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Chapter 19

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Chapter 19

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  1. Chapter 19 Radioactivity and Nuclear energy

  2. This sort of chemistry doesn’t involve the electrons • It’s all wrapped up in the nucleus • Think about nuclear processes and what parts of our lives are they involved in… Nuclear weapons, power plants, radioactive dating, nuclear medicine

  3. The nucleus is small and dense • Consider a ton… • A ton is 2,000 pounds • If you were to build a nucleus the size of a ping pong ball it would weigh 2.5 billion tons • The energies involved in nuclear processes are millions of times greater than those associated with chemical processes. • Consider the energy “crisis” • How is your electricity generated?

  4. mini-review • A nucleus is made of nucleons: Protons (p) and neutrons (n) • The number of protons is equal to the atomic number (Z) • n + p = mass number (A) • nuclei that have the same number of protons but a different number of neutrons are called… isotopes

  5. we will represent the nuclide in this way C 14 (A) mass number 6 (Z) atomic number How many neutrons?

  6. 19.1 radioactive decay • Think • What are the constituents of the nucleus? • What are the charges on those particles? • Are those charges attractive or repulsive? • If so, what is holding the nucleus together? • The larger the nucleus, the greater the repulsion • Nuclei with more than 83 protons are all unstable, and will eventually break up into smaller pieces; this is known as radioactivity

  7. Radioactive Decay: The spontaneous disintegration of a nucleusinto a less massive and/or more stable nucleus, accompanied with particle or electromagnetic emission! • initial nucleus = parent nuclidenew nucleus = daughter nuclides • can be written w/ equations, as long as...

  8. Equations must have: Atomic #left = Atomic #right and Mass #left = Mass #right

  9. Alpha emission:parent  daughter + 

  10. He 4 2  =

  11. Th Ra He 230 226 4 90 88 2 • Write an equation for the alpha decay of Th-230.  + parent alpha daughter

  12. Ra He 226 4 88 2 • Write an equation for the decay of radium-226 by  emission. Rn 222  + 86 parent alpha daughter

  13. Pu He 240 4 94 2 • Write an equation for the decay of plutonium-240 by  emission. U  236 + 92 parent alpha daughter

  14. Po He 218 4 84 2 • The decay products for a nuclear reaction are an alpha particle and polonium-218. What was the parent nuclide? Rn 222  + 86 parent alpha daughter

  15. Beta emission:parent  daughter + – • A an electron particle is often, a beta AND can also be a positron • If it is an electron, the number of neutrons decreases by 1 and the number of protons increases by 1 • If it is a positron, the number of neutrons increases by 1 and the number of protons decreases by 1

  16. e e 0 0 -1 1 – = (Beta particle) + = (Positron)

  17. e C 0 14 + -1 6 parent beta daughter Beta emission N 14 7

  18. e K 0 40 -1 19 parent beta daughter • Potassium-40 undergoes beta emission. Write the equation for this reaction.  Ca 40 + 20

  19. positron emission:parent + + daughter e Na Ne  22 0 22 + 11 1 10

  20. w/ – emission a n p • w/ + emission a p n • after decay, the daughter has less E than parent

  21. Hg 201 80 • electron capturemeans… • An electron is captured by the… Nucleus e Au  201 0 + -1 79

  22. Many nuclei are radioactive. This means they are unstable, and will eventually decay by emitting a particle, transforming the nucleus into another nucleus, or into a lower energy state. • A chain of decays may take place until a stable nucleus is reached.

  23. What is Radioactivity?

  24. 19.2 nuclear transformations • Yes, we can change one element into another = nuclear transformation • How? By bombarding nuclei with neutrons or positive ions we can change the ID of a little critter • easier to throw a nat a nuclei (b/c no charge), but we can throw a positive ion fast enough with a particle accelerator • this is how the transuranium elements are formed (those >U on the Periodic Table)

  25. 19.3 Detection of Radioactivity and the Concept of Half-Life • The most familiar instrument for measuring radioactivity levels is the Geiger counter • The probe contains argon gas. The gas can be ionized by the rapidly moving particles released during radioactive decay • Think: What are the 3 particles? • Discuss with the person next to you, what could be useful applications for a Geiger counter

  26. Half-life (t1/2)is the period of time, for a substance undergoing decay, to decrease by half. • It is the time when the expected value of the number of entities that have decayed is equal to half the original number. • if there is just one radioactive atom with a half-life of 1 second, there will not be "half of an atom" left after 1 second. There will be either zero atoms left or one atom left, depending on whether or not the atom happens to decay. • time for half the parent nuclei to decay = half-life (t1/2)

  27. Example: if 100,000 at beginning then 50,000 after one half life and 25,000 after 2nd half life and 12,500 after 3rd half life; etc... • A half life can be long or short, depending on the isotope involved • There are many natural radioactive materials, some are man-made

  28. example • F-21 has a half-life of approximately 5 seconds. • If there were 20 grams to begin with, how much is there after 5 seconds? • 10g • After 10 seconds? • 5g • After 15 seconds? • 2.5g • After 20 seconds? • 1.25g

  29. example • I-131 has a half-life of 8 days. How much is left after 24 days? • 24 days is how many half-lives? • 24/8 = 3 • 1/2 x 1/2 x 1/2 = 1/8

  30. example • Cr-51 has a half-life of 28 days. How much of a 510-g sample is left after 1 year? • 365 days / 28days per ½ life = 13 half lives • The fraction remaining is 1/2^n 1/2^13 = 1/8192 1/8192 x 510 g = 0.062 g

  31. 19.4 Dating by Radioactivity • C-14 is made in upper atmosphere (from N-14) • C-14 decays (h/l = 5730 y) • C-14 gets absorbed & given off by living critters • When critter dies, C-14 trapped! but still decays! • lets us radiocarbon date something!

  32. know rate of decay? then you can know how long it’s been deadexample: • living critter = 15.3 decays/min/g • if count of dead critter is 7.65 d/m/g then half of C-14 gone (one h/l has gone by) • critter died 5700 y ago

  33. 19.4 • must be ultra pure; not accurate past 60000 y • real old stuff (fossils, rocks) use diff nuclides(e.g. U-238 or K-40)

  34. radiochemical dating is often used to determine the age of bones discovered at archaeological sites. • these bones were estimated to be from about 3000 BC. • NT manuscripts dated this way • could C-14 dating be used to determine the age of a stone disk on leather strands found near the skeleton in the glacier?

  35. 19.5 medical applications of radioactivity • we can use some radioisotopes in the body as tracers • they act chemically just like their no-radioactive buddies • watching where they go we can deduce the health of certain organs • e.g. C-14 and P-32 can trace nutrient pathways in living systems

  36. pick an element that goes to a specific organ (like I to thyroid) • send in astracer(below left) • pick upimage onscanner

  37. why such short h/l?

  38. PET scan above • radioactive tracer in plant to the left

  39. 19.6 nuclear energy • the energy that holds the nucleus together is >million times greater than E in normal chm rxns • tap that and you have an amazing E source • combine two nuclei to make a bigger one = fusion • splitting a big nucleus to make smaller = fission • both yield a “grip” of E

  40. 19.7 nuclear fission • get this to happen to a mol of U-235 and you can get ~26 million times more E than burning a mol of methane