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Nuclear Stability & Radioactive Decay. Notation for a nuclide (specific atom) 12 C 6 The left superscript is the mass number = number of protons + neutrons.The.

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Presentation on theme: "Nuclear Stability & Radioactive Decay. Notation for a nuclide (specific atom) 12 C 6 The left superscript is the mass number = number of protons + neutrons.The."— Presentation transcript:

1 Nuclear Stability & Radioactive Decay

2 Notation for a nuclide (specific atom) 12 C 6 The left superscript is the mass number = number of protons + neutrons.The left superscript is the mass number = number of protons + neutrons. The left subscript is the atomic number = number of protons.The left subscript is the atomic number = number of protons.

3 Isotopes Atoms with identical atomic numbers but different mass numbers. (Two nuclides can have different atomic numbers and different mass numbers.)

4 Nuclear Stability Determined by neutron/proton ratio. –All nuclides with 84 or more protons are unstable. –Light elements (up to atomic number 20): like a neutron/proton ratio of 1. –For heavier elements, the neutron/proton ratio required for stability > 1, and increases as atomic number increases. –Of  2000 known nuclides, only 279 are stable with respect to radioactive decay.

5 Zone of Stability Beta decay Positron emission or electron capture Alpha decay: heavy elements.

6 Radioactive Decay Represented by equations 14 C  14 N + 0 e 67 Original nuclide Decay Mode Decay Product

7 Decay Modes Alpha – common decay mode for heavy nuclides. Mass #  by 4, atomic #  by 2. Tends to slightly increase n/p ratio. Beta – mass # remains constant. –Net effect: neutron changed to proton. So this is a likely decay mode for nuclides whose n/p ratio is too high – decreases n/p ratio.

8 Decay Modes Positron production: net effect: change a proton to a neutron. –Important decay mode for nuclides whose n/p ratio is low – it increases the n/p ratio! Electron capture: inner-orbital electron is captured by the nucleus –Increases neutron-proton ratio

9 Decay series Some radioactive nuclides must go through several decay events to reach a stable (nonradioactive) state. 235 U  207 Pb 238 U  206 Pb

10 Kinetics of Radioactive Decay Can never predict exactly when a specific nuclide will decay. N = # of nuclides Rate = - (  N/  t) = kN i.e., the rate is directly proportional to the # of nuclides in the sample.

11 Rate = - (  N/  t) = kN ln(N/N 0 ) = -kt N = # of nuclides remaining at time t N 0 = # of nuclides at t = 0.

12 Half-Life, t ½ Half-life of a sample = time required for the number of nuclides to reach half the original value, N 0 /2. t ½ = 0.693/k

13 Nuclear Transformations Change of one nuclide into another Target nucleus is bombarded by a “bullet” “Bullet” may be a positive ion or a neutron –Particle accelerators used for + ions –Positive ions must be accelerated to high KE to overcome electrostatic repulsions Cyclotron –Neutrons quite different experimentally. Not repelled by target nuclei.

14 Transuranium Elements Elements 93 – 1** have been synthesized.

15 Uses of Radioisotopes Ratioactive Dating 14 C  0 e + 14 N 67 Continuously produced in atm by: 14 N + 1 n  14 C + 1 H 7061 So, C-14 is incorporated into living plants. As long as it is alive, C-14 to C-12 ratio is constant. When plant dies, 14 C/ 12 C ratio starts to decrease. t ½ = 5730 yrs

16 Geologic History 238 U  206 Pb 9282

17 Medical Applications Radiotracers/Diagnosis –Radioactive nuclide whose pathway in an organism can be traced by monitoring its radioactivity. –I-131 thyroid –Th-201 heart Treatment

18 Thermodynamic Stability of Nucleus Mass of a nucleus is always less than the sum of the masses of the protons and neutrons that make up the nucleus. This difference is a measure of the binding energy Binding energy = energy released when nucleus is formed.

19 Chemical Potential Well Potential Energy Separate Nucleons Stable Nucleus Green Arrow represents binding energy: Energy RELEASED when nucleus is formed.

20 Mass Defect for O-16 8p: (8 X amu) = amu 8n: (8 X amu) = amu 8e: (8 X amu) = amu Total combined mass = Atomic mass of 0-16 = amu  m = amu Use 1 amu = X kg

21 Mass Defect for O-16  m = amu Use 1 amu = X kg  m = X kg  mc 2 =  E = X kg  m 2  s -2  E = X J per O-16 atom & 1.22 X J per nucleon In kJ/mol: 7.4 X 10 8 kJ per nucleon per mol

22 Binding Energy/Nucleon vs. Mass #

23 Units of binding energy Chemists use kJ/(mol  nucleon) Physics uses a different unit: Mev

24 Nuclear Fission Splitting a heavy nucleus into two smaller nuclei. U-235 and Pu-239 are fissionable fuels Reaction initiated by a neutron Many, many possible products

25 Nuclear Fusion Two light nuclei combine to form a heavier, more stable nucleus. Occurs in the stars. Sun: 73% H, 26% He, and 1% other Protons fuse to form He

26 Sun 1 H + 1 H  2 H + 0 e + energy 1 H + 2 H  3 He + energy Then 3 He + 3 He  4 He H + energy 3 He + 1 H  4 He + 0 e + energy

27 Fusion vs. Fission as Energy Source

28 Nuclear vs. Ordinary Chemical & Physical Change Nuclear transformations involve much larger energy changes than ordinary chemical & physical changes –Orders of magnitude larger

29 Risks of radionuclides Somatic damage = damage to the organism itself, resulting in sickness or death Genetic damage = damage to genes


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