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:
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 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.
Isotopes Atoms with identical atomic numbers but different mass numbers. (Two nuclides can have different atomic numbers and different mass numbers.)
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.
Zone of Stability Beta decay Positron emission or electron capture Alpha decay: heavy elements.
Radioactive Decay Represented by equations 14 C 14 N + 0 e 67 Original nuclide Decay Mode Decay Product
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.
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
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
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.
Rate = - ( N/ t) = kN ln(N/N 0 ) = -kt N = # of nuclides remaining at time t N 0 = # of nuclides at t = 0.
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
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.
Transuranium Elements Elements 93 – 1** have been synthesized.
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
Geologic History 238 U 206 Pb 9282
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
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.
Chemical Potential Well Potential Energy Separate Nucleons Stable Nucleus Green Arrow represents binding energy: Energy RELEASED when nucleus is formed.
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
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
Binding Energy/Nucleon vs. Mass #
Units of binding energy Chemists use kJ/(mol nucleon) Physics uses a different unit: Mev
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
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
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
Fusion vs. Fission as Energy Source
Nuclear vs. Ordinary Chemical & Physical Change Nuclear transformations involve much larger energy changes than ordinary chemical & physical changes –Orders of magnitude larger
Risks of radionuclides Somatic damage = damage to the organism itself, resulting in sickness or death Genetic damage = damage to genes