Nuclear Chemistry

1. Nuclear Chemistry Structure and Stability of Nuclei, Fission, Fusion, and Radiation

2. Standards 11. Nuclear processes are those in which an atomic nucleus changes, including radioactive decay of naturally occurring and human-made isotopes, nuclear fission, and nuclear fusion. As a basis for understanding this concept: a. Students know protons and neutrons in the nucleus are held together by nuclear forces that overcome the electromagnetic repulsion between the protons. 11. b. Students know the energy release per gram of material is much larger in nuclear fusion or fission reactions than in chemical reactions. The change in mass (calculated by E = mc2 ) is small but significant in nuclear reactions. 11. c. Students know some naturally occurring isotopes of elements are radioactive, as are isotopes formed in nuclear reactions. 11. d. Students know the three most common forms of radioactive decay (alpha, beta, and gamma) and know how the nucleus changes in each type of decay. 11. e. Students know alpha, beta, and gamma radiation produce different amounts and kinds of damage in matter and have different penetrations. 11. f.* Students know how to calculate the amount of a radioactive substance remaining after an integral number of half-lives have passed. 11. g.* Students know protons and neutrons have substructures and consist of particles called quarks.

5. Nuclear Chemistry has to do with an atom’s nucleus • plural of • nucleus = nuclei What’s a nucleus? Let’s review An atom’s nucleus contains almost all of an atom’s mass, but takes up very little of it’s volume.

6. Subatomic Particlesthe particles that make up an atom • Protons – high mass, positive charge. Found in nucleus. • Neutrons – high mass, no charge. Found in nucleus. • Electrons – low mass, negative charge. Found orbiting around nucleus. (abbreviated e– )

7. Comparison of Masses Electron Proton Neutron

8. An Atom Size of atom Size of nucleus 2 protons = He = helium

9. Basic Electrical Charge Laws + and– : Attract (pull together) –and– : Repel (push away) + and + : Repel (push away) Like charges repel and Oppositesattract

10. So why don’t the protons in the nucleus fly apart? nuclear force is stronger over short distances electrostatic force is stronger over long distances

11. Key • Atomic Number • Number of Protons • Number of Electrons(when atom is neutrally charged) • Property unique to each element 11 Na Sodium 22.99

12. Key 11 Na Sodium 22.99 Average atomic mass* • Weighted Average number of Protons and Neutrons (approximately)

13. Isotopes When atoms have the same number of protons, but different numbers of neutrons, they are called isotopes. Examples: Carbon-14 has 6 protons and 8 neutrons. Carbon-12 has 6 protons and 6 neutrons. Carbon-14 is found more often in living organisms than in non-living matter. It also undergoes radioactive decay – which is why it is used for fossil dating.

14. Isotopes More Examples: Uranium-238 has 92 protons and 146 neutrons. Uranium-235 has 92 protons and 143 neutrons. Uranium-235 is more rare in nature than Uranium-238, but it also undergoes nuclear chain reactions more easily, which is why small amounts of Uranium-235 are used in nuclear weapons and nuclear power plants. Some isotopes are more stable than others.

15. Which isotope is more abundant? Neon-20 Neon-22 How many neutrons in each? Neon-20 Neon-22 20 – 10 = 10 neutrons 22 – 10 = 12 neutrons

16. Which isotope is more abundant? Bromine-79 Bromine-80 How many neutrons in each? Bromine-79 Bromine-80

17. Lead has two isotopes with the following relative amounts: 80% Lead-207 20% Lead-208 What would the average atomic mass calculate to be? 0.8(207) + 0.2(208) = 207.2

18. Chlorine has two stable isotopes: Chlorine-35 Chlorine-37 ??% x 100 –??% 1 – x What are the relative abundances (in %) of each isotope? -2x = -1.55 x(35) + (1-x)(37) = 35.45 -2 -2 35x – 37x + 37 = 35.45 x = 0.775 -2x + 37 = 35.45 –37 –37 -2x = -1.55

19. Chlorine has two stable isotopes: Chlorine-35 Chlorine-37 77.5% 0.775 22.5% 0.225 What are the relative abundances (in %) of each isotope? -2x = -1.55 x(35) + (1-x)(37) = 35.45 -2 -2 35x – 37x + 37 = 35.45 x = 0.775 -2x + 37 = 35.45 –37 –37 -2x = -1.55

21. So why don’t the protons in the nucleus fly apart? nuclear force is stronger over short distances electrostatic force is stronger over long distances

22. Sometimes these forces are overcome. Fission – a nucleus breaks apart. (ex. atomic bomb, nuclear power plants, radioactive decay) Fusion – a nucleus merges with another. (ex. the sun, hydrogen bomb, experimental fusion reactors)

24. Both fission and fusion release radiation. It is called radiation because it radiates out. Symbol Common Types of Radiation γ α (alpha): helium nucleus at high speed. β (beta): electron at high speed. γ (gamma): high energy photon. 4 0 0 He e -1 2 0 harder to block

25. Symbol Other Types of Radiation neutron at high speed. Proton (or hydrogen nucleus) at high speed. β+ positron at high speed (the anti-matter version of an electron). β+ 1 1 0 n p 0 1 1

26. atomic mass 23 22 Na Na 11 11 number of protons

27. We can use conservation ofmass and charge to figure out nuclear reactions just like chemical reactions. + ??? + ??? 4 238 234 0 e He U Th -1 2 90 92

28. We can use conservation ofmass and charge to figure out nuclear reactions just like chemical reactions. + + ??? 238 234 4 234 0 e He Th U Th -1 2 92 90 90

29. We can use conservation ofmass and charge to figure out nuclear reactions just like chemical reactions. + + 0 234 4 234 238 234 He e Th Pa U Th -1 2 92 90 90 91

31. Half-Life • Half-life – how much time has passed when half of the original amount remains. • How much of the original remains after two half-lives? • How much after three half-lives? • How much after four half-lives? 1 16 1 2 1 4 1 8 or 50 % or 25 % or 12.5 % or 6.25 %

32. Table of Half-Lives

33. Table of Half-Lives

34. As a archaeologist you find a dead rat that contains 0.0009 grams of Carbon-14. A rat that died a year ago has 0.01 grams of Carbon-14. How long ago did the rat die? t½ = 5,730 years 0.0009 g 0.01 g 1 11.1 1 2 1 16 1 32 1 64 1 4 1 8 = 2 H.L. 3 H.L. 5 H.L. 6 H.L. 1 H.L. 4 H.L.

35. As a archaeologist you find a dead rat that contains 0.0009 grams of Carbon-14. A rat that died a year ago has 0.01 grams of Carbon-14. How long ago did the rat die? t½ = 5,730 years 3(5,730) = 17,190 years 1 11.1 1 16 1 8 22,920 years 4(5,730) = 17,190 – 22,920 years ago 3 H.L. 4 H.L.

36. Strontium-90 is one of the “fallout” products from atmospheric testing of nuclear weapons. If there were 50,000 grams of Strontium-90 in the U.S. southwest region when the 1963 testing ban began, how much Strontium-90 currently remains? t½ = 28 years 1 2 1 16 1 32 1 64 1 4 1 8 50 yr 28 yr 2013 – 1963 = 50 years 1.8 half-lives = 2 H.L. 3 H.L. 5 H.L. 6 H.L. 1 H.L. 4 H.L.

37. Radioactive Decay Formulas A0 = original amount A = current amount t = current time t½ = half-life 1 2 3 -( )t ln (2) t½ A = A0 e A0 A ln ( ) ln (2) ln (2) ln ( ) t = t½ t½ = t A0 A

38. Energy-Mass Equivalence Sometimes in nuclear change a small amount of the mass disappears. It has been converted to a large amount of energy according to the formula: E = mc2 energy = (mass) × (speed of light)2 J = kg × (m/s)2 speed of light: c = 3 × 108 m/s

40. Fission Nuclear Reaction

41. 4 e– in valence shell

45. Which isotope is more abundant? Lead-207 Lead-208 How many neutrons in each? Lead-207 Lead-208