1. 1 Chemistry 100 Chapter 21 Nuclear Chemistry

2. 2 Nuclear Equations Nucleons: particles in the nucleus: – p + : proton – n 0 : neutron. Mass number: the sume of the number of p + and n 0. Atomic number: the number of p +. Nuclear equations, the total number of nucleons is conserved:

3. 3 Sample Nuclear Equations 4 2 He -  particle 0 -1  -   particle

4. 4 Three Types Of Decay Processes  -radiation – the loss of 4 2 He from the nucleus,  -radiation – the loss of an electron from the nucleus,  -radiation – the loss of high-energy photon from the nucleus.

5. 5 Radioactivity

6. 6 Types of Radioactive Decay Ensure conservation of nucleons – Write all particles with their atomic and mass numbers. Nucleons can undergo decay  -particle emission Electron capture

7. 7 Types of Radioactive Decay

8. 8 Neutron-to-Proton Ratio The proton has high mass and high charge – proton-proton repulsion is large. The cohesive forces in the nucleus are called strong nuclear forces. Neutrons are involved with the strong nuclear force. As more protons are added (the nucleus gets heavier) the proton-proton repulsion gets larger.

9. 9 The ‘Belt of Stability’

10. 10 Radioactive Series A nucleus usually undergoes more than one transition on its path to stability. The series of nuclear reactions that accompany this path is the radioactive series. Nuclei resulting from radioactive decay are called daughter nuclei.

11. 11 An Example Radioactive Decay Series

12. 12 Nuclear Transmutations Nuclear transmutations are the collisions between nuclei. 14 N + 4   17 O + 1 H. The above reaction is written in short-hand notation: 14 N( ,p) 17 O. To overcome electrostatic forces, charged particles need to be accelerated before they react.

13. 13 Nuclear Transmutations

14. 14 Radioactive Half-Lives 90 Sr has a half-life of 28.8 yr. 90 38 Sr  90 39 Y + 0 -1 e Each isotope has a characteristic half-life. Half-lives are not affected by temperature, pressure or chemical composition. Natural radioisotopes tend to have longer half-lives than synthetic radioisotopes.

15. 15 Rates of Radioactive Decay

16. 16 Rates of Radioactive Decay

17. 17 Carbon Dating Carbon-14 is used to determine the ages of organic compounds – We assume the ratio of 12 C to 14 C has been constant over time. For us to detect 14 C the object must be less than 50,000 years old. The half-life of 14 C is 5,730 years.

18. 18 Rates of Radioactive Decay Radioactive decay is a first order process: Rate = kN N – the number of radionuclides k – the first order rate constant

19. 19 Detection of Radioactivity

20. 20 Radiotracers Radiotracers are used to follow an element through a chemical reaction. Photosynthesis has been studied using 14 C: – The carbon dioxide is said to be 14 C labeled.

21. 21 Einstein showed that mass and energy are proportional: E = mc 2 The mass of a nucleus is less than the mass of their nucleons. – the mass defect! Binding energy is the energy required to separate a nucleus into its nucleons. Since E = mc 2 the binding energy is related to the mass defect. Energy Changes in Nuclear Reactions

22. 22 Nuclear Binding Energies

23. 23 Nuclear Fission Splitting of heavy nuclei is exothermic for large mass numbers. Consider a neutron bombarding a 235 U nucleus:

24. 24 A Nuclear Fission Process

25. 25 Chain Reactions The number of fissions and the energy increase rapidly - eventually, a chain reaction forms. The minimum mass of fissionable material is required for a chain reaction – critical mass.

26. 26 The Fission Process

27. 27 The Fission Process For subcritical masses, the neutrons escape and no chain reaction occurs. At critical mass, the chain reaction accelerates. Anything over critical mass is called supercritical mass. Critical mass for 235 U is about 1 kg.

28. 28 Atomic Bombs

29. 29 Nuclear Reactors Use a subcritical mass of 235 U (enrich 238 U with about 3% 235 U) Enriched 235 UO 2 pellets are encased in Zr or stainless steel rods. Control rods are composed of Cd or B, which absorb neutrons. Moderators are inserted to slow down the neutrons. Natural abundance uranium used as a fuel souce. Enriched 235 UO 2 pellets are encased in Zr rods. Heavy water is used as the moderator and the coolant. Heat produced in the reactor core is removed by a cooling fluid to a large tank of water (producing steam). Steam drives an electric generator.

30. 30 A Schematic Nuclear Reactor

31. 31 Nuclear Fusion Light nuclei can fuse to form heavier nuclei. Most reactions in the Sun are fusion. Fusion products are not usually radioactive, so fusion is a good energy source. Also, the hydrogen required for reaction can easily be supplied by seawater. However, high energies are required to overcome repulsion between nuclei before reaction can occur. High energies are achieved by high temperatures: the reactions are thermonuclear.

32. 32 Fusion of tritium and deuterium requires about 40,000,000K: 2 1 H + 3 1 H  4 2 He + 1 0 n These temperatures can be achieved in a nuclear bomb or a tokamak. A tokamak is a magnetic bottle: strong magnetic fields contained a high temperature plasma so the plasma does not come into contact with the walls. (No known material can survive the temperatures for fusion.) To date, about 3,000,000 K has been achieved in a tokamak. Nuclear Fusion

33. 33 Biological Effects of Radiation The penetrating power of radiation is a function of mass. –  -radiation (zero mass) penetrates deeply –  -radiation penetrates much further than  - radiation Radiation absorbed by tissue causes excitation (nonionizing radiation) or ionization (ionizing radiation). Ionizing radiation is much more harmful than nonionizing radiation.

34. 34 Biological Effects of Radiation

35. 35 Biological Effects of Radiation Most ionizing radiation interacts with water in tissues to form H 2 O +. The H 2 O + ions react with water to produce H 3 O + and OH. OH has one unpaired electron. It is called the hydroxy radical. Free radicals generally undergo chain reactions.

36. 36 The SI unit for radiation is the becquerel (Bq). 1 Bq is one disintegration per second. The curie (Ci) is 3.7  10 10 disintegrations per second. (Rate of decay of 1 g of Ra.) Absorbed radiation is measured in the gray (1 Gy is the absorption of 1 J of energy per kg of tissue) or the radiation absorbed dose (1 rad is the absorption of 10 -2 J of radiation per kg of tissue). Radiation Doses

37. 37 The Relative Biological Effectiveness Not all forms of radiation have the same effect, Account for the differences using RBE (relative biological effectiveness –  for  - and  -radiation and 10 for  radiation). rem (roentgen equivalent for man) = rads.RBE SI unit for effective dosage is the Sievert (1Sv = RBE.1Gy = 100 rem).

38. 38 Radiation Doses

39. 39 Radon The nucleus 222 86 Rn is a product of 238 92 U. Radon exposure accounts for more than half the 360 mrem annual exposure to ionizing radiation. Rn is a noble gas so is extremely stable. The half-life of is 3.82 days. It decays as follows: 222 86 Rn  218 84 Po + 4 2 He

40. 40 Biological Effects of Radon The  -particles produced have a high RBE. Therefore, inhaled Rn is thought to cause lung cancer. The picture is complicated by realizing that 218 Po has a short half-life (3.11 min) also: 218 84 Po  214 82 Pb + 4 2 He The 218 Po gets trapped in the lungs where it continually produces  -particles. The EPA recommends 222 Rn levels in homes to be kept below 4 pCi per liter of air.