13Nuclear - Nuclear Chemistry The nuclei of some unstable...

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Nuclear Chemistry The nuclei of some unstable isotopes undergo change by releasing energy and particles collectively known as radiation Spontaneous nuclear reactions: Radioactive Decay 5 kinds 1) Emission of $ - particles : 4 2 He e.g. 238 92 U " 234 90 Th + 4 2 He In air, $ - particles travel several cm. In Al, $ - particles travel 10 -3 mm . 2) Emission of ! -particles : 0 –1 e = electrons . e.g. 131 53 I " 131 54 Xe + 0 –1 e ! - particle emission converts a neutron to a proton: 1 0 n " 1 1 p + 0 –1 e In air, ! -particles travel 10m. In Al, ! -particles travel 0.5mm. Emission of ! -particles 3) Emission of # -rays : 0 0 # # - ray emission changes neither atomic number nor mass. In Al, # -particles travel 5-10 cm. Emission of # -rays
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4) Emission of positrons ( ! + -particles): 0 +1 e e.g. 11 6 C " 11 5 B + 0 1 e Positron emission converts a proton to a neutron: 1 1 p " 1 0 n + 0 1 e Positrons have a short lifetime because they recombine with electrons and annihilate: 0 1 e + 0 –1 e " 2 0 0 # Emission of positrons 5) Electron Capture: an electron from the orbitals surrounding the nucleus can be captured: e.g. 81 37 Rb + 0 –1 e " 81 36 Kr Electron capture converts a proton to a neutron: 1 1 p + 0 –1 e " 1 0 n Electron Capture
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Fill in the blanks 239 94 Pu " 4 2 He + ? 234 91 Pr " 234 92 U + ? 1. 1 1 p 2. 0 –1 e 3. 1 0 n 4. 4 2 He 192 77 Ir + ? " 192 76 Os 18 9 F " 18 8 O + ? Because the mechanism is unimolecular, nuclear decay is always a first order process. Decay Rate = -dN/dt = kN where: k is a constant, N is the number of decaying nuclei. Integrated rate law: ln[N(t)/N 0 ] = -kt N(t) = N 0 e -kt where N 0 is the number of radioactive nuclei at t=0. NUCLEAR DECAY KINETICS
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Half-Life : the time required for half of a radioactive sample to decay. N(t 1/2 ) = N 0 /2 ln(N/N 0 ) = -kt k = 0.693/t 1/2 ; t 1/2 = 0.693/k Examples: Isotope t 1/2 Decay 238 92 U 4.5x10 9 yr $ 235 92 U 7.1x10 8 yr $ 14 6 C 5.7x10 3 yr ! Half-Life
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13Nuclear - Nuclear Chemistry The nuclei of some unstable...

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