denoted by the symbol . • Both the subscript and superscript are ‘0’.

  41. Types of Radiation • Thus, the emission of γ rays does not change the atomic # or mass # of a nucleus. • γ rays almost always accompany α and β radiation, as they account for most of the energy loss that occurs as a nucleus decays.

  42. Types of Radiation • e.g. γ rays accompany the α-decay rxn of U-238. • The 2 in front of the γ symbol indicates that 2 γ rays of different frequencies are emitted. • Because γ rays have no effect on mass # or atomic #, it is customary to omit them from nuclear eqns.

  43. Radioactive Decay • Of all the known isotopes, only about 17% are stable and don’t decay spontaneously.

  44. Beta Decay • A radioisotope that lies above the band of stability is unstable because it has too many n relative to its # of p+. e.g. unstable has a n0 / p+ ratio of 1.33 : 1, whereas stable elements of similar mass, such as and , have n0 / p+ ratios ≈1:1

  45. Beta Decay It is not surprising then that undergoes beta decay, as this type of decay decreases the # of n0 in the nucleus. Note that the atomic # of the product nucleus, , has increased by 1. • The N-14 atom now has a stable n0 / p+ratio of 1 : 1.