Radioactivity and Nuclear Energy: Exploring the Nucleus

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 • Nuclear weapons, power plants, radioactive dating all included here

3. The nucleus is unbelievably small and dense • A ping pong ball size of nuclear stuff would weigh 2.5 billion tons • and the energies involved here are orders of ten greater than “normal” chemistry involving the electrons (think: nuclear weapon)

4. mini-review: • nucleus is made of nucleons: protons and neutrons • # protons = atomic number (Z) • n + p = mass number (A) • nuclei with same protons but different neutrons = isotopes

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

7. 19.1 radioactive decay • many nuclei spontaneously decay •  nuclear reactions are exothermic (lose E) • nuclei want to be stable (happy); they decay (fall apart) until they are • but that makes them radioactive • they decay in nuclear rxns…

8. Radioactive Decay: The spontaneous disintegration of a nucleus into a slightly lighter & more stable nucleus, accompanied with particle or electromagnetic emission! (Wow) • initial nucleus = parent nuclidenew nucleus = daughter nuclides • can be written w/ equations, as long as...

9. Atomic #left = Atomic #right and Mass #left = Mass #right

10. Alpha emission:parent  daughter + 

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 + –

16.  e C 0 14 + -1 6 parent beta daughter N 14 7

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

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

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

20. Hg 201 80 • electron capture means… well.. an electron was captured • and a gamma ray is always tossed out  e Au  0 201 0 + + -1 79 0

23. these critters are often not happy even after they’ve decayed and continue to do so in a decay series…

25. What is Radioactivity?

26. 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)

29. 19.3 Detection of Radioactivity and the Concept of Half-Life • Most popular is geiger counter • Ar gets ionized by particles and sets off a “click” • These instruments can be used to determine…

30. Half-life helps us to figure out how long a radioactive sample has been ticking off • Decay is random, can’t predict which one will blow, but… • can predict how long for a huge sample to decay (e.g. 1/2 of it) • time for half the parent nuclei to decay = half-life (t1/2)

32. 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 billionths of a second to billions of years; depends entirely on element • There are many natural radioactive material, some are man-made

34. example • F-21 has a half-life of approximately 5 seconds. How much is left after 1 minute? • 1 minute = 12 half-lives • 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 x 1/2 = 1/4096 = 0.000 24 • If there were 20 grams to begin with, how much is there now? • 20 g x 0.000 24 = 0.0049 g

35. example • I-131 has a half-life of 8 days. How much is left after 32 days? • 32 days is 4 half-lives • 1/2 x 1/2 x 1/2 x 1/2 = 1/16

36. example • Cr-51 has a half-life of 28 days. How much of a 510-g sample is left after 1 year? • 365 days is 13 half-lives = 1/8192 • 1/8192 x 510 g = 0.062 g

38. 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!

39. 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

40. 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)

41. 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?

43. 19.4 EOC questions • 27

44. 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

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

46. why such short h/l?

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

50. 19.5 EOC questions • 31