Chapter 19 Nuclear Chemistry.ppt

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

2、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 di

3、fferent mass numbers (#s of neutrons) Nuclide = a particular isotope,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 m

4、ust 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 heavyMagic Numbers: Z = even, (A-Z) = even stable Z = odd, (A-Z) = odd unstable Proton or Neutron numbers of 2, 8, 20, 28, 50, 82

5、, 126 very stable,Calcium-40 is“Doubly Magic”,C. Types of Radioactive Decay Decay involving the change in mass number of the nucleusa-particle production: loss of a helium nucleus; very commonSpontaneous Fission: splitting of a heavy nuclide into about equal parts; rareDecay when mass number stays t

6、he sameb-particle production: loss of an electron Fairly common for nuclides where Neutrons:Protons 1.0 Nucleus doesnt contain electrons; loss of energy that becomes electron Net effect: changes a neutron to a proton (Z increases by +1),g-ray production: loss of a high energy photon Can accompany ot

7、her decay types Way for nucleus in an excited state to return to ground statePositron 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 Elect

8、ron; collision with an electron leads to annihilationElectron 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,214,The Kinetics of Radioactive Decay Rate of Decay = - change in number

9、of nuclides per unit time Radioactive nuclides decay at a rate proportional to the size of the sampleThis 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 Mol

10、ybdenum-99 is 67.0 h. How much of a 1.000 mg sample is left after 335 h?,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-14Carbon-14 is continuously produced in the a

11、tmosphere by neutrons from spaceThese 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

12、.1/min. Fresh wood is 13.6/min. t1/2 = 5730 y.,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 Bomb

13、arding 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 BombardmentPositive-Ion BombardmentMedical Uses Radiotracers = radioactive nuclides introduced to an organism to follow pathway Iodine-131 is

14、used to diagnose thyroid gland problems Thallium-201 and Technetium-99 diagnose heart damage PET scan = Positron Emission Tomography,Targeted Imaging: PET,Energy Production Fission = splitting a heavy nuclide into 2 lighter, more stable ones (DH = -) Uranium fission provides electrical powerb) 3.5 x

15、 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 lossChain reaction: neutrons produced can cause more reactions Subcritical: 1 neutron/reaction causes another fission (explosion)Nuclear Reactor: Fission heats w

16、ater, runs turbine, make electricity Reactor core: enriched uranium (3% U-235) sustains the reaction Control rods absorb neutrons to regulate the reactionBreeder Reactor: produces its own fissionable Pu-239 from U-238 Pu-239 is toxic and flames in air, so U.S. doesnt use, France does,2) Fusion = com

17、bining 2 light nuclides to form a heavier, more stable one (DH = -) Stars produce their heat through this processWould 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 repulsi

18、on (+/+) 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 e

19、nergy = 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,REM = Roentgen Equivalent for Man = normalizes radiation

20、 effects for different types of radiation exposure Short term effects of radiation exposureThere 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 dont know which model is correct, follow linear,REM,