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  • Chapter 19 - Nuclear Chemistry Nuclear Stability and Modes of Decay

  • History and

    Discovery of Radioactivity

  • The Discovery of Radioactivity (1896)

    Antoine-Henri Bequerel designed experiment to determine whether phosphorescent minerals also gave

    off X-rays.

  • Bequerel determined that the minerals contained uranium

    uranic rays Production of uranic rays did not

    require exposure to outside energy.

    Energy was being produced from nothing !!!

    Bequerel discovered that certain minerals were constantly producing

    penetrating energy rays

    like X-rays not related to fluorescence

  • Marie Curie (1867-1934) broke down these minerals and used an

    “electroscope” to detect uranic rays.

    She discovered the rays were emitted from specific elements.

    She also discovered new elements 1. radium named for its green phosphorescence 2. polonium named for her homeland

    She coined the name “radioactivity”

    The Curies

  • Other Properties of Radioactivity

    Can “ionize” matter (cause matter to become charged)

    (basis of Geiger Counter)

    High energy

    Can penetrate matter

    Can cause phosphorescent chemicals to glow

    (basis for the scintillation counter)

  • ElectroscopeElectroscope + + + +

    + +

    When charged, the metal

    foils spread apart due to like charge repulsion

    When exposed to ionizing

    radiation, the radiation knocks electrons off the

    air molecules, which jump onto the foils and

    discharge them, causing

    them to drop down.

    Electroscope

    + + + +

    + +

    When charged, the metal

    foils spread apart due to like charge repulsion

    When exposed to ionizing

    radiation, the radiation knocks electrons off the

    air molecules, which jump onto the foils and

    discharge them, causing

    them to drop down.

    Ionizing radiation knocks electrons off the air molecules, which jump onto the foils and

    discharge them.

    When charged, the metal foils spread apart due to like charge

    repulsion.

    Ionizing radiation

  • Rutherford (1871-1937) Discovered Three Types of RadiationRutherford’s Experiment

    ++++++++++++

    --------------

    α

    γ β

  • Types of Radioactive Rays

    “Alpha” Rays (𝛂)

    charge of +2 and mass of 4 amu
 essentially the nucleus of a helium atom

    “Beta” Rays (β)

    charge of -1 and negligible mass high-energy electrons

    “Gamma” Rays (𝛄) electromagnetic radiation, not 𝛂 or β

  • Penetrating Ability of Radioactive Rays Penetrating Ability of Radioactive

    Rays

    α β γ

    0.01 mm 1 mm 100 mm

    Pieces of Lead Pieces of Lead

  • Nuclear Chemistry

    Nuclear reaction – process that alters the number of neutrons and protons in the

    nucleus of an atom.

    Radionuclide – an unstable nuclide that undergoes radioactive decay.

    Radioactive decay – the spontaneous disintegration of unstable particles

    accompanied by the release of radiation.

  • Binding Energy and

    Nuclear Stability

  • What Causes Nuclei to Break Down?

    The particles in the nucleus are held together by a very strong attractive force between nucleons,

    the strong force, which acts only over very short distances.

  • Neutrons and Protons are Held Together by the “Strong Force.”

    Neutrons play an important role in

    stabilizing the nucleus. They add to the strong

    force, but don’t repel each other like protons.

  • The “Valley of Stability” and the N/Z Ratios (neutrons/protons)

    For Z =1-20, stable N/Z ratio = 1

    For Z = 20-40, stable N/Z ratio ≈1.25

    For Z = 40-80, stable N/Z ratio ≈1.5

    For Z > 84, there are no stable nuclei

    Valley of Stability

    for Z = 1 ⇒ 20,

    stable N/Z ≈ 1

    for Z = 20 ⇒ 40,

    stable N/Z approaches 1.25

    for Z = 40 ⇒ 80,

    stable N/Z approaches 1.5

    for Z > 83,

    there are no stable nuclei

  • Mass Defect (Δm)

    He nucleus

    2 neutrons + 2 protons = 6.69510 × 10–27 kg

    Mass of 4He = 6.64465 × 10–27 kg

    Δm = 5.045 × 10–29 kg

    E = mc2

    BE = 4.54 x 10-12 J/atom

    BE = 1.13 x 10-12 J/nucleon

  • Binding Energy

  • Binding Energy

  • Unstable Nuclei and

    Modes of Radioactive Decay

  • Review of Nuclear Structure

    Every atom of an element has the same number of protons

    designated by the atomic number “Z”

    Atoms of the same element may have different numbers of neutrons

    called “isotopes” have different atomic masses

    XAZ symbol mass number

    atomic number mass # = protons + neutrons

    Isotopes and “Nuclides” are represented symbolically:

  • Radioactivity

    Unstable radioactive nuclei spontaneously decompose into smaller nuclei through “radioactive decay.”

    PARENT NUCLIDE ————> DAUGHTER NUCLIDE(S)

    All nuclides with 84 or more protons are radioactive

    PARTICLE(S) and/or ENERGY

  • Important Atomic SymbolsImportant Atomic Symbols Particle Symbol Nuclear

    Symbol

    proton p+

    neutron n0

    electron e-

    alpha α

    beta β, β

    positron β, β+

  • Transmutation

    Atoms of one element are changed into atoms of a different element.

    The number of protons in the nucleus changes.

    Nuclear Equations

    •  we describe nuclear processes with nuclear equations

    •  use the symbol of the nuclide to represent the nucleus

    •  atomic numbers and mass numbers are conserved  use this fact to predict the daughter nuclide if you know

    parent and emitted particle

    We describe the process with nuclear equations.

  • Transmutation

    •  Rutherford discovered that during the radioactive process, atoms of one element are changed into atoms of

    a different element - transmutation

     Dalton’s Atomic Theory statement 3 bites the dust

    •  in order for one element to change into another, the number of protons in the nucleus must change

    Nuclear Equations

    In nuclear equations, atomic numbers and mass numbers are conserved.Nuclear Equations

    •  we describe nuclear processes with nuclear equations

    •  use the symbol of the nuclide to represent the nucleus

    •  atomic numbers and mass numbers are conserved  use this fact to predict the daughter nuclide if you know

    parent and emitted particle

    238 = 234 + 4 92 = 90 + 2

    conservation of nucleons

    conservation of charge

  • Alpha Emission

    An 𝛂 particle contains 2 protons and 2 neutrons. a helium nucleus

    The “most ionizing”, but “least penetrating” of radiation typesAlpha Emission

    •  an α particle contains 2 protons and 2 neutrons

     helium nucleus

    •  most ionizing, but least penetrating

    •  loss of an alpha particle means  atomic number decreases by 2

     mass number decreases by 4

  • “Radium-222 decays by alpha emission”

    Alpha Emission •  an α particle contains 2 protons

    and 2 neutrons

     helium nucleus

    •  most ionizing, but least penetrating

    •  loss of an alpha particle means  atomic number decreases by 2

     mass number decreases by 4

    Alpha Emission

    Loss of an 𝛂 particle means atomic number decreases by 2 mass number decreases by 4

  • Beta Emission

    A beta particle is like an electron

    moves much faster (has more energy) produced in the nucleus

    In β decay, a neutron changes into a proton

    Important Atomic Symbols Particle Symbol Nuclear

    Symbol

    proton p+

    neutron n0

    electron e-

    alpha α

    beta β, β

    positron β, β+

  • Beta Emission

    “Thorium-234 decays by beta emission”

    Beta Emission

    •  a β particle is like an electron  moving much faster

     produced from the nucleus

    •  when an atom loses a β particle its  atomic number increases by 1

     mass number remains the same

    •  in beta decay, a neutron changes into a proton

    Loss of an β particle means atomic number increases by 1 mass number remains the same

  • Gamma Emission

    No change in composition of the nucleus

    Occurs after the nucleus undergoes some other type of decay and the remaining particles rearrange

    Gamma Emission

    •  gamma (γ) rays are high energy photons of light •  no loss of particles from the nucleus •  no change in the composition of the nucleus

     Same atomic number and mass