Chapter 19 Nuclear Chemistry

1. 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

2. 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”

3. 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)

4. 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

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7. 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?

8. 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.

9. 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

10. Targeted Imaging: PET

11. 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

13. 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

14. 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