1. NUCLEAR CHEMISTRY Chapter 25

2. Nuclear Chemistry  The study of nuclear reactions and their uses in chemistry  Radiation -  The penetrating rays & particles emitted by a radioactive source

3.  Radioactivity -  Process that occurs when nuclei change spontaneously  Radioisotopes -  Atoms containing radioactive nuclei  Recall… What are isotopes?  Atoms with the same atomic number but different mass numbers

4. How do nuclear reactions differ from chemical reactions?  Chemical Reactions  Atoms become stable by transferring or sharing electrons  Rate can be changed by catalysts, temperature, pressure, etc.  Mass & charge conserved  Nuclear Reactions  Nuclei become stable by undergoing changes that release energy  Rate cannot be changed  Charge conserved, mass almost conserved A very small amount of mass is converted into energy

5. Nuclear Stability  Most nuclei are stable  The stability of a nucleus depends on its neutron to proton ratio  The stable nuclei are in the ‘belt of stability’

6. Radioactive Decay  Occurs in atoms with too many or too few neutrons  The nucleus will attempt to become more stable by releasing energy through radiation  Transmutation -  when the unstable nucleus of one element is transformed into a stable nucleus of a different element  Occurs naturally and artificially

7. Types of Radioactive Decay  Alpha particles, α  Beta particles, β  Positron emission  Gamma rays, γ

9. Alpha Decay - α  Gives off alpha particles (helium nuclei)  Occurs when the neutron: proton ratio is low  A radioactive element gives off 2 protons and 2 neutrons  The charge on the nucleus decreases by 2  Alpha particles do not travel far & are not very penetrating due to their large mass and charge  Sheet of paper or surface of skin stops them

10.  Atomic # -  # of protons -  Mass # -  # of neutrons - 240 94 Pu → 236 92 U + 4 2 He ( α ) ↓ by 2 ↓ by 4 ↓ by 2

11. Beta Decay - β  Gives off beta particles (electrons)  Occurs when the neutron: proton is high  A neutron breaks apart into a proton, which stays in the nucleus, and an electron which is released  1 0 n → 1 1 H + 0 -1 e ( β )  Charge on the nucleus increases by 1  More penetrating than α particles  Can pass through paper, but are stopped by aluminum foil or thin pieces of wood

12.  Atomic # -  # of protons -  Mass # -  # of neutrons - 228 88 Ra → 228 89 Ac + 0 -1 e (β) ↑ by 1 ↓ by 1 Same

13. Positron Emission  Gives off a positron (positive electron)  Occur when the neutron:proton is low  A proton breaks apart into a neutron, and a unit of positive charge  1 1 p → 1 0 n + 0 +1 e (positron)  Charge on the nucleus decreases by 1

14.  Atomic # -  # of protons -  Mass # -  # of neutrons - 22 11 Na → 22 10 Ne + 0 +1 e ↓ by 1 ↑ by 1 Same

15. Gamma Decay - γ  Gives off high energy photons called gamma rays  No mass or charge  Very dangerous - extremely penetrating  Pass easily through paper, wood, & the human body  Some can be stopped by several meters of concrete or several centimeters of lead

16.  Atomic # -  # of protons -  Mass # -  # of neutrons -  Nucleus changes only in its energy state 240 94 Pu → 240 94 Pu NC

17. Summary of Radiation ParticleMassChargeSymbolPenetrating Power Alpha4 amu2+ 4 2 He, α Low Beta0 amu1- 0 -1 e, βModerate Positron0 amu1+ 0 +1 eModerate Gamma Ray 0 amunoneγHigh

18. Nuclear Equations  Similar to chemical equations…  Mass and charge must balance on both sides  14 7 N + 4 2 He → 17 8 O + 1 1 H  By using the concept of conservation of charge and mass number, you can identify a missing particle in an equation.

19. Sample Exercises 1. What product is formed when radium-226 undergoes alpha decay? 226 88 Ra  4 2 He + 222 86 Rn 2. What element undergoes alpha decay to form lead-208? ?  4 2 He + 208 82 Pb 3. What product is formed when Mg-27 decays by beta emission? 27 12 Mg  0 -1 e + 27 13 Al ? = 212 84 Po

20. 4. 3 1 H  _____ + 0 -1 e 5. 9 3 Li  9 4 Be + _____ 6. 14 6 C  0 -1 e + _____ 7. 241 95 Am  4 2 He + _____ 8. 16 7 N  16 6 C + _____ 9. What forms when francium-220 decays? 220 87 Fr  4 2 He + 216 85 At 10. What forms when potassium-37 decays? 3 2 He 0 -1 e 14 7 N 237 93 Np 0 +1 e Positron decay 37 19 K  0 +1 e + 37 18 Ar α decay

21. 11. What forms when potassium-42 decays? 12. 6 3 Li + 1 0 n  4 2 He + _____ 13. 27 13 Al + 4 2 He  _____ + 0 1 e 14. 27 14 Si  0 -1 e + _____ 15. 214 83 Bi  4 2 He + _____ 16. 66 29 Cu  66 30 Zn + _____ 17. 235 92 U  90 38 Sr + _____ + 1 0 n + 4 0 -1 e β decay 42 19 K  0 -1 e + 42 20 Ca 31H31H 31 14 Si 27 15 P 210 81 Tl 0 -1 e 144 58 Ce

22. Half-Life  Time required for 1/2 of the nuclei of a radioactive sample to decay into products Half-Lives of Radioactive Nuclides

23.  After each half-life, 1/2 of the existing radioactive atoms have decayed into atoms of a new element

24. Table N  Each element has a characteristic half-life  Anywhere from a fraction of a second to billions of years  ½ life is constant

25. The Decay of a 10.0g Sample of Strontium-90 Over Time

26. Uses for Half-Life  Dating  Nuclear medicine

27. Half-Life Calculations  No formula; make a Mass vs. Time chart  Always start with time = 0  If no mass is given, start with 1 = 1 whole MassTime

28. Sample Problems 1. Most chromium atoms are stable, but Cr-51 is an unstable isotope with a half-life of 28 days. a. What fraction of a sample of Cr-51 will remain after 168 days? b. If a sample of Cr-51 has an original mass of 52.0g, what mass will remain after 168 days? Make a Mass vs. Time chart Always start with time = 0 If no mass is given, start with the number 1, for 1 whole a. 1/64b.0.825g

29. 2. How much was present originally in a sample of Cr-51 if 0.75mg remains after 168 days? 1/2 life = 28 days 168 days = 6 half-lives Previously known: Work backwards… the sample will double from one ½ life to the next 48 mg

30. 3. According to Reference Table N, how much of a 100. microgram ( μ g) sample of nitrogen-16 will remain after 28.52 seconds of decay? 1/2-life of N-16 = 7.13 seconds Look up on Table N: Make a Mass vs. Time chart 6.25 μg

31. 4. In 5.49 seconds, 1.20g of Ar-35 decay to leave only 0.15g. What is the half-life of Ar-35? 1.83 seconds 5. Na-24 has a half-life of 15 hours. How much Na- 24 will remain in an 18.0g sample after 60 hours? 1.125g 1.13g

32. 6. How many half-lives are required for a radioisotope to decay to 1/32 of its initial value? Five 7. What fraction of 226 Ra will be left after 4797 years? 1/8

33. Artificial Transmutations  Particle Accelerators -  Collision of a proton or α particle with a target nucleus  Uses magnetic or electrostatic fields to accelerate particles & overcome the repulsive forces

34. An Aerial View of Fermilab, a high energy particle accelerator

35.  Neutron Collisions -  Occurs when a neutron collides with a target nucleus  Used to prepare radioactive nuclei from stable nuclei 238 92 U + 1 0 n → 239 92 U 59 27 Co + 1 0 n → 60 27 Co 32 16 S + 1 0 n → 32 15 P + 1 1 H

36. Nuclear Fission  Splitting of the nucleus into smaller fragments  Occurs when the nucleus is bombarded with neutrons  Uranium-235 and Plutonium- 239 are the only fissionable isotopes  1 0 n + 235 92 U → 142 56 Ba + 91 36 Kr + 3 1 0 n + energy Fission Animation

37.  Releases huge amounts of energy  Very exothermic  1 kg U-235 = 20,000 tons of dynamite  Atomic bombs & nuclear reactors Nuclear Test

38. (a) Controlled Reaction

39. (a) Controlled Reaction

40. (a) Controlled Reaction

41. (a) Controlled Reaction

42. (a) Controlled Reaction

43. (a) Controlled Reaction

44. (a)(b) Controlled ReactionUncontrolled Reaction

45. (a)(b) Controlled ReactionUncontrolled Reaction

46. (a)(b) Controlled ReactionUncontrolled Reaction

47. (a)(b) Controlled ReactionUncontrolled Reaction

48. (a)(b) Controlled Reaction Uncontrolled Reaction Uncontrolled Mousetrap Animation

49. Nuclear Reactors  Uses controlled fission  Energy from fission rx heats the coolant  Heated coolant is used to produce steam  Steam turns a turbine  Turbine drives a generator to produce electricity How a nuclear reactor works

50. Nuclear Fusion  Light nuclei combine to produce a nucleus of greater mass  Solar fusion – H nuclei fuse to make He nuclei  2 1 H + 3 1 H  4 2 He + 1 0 n + energy Fusion in the Sun

51. Fusion as an Energy Source  Produces more energy than fission  Occurs only at high temperatures (over 40,000,000ºC)  More appealing than fission because:  Availability/low cost of light isotopes  Products are generally not radioactive

52. Detecting Radiation  Radiation cannot be seen, heard, felt, or smelled  Ionizing radiation -  radiation with enough energy to knock electrons off atoms of the bombarded substance to produce ions  Can be detected by Geiger counters, scintillation counters, and film badges

53. Geiger Counter  Gas filled metal tube used to detect radiation

55. Scintillation Counter  Phosphor-coated surface used to detect radiation

56. Film Badge  Several layers of photographic film  People who work near radiation sources wear them and then develop the film to determine the amount of radiation they have been exposed to

57. Using Radiation  Medicine -  Diagnostic tools  Treatment for cancer  Help to determine mechanisms for chemical reactions  Trace movements of atoms in biological systems

58.  Radiation in the body should:  Have a short half-life  Be quickly eliminated from the body A Pellet Containing Radioactive I-131

60. Other Uses of Radiation  Tracers -  Any radioisotope used to follow the path of a substance  Ex. Used in agriculture to test the effects of herbicides, pesticides, and fertilizers  Dating -  Using half-lives, ‘age’ of objects can be determined  C-14 → C-12:  U-238 → Pb-206:  Industrial Applications –  Used to measure the thickness or strength of a material based on radioactive absorption ‘living’ things rocks & geological formations

61.  Food Irradiation –  Kills insects, bacteria, & mold  Prevents the ‘sprouting’ of fruits & vegetables

62. Household Smoke Detector  Contains a small amount of 241 Am  α particles from 241 Am, knock e - off the air, ionizing O 2 and N 2  When smoke particles get in the way, the smoke attaches to the ions & neutralizes them  The drop in current is detected by an electric circuit, causing it to sound an alarm