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Lecture 15

Nuclear Chemistry

Shaun Williams, PhD

Radioactivity

Radioactivity

Nucleons

Nuclides

Band of Stability

  • Of the more than 3,000 nuclides known, about 250 are stable
  • The rest decompose over a period of time, emitting radiation in the process of creating new nuclides
  • The stable nuclides have approximately equal numbers of protons and neutrons (\(N/Z\) ratio = 1) in the lighter elements (\(Z\) = 1 to 20) and more neutrons than protons in the heavier elements (\(N/Z\) ratio > 1).
A plot of number of neutrons versue number of protons with points for all the stable isotopes.

Radiation

Radiation aimed between two electrified metals plates can have three reactions: it can bend towards the positive plate, beta-particles, it may not bend, gamma rays, and it could bend towards the negative plate, alpha particles.

Properties of Types of Radiation

Type Notation Mass Charge Penetration into Al
Alpha \(\chem{{}^4_2\alpha},\, \chem{{}^4_2He^{2+}}\) 4 2+ 0.01 mm
Beta (electron) \(\chem{{}_{-1}^0\beta^-}\) ~0 1- 0.5-1.0 mm
Beta (positron) \(\chem{{}_1^0\beta^+}\) ~0 1+ (Reacts with electrons)
Gamma \(\chem{\gamma}\) 0 0 50-110 mm

Types of Radiation

  • Alpha particles
    • Nuclei of helium-4 atoms
    • Contain 2 protons and 2 neutrons
    • Least harmful to animal and human tissue
  • Gamma rays
    • High energy electromagnetic radiation: energy without charge or mass
    • Highest energy and most penetrating type of radiation
A graphic showing that alpha particles are stopped by our skin, beta particles can pentrate into the tissue, and gamma rays can pass entirely through the body.

Types of Radiation (cont.)

  • Beta particles
    • Small, charged particle that can be emitted from unstable atoms at speeds approaching the speed of light
    • Penetrate through skin into tissue
    • 2 types of beta particles:
      • Positron - Same mass as an electron with an opposite charge
      • Electron
A graphic showing that alpha particles are stopped by our skin, beta particles can pentrate into the tissue, and gamma rays can pass entirely through the body.

Nuclear Reactions

Alpha Particle Emission

A graphic showing the ejection of an alpha particle from a nucleus.

Beta Particle (Electron) Emission

A graphic showing the ejection of an beta-minus particle from a nucleus.

Beta Particle (Positron) Emission

A graphic showing the ejection of an beta-plus particle from a nucleus.

Electron Capture

Gamma Ray Emission

A graphic showing the ejection of an gamma ray from a nucleus.

Nuclear Bombardment Reactions

Particle Accelerators

A photograph of the ring of a particle accelerator.

Sponeaneous Nuclear Decay Reactions

  • The tendency for the neutron/proton (\(N/Z\)) ratio to move toward the band of stability, explains the nuclear reactions of naturally radioactive nuclides.
  • For every process except \(\gamma\) emission, the change that occurs for an unstable nuclide takes it closer to the observed band of stability.
  • Radioactive nuclides convert spontaneously over time to form stable nuclides.
A plot of neutron number versus proton number showing the stable isotopes.

Nuclear Instability

Reason for Nuclear Instability Radioactive Process Emitted Radiation Change in \(N/Z\) Ratio
Excess Mass Alpha decay \(\chem{{}^4_2\alpha}\) Slight increase
\(N/Z\) too high Beta decay \(\chem{{}^0_{-1}\beta^-}\) Decrease
\(N/Z\) too low Positron emission \(\chem{^0_1\beta^+}\) Increase
\(N/Z\) too low Electron capture - Increase
Energetically excited Gamma emission \(\chem{\gamma}\) ray None

Radioactive Decay Series

  • In heavier elements, often the product of radioactive decay is itself radioactive.
  • In such cases, a series of alpha and beta decay steps ultimately leads to a stable nuclide.
  • Accounts for most of the radioactive decay among elements 83 through 92.
A plot of mass number versus atomic number showing the pathway taken by uranium-238 to reach the stable lead-206 isotope.

Rates of Radioactive Decay

Detecting Radiation

A photograph of a Geiger-Muller counter.

Half-Life

A plot of radiation intensity versus time showing the exponential decay of radioactivity.

Archeological Dating

Medical Applications of Isotopes

Medical Applications

A PET scan of a healthy brain showinga lot of colors and an abnormal brain showing very little color.

Medical Applications (cont.)

Biological Effects of Radiation

Biological Effects of Radiation - Radon

The Fear of Nuclear Energy...

Before and After...

Areal photographs of Bakini Atol before and after the Ivy Mike test showing the evaporated island.

Areal photograph of the evaporated island in the Bakini Atol showing the evaporated island.

Comparison

A schematic of the ivy mike test over Hickory, NC.

Nuclear Energy

Fission

Fission of Uranium-235

A graphic showing a neutron being absorbed by uranium-235 resulting in the fission of the isotope.

Chain Reactions

Fission Reactors

  • Nuclear power plants use fission to produce electric energy
  • If the chain reaction is going too quickly, movable control rods made of these elements are inserted into a core of uranium fuel in fission reactors
A diagram showing the parts of a nuclear reactor core including the boron-10 control rods, moderator, uranium fuel rods, and the coolant.

A graphic showing a complete nuclear reactor with core, steam generator, electrical generator, condenser, and cooling tower.

Fusion Reactions

Fusion Reaction Terms

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