Nuclear Chem

Nuclear Chem - Nuclear Chemistry I. Radioactivity. A....

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1 Nuclear Chemistry I. Radioactivity. A. Review Isotopic Notation. 1. Proton, neutron and electron model. a. Very approximate and incorrect model. However, it can be used to keep tract of, but not explain, radioactive decay and nuclear stability. Mass Charge Particle Symbol kg amu (u) Coulombs Electron Units Proton 1 1 H 1.67252x10 -27 1.007276 1.6022x10 -19 +1 Neutron 1 0 n 1.67496x10 -27 1.008665 0 0 Electron 0 -1 e 9.1095x10 -31 0.000549 -1.6022x10 -19 -1 2. Protons and neutrons comprise the nucleus = nucleons. 3. Atomic number = Z = Charge on nucleus in electron charge units = number of protons in the nucleus. Mass number = A = Total number of nucleons in nucleus = mass in amu rounded to nearest whole number = number of protons + number of neutrons. 4. Isotopic notations. A Z (symbol) Examples: 14 6 C { 6 protons 8 neutrons } in nucleus + 6 electrons outside of nucleus 238 92 U { 92 protons 146 neutrons } in nucleus + 92 electrons outside of nucleus. B. Radioactive Decay. 1. Neutrons are important in stabilizing the nucleus. In general, the greater the number of protons in the nucleus, the greater the number of neutrons required to stabilize it. a. For the lighter elements, up to Z = 20 (Ca) the stable, nonradioactive, nuclei have neutron to proton ratios, n/p = 1. b. As Z increases the n/p ratio increases to a value of approximately 2.5 for 209 83 Bi. c. All isotopes with Z > 83 are radioactive. 2. Isotopes outside this "belt of stability" will undergo radioactive decay and change there n/p ratio until a stable isotope is formed.
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2 Proton/Neutron Ratio and Stability
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3 3. Types of radioactive decay. a. Alpha particle decay. 1) Alpha particle is a helium-4 nucleus. Symbol is 4 2 He or 4 2 α . 2) In alpha particle decay, Z goes down by 2 and A goes down by 4. Alpha decay is the only way a radioactive isotope can decrease its mass number. 3) Example: 238 92 U ----> 4 2 He + 234 90 Th Note in a balanced nuclear equation the sum of the Z's and the sum of the A's on both sides of the equation must be the same. 4) Very important in the decay of the heavier elements b. Beta decay. 1) Beta particle is an electron that is emitted from the nucleus. Symbol is 0 -1 e or β - . 2) In beta decay, Z goes up by one and A remains unchanged. A neutron is converted into a proton ( 1 0 n ---> 1 1 H + 0 -1 e ). 3) Example; 14 6 C -----> 0 -1 e + 14 7 N c. Gamma decay. 1) Gamma ray is a very high energy photon. Symbol 0 0 γ or γ . 2) In gamma decay, neither Z nor A changes. Gamma radiation accompanies alpha and beta decay. It is the way a nucleus can lose energy. 3) Example: 60 Co * ---> 60 Co + γ where the asterisk ( * ) denotes an excited state of the nucleus. d. Positron emission. 1) Positron is a particle that is identical to an electron except that it has a positive charge. It is the antimatter equivalent of the electron. Symbol 0 1 e or β + . 2) In positron emission, Z goes down by one and A remains unchanged. A proton is converted into a neutron ( 1 1 H ---> 1 0 n + 0 1 e).
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This note was uploaded on 04/07/2008 for the course CHEM 1304 taught by Professor Prof.maguire during the Spring '08 term at SMU.

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Nuclear Chem - Nuclear Chemistry I. Radioactivity. A....

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