Nuclear Chemistry. Chapter 23

Transcription

1 Nuclear Chemistry Chapter 23

2 n/p too large beta decay X Y n/p too small positron decay or electron capture

3 Nuclear Stability Certain numbers of neutrons and protons are extra stable n or p = 2, 8, 20, 50, 82 and 126 Like extra stable numbers of electrons in noble gases (e - = 2, 10, 18, 36, 54 and 86) Nuclei with even numbers of both protons and neutrons are more stable than those with odd numbers of neutron and protons All isotopes of the elements with atomic numbers higher than 83 are radioactive All isotopes of Tc and Pm are radioactive

4 Atomic number (Z) = number of protons in nucleus (element symbols) = charge (particles) Mass number (A) = number of protons + number of neutrons = atomic number (Z) + number of neutrons Mass Number Atomic Number A Z X Element Symbol proton 1 1 p or 1 1 H neutron 1 n 0 electron 0 e -1 or 0-1 positron 0 e +1 or 0 +1 particle 4 He 2 or 4 2 A Z Other relevant particles of interest: Gamma photon neutrino Anti-neutrino 0

5 Balancing Nuclear Equations 1. Conserve mass number (A). The sum of protons plus neutrons in the products must equal the sum of protons plus neutrons in the reactants. 235 U 1 0 n 92 + Cs Rb 1 n = x1 2. Conserve atomic number (Z) or nuclear charge. The sum of nuclear charges in the products must equal the sum of nuclear charges in the reactants. 235 U 1 0 n 92 + Cs Rb 1 n = x0

6 212 Po decays by alpha emission. Write the balanced nuclear equation for the decay of 212 Po. alpha particle - 4 He 2 or Po 4 He + A X 84 2 Z? 212 = 4 + A A = = 2 + Z Z = Po 4 He Pb

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8 Beta decay Nuclear Stability and Radioactive Decay 14 C 14 N K 40 Ca Decrease # of neutrons by 1 Increase # of protons by 1 1 n 1 p Positron decay 11 C 11 B K 38 Ar Increase # of neutrons by 1 Decrease # of protons by 1 1 p 1 n and have A = 0 and Z = 0

9 Nuclear Stability and Radioactive Decay Electron capture decay 37 Ar + 0 e 37 Cl Fe + 0 e 55 Mn Increase # of neutrons by 1 Decrease # of protons by 1 Alpha decay 1 p + 0 e 1 n Po 4 He Pb Decrease # of neutrons by 2 Decrease # of protons by 2 Spontaneous fission 252 Cf In n

10 Nuclear binding energy (BE) is the energy required to break up a nucleus into its component protons and neutrons. E = mc 2 BE + 19 F 9 1 p n BE = 9 x (p mass) + 10 x (n mass) 19 F mass BE (amu) = 9 x x BE = amu 1 amu = 1.49 x J BE = 2.37 x J binding energy per nucleon = binding energy number of nucleons = 2.37 x J 19 nucleons = 1.25 x J

11 Nuclear binding energy per nucleon vs Mass number nuclear binding energy nucleon nuclear stability

12 Kinetics of Radioactive Decay N daughter rate = - N t - N t rate = N = N N = the number of atoms at time t N 0 = the number of atoms at time t = 0 is the decay constant First Order Equations: ln N t t N0 ln N N e t t 0 ln t o ( ) N N t ln or or t1 t t 2

13 Kinetics of Radioactive Decay [N] = [N] 0 exp(- t) ln[n] = ln[n] 0 - t [N] ln [N]

14 Radiocarbon Dating 14 N + 1 n 14 C + 1 H C 14 N t ½ = 5730 years Uranium-238 Dating U Pb t ½ = 4.51 x 10 9 years

15 Nuclear Transmutation 14 N O + 1 p Al P + 1 n N + 1 p 11 C Cyclotron Particle Accelerator

16 Nuclear Transmutation Darmstadtium Roentgenium Copernicium Ds Rg Cn Pb Ni Ds n Bi 28 Ni 111Rg 0 n Pb Zn Cn Cn n

17 Nuclear Fission 235 U + 1 n 90 Sr Xe n + Energy Energy = [mass 235 U + mass n (mass 90 Sr + mass 143 Xe + 3 x mass n )] x c 2 Energy = 3.3 x J per 235 U = 2.0 x J per mole 235 U Combustion of 1 ton of coal = 5 x 10 7 J

18 Nuclear Fission Representative fission reaction 235 U + 1 n 90 Sr Xe n + Energy

19 Nuclear Fission Nuclear chain reaction is a self-sustaining sequence of nuclear fission reactions. The minimum mass of fissionable material required to generate a self-sustaining nuclear chain reaction is the critical mass. Non-critical Critical

20 Schematic Diagram of a Nuclear Reactor

21 Nuclear Fission 35,000 tons SO 2 Annual Waste Production 4.5 x 10 6 tons CO x 10 6 ft 3 ash 70 ft 3 vitrified waste 1,000 MW coal-fired power plant 1,000 MW nuclear power plant

22 Nuclear Fission Hazards of the radioactivities in spent fuel compared to uranium ore From Science, Society and America s Nuclear Waste, DOE/RW-0361 TG

23 Chemistry In Action: Nature s Own Fission Reactor Natural Uranium % U % U-238 Measured at Oklo % U-235

24 Nuclear Fusion Fusion Reaction 2 H + 2 H 3 H + 1 H H + 3 H 4 He + 1 n Li + 2 H 2 4 He Energy Released 6.3 x J 2.8 x J 3.6 x J Tokamak magnetic plasma confinement

25 Radioisotopes in Medicine 1 out of every 3 hospital patients will undergo a nuclear medicine procedure 24 Na, t ½ = 14.8 hr, emitter, blood-flow tracer 131 I, t ½ = 14.8 hr, emitter, thyroid gland activity 123 I, t ½ = 13.3 hr, ray emitter, brain imaging 18 F, t ½ = 1.8 hr, emitter, positron emission tomography 99m Tc, t ½ = 6 hr, ray emitter, imaging agent Brain images with 123 I-labeled compound

26 Radioisotopes in Medicine Research production of 99 Mo 98 Mo + 1 n 99 Mo Commercial production of 99 Mo 235 U + 1 n 99 Mo + other fission products Bone Scan with 99m Tc 99 Mo 99m Tc t ½ = 66 hours 99m Tc 99 Tc + -ray t ½ = 6 hours

27 Geiger-Müller Counter

28 Biological Effects of Radiation Radiation absorbed dose (rad) 1 rad = 1 x 10-5 J/g of material Roentgen equivalent for man (rem) 1 rem = 1 rad x Q Quality Factor -ray = 1 = 1 = 20

29 Chemistry In Action: Food Irradiation Dosage Up to 100 kilorad kilorads 1000 to 10,000 kilorads Effect Inhibits sprouting of potatoes, onions, garlics. Inactivates trichinae in pork. Kills or prevents insects from reproducing in grains, fruits, and vegetables. Delays spoilage of meat poultry and fish. Reduces salmonella. Extends shelf life of some fruit. Sterilizes meat, poultry and fish. Kills insects and microorganisms in spices and seasoning.