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Nuclear Chemistry. The study of changes to the nucleus of the atom. The Nucleus. Comprised of protons and neutrons ( nucleons ). # protons = atomic number. # protons + neutrons = mass number. Isotope Review. Isotope : Atoms of the same element with different numbers of neutrons . - PowerPoint PPT Presentation


  • Nuclear ChemistryThe study of changes to the nucleus of the atom.

  • The NucleusComprised of protons and neutrons (nucleons).# protons = atomic number.# protons + neutrons = mass number

  • Isotope ReviewIsotope: Atoms of the same element with different numbers of neutrons.

    Have different levels of abundance in nature.

    Some isotopes or nuclides of an element can be unstable, or radioactive.

    Note: We will be talking about isotopes very specifically in this unit. We will not be using the average atomic mass you see on the Ref tables.

  • What is Radioactivity?Radioactivity: the decay of the nucleus by emitting particles and/or energy in order to become more stable.

  • What Causes an Isotope to be Radioactive and Decay?Proton : Neutron ratio in nucleus

  • Neutron-Proton RatiosPositive protons in the nucleus repel each other.

    Neutrons play a key role stabilizing the nucleus.

  • Neutron-Proton RatiosFor smaller nuclei (atomic # below 20) stable nuclei have a neutron-to-proton ratio close to 1:1.

  • Neutron-Proton RatiosAs nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

  • Stable Nuclei

    There are no stable nuclei with an atomic number greater than 83.

  • Early Pioneers in RadioactivityRoentgen:Discoverer of X-rays 1895Becquerel:Discoverer of Radioactivity 1896The Curies:Discoverers of Radium and Polonium 1900-1908Rutherford:Discoverer Alpha and Beta rays 1897Marie Curie 3 parts 7 minutes each

  • Ernest Rutherford discovered three types of radioactive emissions by using a magnetic field.

  • Reference Table OShows the symbols of some of the different particles used in nuclear chemistry.

    Top # = massBottom # = charge

  • Types ofRadioactive Decay

    Cloud Chamber:

  • Alpha DecayAn -particle is emitted (basically a helium nucleus)

  • Heaviest type of emissionMass of 4Charge of +2

  • Beta Decay A - particle is emitted (a high energy electron)

  • Wait a ticHow does a nucleus give off an electron!

    Neutron splits into a proton and electron.

    n p+ + e-

    Proton stays behind and electron shoots out of nucleus.

  • Positron EmissionA positron is emitted (a particle that has the same mass as but opposite charge than an electron)

  • Positrons are a type of antimatter.

    Quickly destroyed as soon as they come in contact with an electron.

  • Gamma EmissionHigh-energy radiation that almost always accompanies the loss of a nuclear particle.It is NOT a particle, it is pure energy.No mass or charge.Not affected by a magnetic field.

  • Dont make me madMs. Nelson on a bad day

  • Electron CaptureAn electron close tonucleus get captured.It combines with aproton to make a neutron.

  • Penetrating PowerPenetrating Power: how far radiation can travel through material.

    Protection requires different degrees of shielding.

    Alpha paper or skinBeta aluminum foilGamma thick lead

  • Ionizing Ability: how well radiation strips electrons from other atoms and molecules creating ions.Can cause mutations, and cell destruction

    Alpha - HighestBeta - MiddleGamma Low

    Ionizing Ability

  • Damage to CellsBecause of high ionizing ability, Alpha and Beta cause most damage inside the human body.

    Gamma rays are less ionizing but protection against gammas requires thicker shielding.

  • Measuring RadioactivityOne can use a device like a Geiger counter to measure the amount of activity present in a radioactive sample.

  • Natural Transmutation (Decay)Spontaneous transmutation of a radioisotope into another element.Doesnt require the input of outside energy.Occurs at a specific rate that we can measure. (Half Life)

  • Artificial TransmutationThe change of one element to another artificially by bombarding it with other particles.

    These equations always have 2 reactants on the left (as opposed to natural decay)

    Artificial TransmutationNatural Transmutation

  • How do We Bombard Nuclei?Particle Accelerators:Speed up charged particles in a magnetic field to collidewith nuclei

    Neutrons and gamma radiation cant be accelerated as theyhave no charge!

  • Typical Particle AcceleratorEnormous, with circular tracks with radii that are miles long.

  • Brookhaven Accelerator

  • Balancing Nuclear Equations

    Mass and charge are conserved

    Balance so that the mass (top #s) and charge (bottom #s) equal each other.

  • Typical Test Questions

  • Radioactive Series

    Decay Series: very large radioactive nuclei undergo a series of decays until they form a stable nuclide (often a nuclide of lead).

  • Transuranium Elements: Elements beyond uranium (largest natural element)Atomic numbers greater than 92Artificially created through nuclear bombardment

    Video: Islands of Stability (13 minutes)

  • Half Life

    Amount of time for half a radioactive sample to decay.

    Length of a half life cannot be changed.

    Ranges from milliseconds to billions of years. (See Table N)

    Radioactivity decreases with time.

  • Radioactive DatingRate of decay is constant over time.

    Measure amount of radioisotope remaining in sample to determine age.

    C-14 is used to date organic material up to 60,000 years old.

    U-238 is used to date extremely old geological formations

    Carbon 14 Dating:n (2 minutes)

  • Reference Table NDecay mode: type of particle emitted by natural decay

    Half Life: length of time for half of the atoms in a sample to undergo natural decay.

  • Half Life Formula:

    # Half Lives = Total Time Elapsed Time of One Half Life (t1/2 )

  • Half Life ProblemEx: If 500 grams of I-131, t1/2 = 8 days, decays for 32 days, how much would remain?

    32 days = 4 half lives8 days

    500 g 250 g 125 g 62.5 g 31.25 grams

  • Half Life ProblemEx: If 300g of a radioisotope decays to 37.5g in 120 days, what is the t1/2 ?

    300 150 75 37.5g3 half lives

    3 half lives = 120 days t1/2 = 40 days t1/2

  • Half Life ProblemEx: What fraction of a sample of I-131 remains after 24 days of decay? t1/2 = 8 days

    24 days = 3 half lives 8 days

    Start End1 1/8

  • Half Life ProblemEx: If 60 g of N-16 remains in a sample. How many grams were present 28 seconds ago? t1/2 = 7 sec.

    28 sec = 4 half lives AGO7 sec

    We double going back in time 60 120 240 480 960 grams

  • Honors Half Life EquationsRadioisotopes each have a unique half-life.Each will decay at a specific rate over time.Use the rate constant k to denote a specific rate constant for an isotope in half-life problems.k = .693t1/2

  • log N0 = k x tN 2.3

    N0 = original quantityN = final quanityt = total timek = decay constant (.693)t1/2

  • Use the graph to see how much time it takes for half the nuclei to decay

    Half Life Graph

  • Energy in Nuclear ReactionsNuclear reactions yield more energy than chemical reactions

    When changes happen to the nucleus, some matter is converted to energy.

    Einsteins famous equation, E = mc2, allows us to calculate this energy.

  • Energy in Nuclear ReactionsE = energy in Joulesm = mass (lost) in kilogramsc = the speed of light (3 x 108 meters/sec)

  • Energy in Nuclear ReactionsEx: The mass change for the decay of 1 mole of uranium-238 is 0.0046 g.

    The change in energy, E, is thenE = (m) c2E = (4.6 106 kg)(3.00 108 m/s)2E = 4.1 1011 Joules

  • Mass Defect (Honors)The difference between the mass of an atom and the sum of the masses of the individual protons and neutrons in its nucleus.

    The "vanishing" mass of the protons and neutrons is converted to energy.

  • Nuclear Fission = Splitting the Nucleus

  • Nuclear FissionLarge nuclei are split (basically in half), producing various fission products and large amounts of energy.

    Total mass after splitting is less than what you started with.

    Matter is converted to energy.

  • Recognize this Reaction

  • Nuclear Chain Reaction:Bombard nuclide with a neutron.Nuclei split releasing more neutrons that strike other nuclei, and so on and so on....

  • Critical Mass: minimum amount of fissionablematerial present for the chain reaction to be sustained.

  • Controlled vs. Uncontrolled FissionControlled Chain Reaction: occurs in nuclear reactors or power plants.Some of the free neutrons are removed

    Uncontrolled Chain Reaction: occurs in nuclear bombs or atomic bombs.

    Video Clip: Uncontrolled Fission

  • Nuclear Power Plants

  • Nuclear ReactorGenerates heat through controlled nuclear fission to produce steam that turns a turbine connected to an electric generator.

  • Major Parts of a Nuclea