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Chapter 24 Nuclear Chemistry

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  1. Chapter 24 Nuclear Chemistry • 24.1 Nuclear Radiation • 24.2 Radioactive Decay (includes decay rates & radiochemical dating) • 24.3 Nuclear Reactions (Transmutation • Part only) • 24.4 Applications & Effects of Nuclear Reactions (except for radiation dose and intensity/distance)

  2. Section 24.1 Nuclear Radiation Under certain conditions, some nuclei can emit alpha, beta, or gamma radiation. • Summarize the developments that led to the discovery and understanding of nuclear radiation, including the names of the important scientists and the nature and significance of their contributions. • Distinguish between chemical and nuclear reactions. • Identify alpha, beta, and gamma radiations in terms of composition and key properties. • Rank the penetrating power of the various types of radiation. • Predict the effect of an electric field on the path of the various types of radiation.

  3. Section 24.1 Nuclear Radiation Key Concepts • Wilhelm Roentgen discovered X rays in 1895. • Henri Becquerel, Marie Curie, and Pierre Curie pioneered the fields of radioactivity and nuclear chemistry. • Gamma radiation has the most and alpha particles the least penetrating power of the 3 basic types of nuclear radiation.

  4. Chemical vs Nuclear Reactions

  5. Chemical vs Nuclear Reactions

  6. Classifying • Classify each of the following as a chemical reaction, a nuclear reaction, or neither: • Thorium emits a beta particle • Two atoms share electrons to form a bond • A sample of pure sulfur releases heat as it slowly cools • A piece of iron rusts ? Nuclear Chemical Neither Chemical

  7. Discovery of Radioactivity • Wilhelm Roentgen (Germany), 1895: invisible rays emitted when electrons bombarded surface of certain materials • Rays caused photographic plates to darken • Roentgen called these high energy rays called X rays • Roentgen in 1901 became first Nobel laureate in physics for this discovery

  8. Discovery of Radioactivity Antoine-Henri Becquerel 1896 (France) - experiment to determine if phosphorescent minerals also gave off X-rays Image of Becquerel's photographic plate which has been fogged by exposure to radiation from a uranium salt. The shadow of a metal Maltese Cross placed between the plate and the uranium salt is clearly visible. http://en.wikipedia.org/wiki/Henri_Becquerel

  9. Becquerel discovered that certain minerals were constantly producing penetrating energy rays he called uranic rays like X-rays, but not related to fluorescence Determined that all minerals that produced these rays contained uranium rays were produced even though mineral was not exposed to outside energy Energy apparently being produced from nothing?? Discovery of Radioactivity

  10. Discovery of Radioactivity • Henri Becquerel • uranium saltK2UO2(SO4)2 • Darkened photographic plates– even when not exposed to light –

  11. Discovery of Radioactivity • Marie Curie (Polish born French physicist/chemist) ~ 1896-1898 • Named process by which materials give off such rays radioactivity • Emitted rays and particles she named radiation • Developed device to measure radioactivity

  12. Detecting Radiation: Electroscope +++ +++ When positively charged, metal foils in electroscope spread apart due to like charge repulsion When exposed to ionizing radiation, radiation knocks electrons off air molecules, which jump onto foils and discharge them, causing them to drop down

  13. Madam Curie Used electroscope to detect uranic rays in samples Discovered new elements by detecting their rays radium named for its green phosphorescence polonium named for her homeland Since these rays were no longer just a property of uranium, she changed name from uranic rays to radioactivity

  14. Discovery of Radioactivity • Curies in 1898, by processing several tons of uranium ore (pitchblende), identified 2 new radioactive elements: polonium, radium • Curies shared 1903 Nobel prize in physics with Becquerel • Marie awarded 1911 Nobel prize in chemistry for work with polonium & radium • Died in 1934 from effects of radiation

  15. Discovery of Radioactivity

  16. Properties of Radioactivity Can ionize matter (cause uncharged matter to become charged) basis of Geiger Counter and electroscope Has high energy Can penetrate matter Causes phosphorescent materials to glow basis of scintillation counter

  17. 3 Common Types of Radiation • Alpha particles • Beta particles • Gamma rays • (two more types described in next section)

  18. + Alpha Radiation • Alpha particle 42He+2 • 2 protons & 2 neutrons = nucleus of helium-4 atom • +2 charge

  19. +  Beta Radiation • 0-1b Beta particles – fast moving electrons • Originate from decay of a neutron

  20. neutron proton W– boson neutrino electron Beta Decay In Neutron • Particle Symbol Relative mass • Electron e- 1/1840 • Proton p+ 1.000 • Neutron n0 1.001 • Matter changed to energy plus other matter • Neutron (made of quarks - fundamental) does not “contain” an electron (a lepton) Example of weak force, of which W– is a boson

  21. Gamma Radiation • 00g High energy radiation; massless • Except for very unusual cases, gamma radiation always accompanies alpha and beta decay – few “pure” gamma emitters

  22. Characteristics of Alpha, Beta, and Gamma Radiation

  23. Alpha, Beta, Gamma Properties

  24. Penetrating Ability of Radioactive Rays a g b 0.01 mm 1 mm 100 mm Thickness of Lead

  25. Effect of Electric Field on Trajectory of Subatomic Particles Lead Block b 1- charge Positive plate Hole g 0 charge a 2+ charge Radioactive Source Negative plate

  26. X-Rays • Not generated by nuclear processes (get by bombarding materials with electrons) • Like gamma rays – form of high energy electromagnetic radiation (gamma has higher energy) • Both X and gamma rays highly penetrating & can be very damaging to living tissue

  27. Practice • Nuclear Radiation • Problems 1-5, page 864 • Problems 34-41, page 894

  28. Chapter 24 Nuclear Chemistry • 24.1 Nuclear Radiation • 24.2 Radioactive Decay (includes decay rates & radiochemical dating) • 24.3 Nuclear Reactions (Transmutation • Part only) • 24.4 Applications & Effects of Nuclear Reactions (except for radiation dose and intensity/distance)

  29. Section 24.2 Radioactive Decay Unstable nuclei can break apart spontaneously, changing the identity of atoms. • Explain why certain nuclei are radioactive while others are stable. • Predict the type of radiation an unstable nucleus will emit. • Apply your knowledge of radioactive decay to write balanced nuclear equations. • Solve problems involving radioactive decay rates. • Explain the basis for the technique of radiochemical dating, especially carbon dating. • Describe the decay processes of positron emission and electron capture.

  30. Section 24.2 Radioactive Decay Key Concepts • Radioisotopes emit radiation to attain more-stable atomic configurations. • Atomic number and mass number are conserved in nuclear reactions. • Radiochemical dating is a technique for determining the age of an object by measuring the amount of certain radioisotopes remaining in the object.

  31. Section 24.2 Radioactive Decay Key Concepts • A half-life is the time required for half of the atoms in a radioactive sample to decay. The number of nuclei N remaining after a certain number of half-lives n or after some time t can be calculated from:

  32. Nuclear Reactions • Involve a change in atom’s nucleus • Radioactive materials spontaneously emit radiation • Called radioactive decay • Do this because a radioactive nucleus is unstable • Gain stability by losing energy

  33. Pencil Analogy for StabilityGravitational Potential Energy

  34. Forces Between Nucleons • Green: • Strong Force (attractive) • Purple: • EM Force (repulsive for protons)

  35. Nuclear Stability - Forces • Nucleons (protons, neutrons) held together by strong force • Overcomes electrostatic repulsion by protons • Neutrons don’t have repulsion • Stability tied to neutron/proton ratio (n/p) • High atomic number nuclei need relatively more neutrons for stability • Range for stable nuclei: 1:1 light to 1.5:1 heavy (Pb, AN 82)

  36. Neutron-to-Proton Ratio Shaded region corresponds to “band” or “belt” of stability

  37. Nuclear Stability • Radioactive nuclei are found outside band of stability – above/below/beyond • Undergo decay to gain stability • All elements with atomic number (AN) > 82 (lead) are radioactive • Isotopes of elements with AN ≤ 82 but outside band of stability are radioactive

  38. Nuclear Stability – Decay Series • Various decay types change n/p in different ways • Unstable nuclei lose energy through radioactive decay in order to form a nucleus with a stable n/p ratio • Eventually, radioactive atoms undergo enough decays to form stable atoms • Lead-206 is final decay product of Uranium-238 (14 steps)

  39. Decay of 238U to 206Pb

  40. Practice • Nuclear Stability • Problems 12 - 14 page 874 • Problems 42, 45 – 48, 50 page 894

  41. Nuclear Equations Atomic number (AN) and mass numbers (MN) are shown Atomic and mass numbers are conserved AN: 88 = 86 +2 MN: 226 = 222 + 4

  42. 5 Types of Radiation • Alpha • Beta • Positron Emission * • Electron Capture * • Gamma • * New in this section

  43. n/p: 138/88=1.57 136/86 =1.58 Alpha Radiation Alpha particle emission changes the element Leaves n/p about the same (for heavier elements) In example below, start with radium, end up with radon

  44. n/p: 8/6=1.43 7/7 =1.00 Beta Radiation • 0-1b Beta particles – fast moving electrons • Originate from decay of neutron • Beta emission changes element • Lowers n/p • In example below, start with carbon, end up with nitrogen

  45. Positron Emission (+ Decay)

  46. Positron Emission (+ Decay) + + + + + + + + + + Neutron-deficient isotopes can decay by proton decay (emitting positrons – antiparticle of electron) anti-neutrino • Net effect: one proton replaced by • neutron • anti-neutrino • positron positron

  47. Electron Capture • Like positron emission, also reduces number of protons (increase n/p) • Nucleus draws in surrounding electron (usually from lowest energy level) • Electron combines with proton to form neutron with X-ray emission • 11p + 0-1e  10n + X-ray • 8137Rb + 0-1e  8136Kr + X-ray

  48. +  Decay Processes that Increase n/p Electron Capture • Positron Emission

  49. Particle Changes Beta Emission: neutron  proton Positron Emission: proton neutron Electron Capture: proton neutron

  50. Decay Process Summary