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Chapter 19

Chapter 19. Radioactivity and Nuclear energy. 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.

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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

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