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Chapter 19 Nuclear Chemistry. Properties of the Nucleus Chemist’s View: Seat of positive charge and mass in atoms and molecules Not very important to chemical reactivity; valence electrons are key Nuclear Characteristics Very small size: about 1 x 10 -13 cm (Whole atom = 1 x 10 -8 cm)

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Chapter 19 Nuclear Chemistry


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chapter 19 nuclear chemistry
Chapter 19 Nuclear Chemistry
  • Properties of the Nucleus
    • Chemist’s View:
      • Seat of positive charge and mass in atoms and molecules
      • Not very important to chemical reactivity; valence electrons are key
    • Nuclear Characteristics
      • Very small size: about 1 x 10-13 cm (Whole atom = 1 x 10-8 cm)
      • Very high density: 1.6 x 1014 g/cm3
      • Very high energy processes (106 time greater than typical chemical reactions)
      • Components = “Nucleons”
        • Protons = +1 charge, 1 mass unit (Atomic Number = Z = # of protons)
        • Neutrons = 0 charge, 1 mass unit
        • Mass Number = A = sum of neutrons + protons
        • Isotopes = same atomic number but different mass numbers (#’s of neutrons)
        • Nuclide = a particular isotope
slide2
II. Nuclear Stability and Radioactive Decay
  • Thermodynamic Stability = potential energy of the nucleus compared to separate parts
  • Kinetic Stability = Probability that the nucleus will undergo Radioactive Decay
    • Example:
    • Both A and Z must be conserved (must be the same on both sides of equation)
    • Zone of Stability
      • All nuclides with Z > 84 unstable
      • (A-Z):Z ratio = 1 stable if light
      • (A-Z):Z ratio > 1 stable if heavy
      • Magic Numbers:
        • Z = even, (A-Z) = even stable
        • Z = odd, (A-Z) = odd unstable
        • Proton or Neutron numbers of

2, 8, 20, 28, 50, 82, 126 very stable

Calcium-40 is“Doubly Magic”

slide3
C. Types of Radioactive Decay
  • Decay involving the change in mass number of the nucleus
    • a-particleproduction: loss of a helium nucleus; very common
    • Spontaneous Fission: splitting of a heavy nuclide into about equal parts; rare
  • Decay when mass number stays the same
    • b-particleproduction: loss of an electron
      • Fairly common for nuclides where Neutrons:Protons > 1.0
      • Nucleus doesn’t contain electrons; loss of energy that becomes electron
      • Net effect: changes a neutron to a proton (Z increases by +1)
slide4
g-ray production: loss of a high energy photon
    • Can accompany other decay types
    • Way for nucleus in an excited state to return to ground state
  • Positron production: loss of mass of an electron, but positive charge
    • Occurs for nuclides with Neutron:Proton ratio < 1.0
    • Net effect is change of a proton to a neutron (Z changes by -1)
    • Positron is the Antiparticle of an Electron; collision with an electron leads to annihilation
  • Electron capture: an inner orbital electron is captured by the nucleus
    • Always produces g-rays as well
    • The ideal reaction for an alchemist, but too slow to be useful

Examples

slide7
The Kinetics of Radioactive Decay
    • Rate of Decay = - change in number of nuclides per unit time
      • Radioactive nuclides decay at a rate proportional to the size of the sample
      • This is the same as a first order rate law
      • Integrated first order rate law and half life equation work too!
      • Example: Technicium-99 is used for medical imaging. k = 0.116/h. t1/2 =?
      • Example: t1/2 of Molybdenum-99 is 67.0 h. How much of a 1.000 mg sample is left after 335 h?
slide8
B. Carbon Dating
  • Archeological technique to determine the age of artifacts
  • Willard Libby received the Nobel Prize in Chemistry for this work
  • Based on the radioactive decay of carbon-14
  • Carbon-14 is continuously produced in the atmosphere by neutrons from space
    • These processes have reached equilibrium: no net change in [carbon-14]
    • Plants take up the carbon as CO2 while alive, but stop when they die
    • Ratio of 14C to 12C begins to get smaller as soon as the plant dies
    • t1/2 = 5730 years for the decay of 14C
  • Example: 14C decay is 3.1/min. Fresh wood is 13.6/min. t1/2 = 5730 y.
slide9
Applications of Nuclear Reactions
    • Nuclear Transformations
      • Particle accelerators: device to propel particles at high speed
        • Linear accelerator uses changing electric fields
        • Cyclotron uses oscillating voltage to accelerate; magnets cause circular path
      • Bombarding Nuclides with other nuclides or particles can lead to new Nuclides
      • Most of the “trans-Uranium” elements were synthesized this way (Z = 93-112)
        • Neutron Bombardment
        • Positive-Ion Bombardment
    • Medical Uses
      • Radiotracers = radioactive nuclides introduced to an organism to follow pathway
        • Iodine-131 is used to diagnose thyroid gland problems
        • Thallium-201 and Technetium-99 diagnose heart damage
      • PET scan = Positron Emission Tomography
slide11
Energy Production
    • Fission = splitting a heavy nuclide into 2 lighter, more stable ones (DH = -)
      • Uranium fission provides electrical power

b) 3.5 x 10-11 J/nuclide = 2.1 x 1013 J/mol of energy is given off by loss of mass

      • E = mc2 is used to calculate the amount of energy from the mass loss
      • Chain reaction: neutrons produced can cause more reactions
        • Subcritical: < 1 neutron/reaction causes another fission (rxn dies out)
        • Critical: = 1 neutron/reaction causes another fission (rxn sustained)
        • Supercritical: > 1 neutron/reaction causes another fission (explosion)
      • Nuclear Reactor: Fission heats water, runs turbine, make electricity
        • Reactor core: enriched uranium (3% U-235) sustains the reaction
        • Control rods absorb neutrons to regulate the reaction
      • Breeder Reactor: produces its own fissionable Pu-239 from U-238

Pu-239 is toxic and flames in air, so U.S. doesn’t use, France does

slide13
2) Fusion = combining 2 light nuclides to form a heavier, more stable one (DH = -)
      • Stars produce their heat through this process
      • Would be great energy source on Earth
        • Lots of small nuclei to use as fuel
        • But, only takes place at high temperatures (40,000,000 Kelvins)
        • High temperature overcomes strong nuclear repulsion (+/+)
        • E = mc2 (4.03298 amu in; 4.00260 amu out)
  • Effects of Radiation
    • Damage to organisms
      • Somatic damage = damage to the organisms itself (sickness or death)
      • Genetic damage = damage to genetic material (offspring are effected)
    • Factors controlling radiation effects
      • Energy of the radiation: higher energy = more damage (1 Rad = 0.01 J/kg)
      • Penetrating ability: g-ray > b-particle (1cm) > a-particle (stopped by skin)
      • Ionizing ability: removing electrons; a-particle >> g-ray
      • Chemical properties: Kr-85 inert, excrete quickly; Sr-90 replaces Ca, stays
slide14

REM

  • REM = Roentgen Equivalent for Man = normalizes radiation effects for different types of radiation exposure
    • Short term effects of radiation exposure
    • There are natural and man-made sources of radiation exposure
  • Models for radiation exposure damage
    • Linear model: any exposure is bad, minimize all exposures
    • Threshold model: no damage unless a certain amount of exposure occurs
    • Better safe than sorry: we don’t know which model is correct, follow linear