Chem 2 Ch 21 Nuclear Chemistry Notes

The Nucleus

• Composed of nucleons: protons and neutrons.
• Atomic number: Number of protons in an atom.
• Mass number: Total number of protons and neutrons.

Isotopes

• Atoms of the same element can have different masses due to varying neutron numbers.
• Example: Naturally occurring isotopes of uranium include:
  • Uranium-234
  • Uranium-235
  • Uranium-238

Radioactivity

• Radioactive nuclei change spontaneously, emitting radiation.
• Such nuclei are referred to as radionuclides.
• Decay of radionuclides can lead to the formation of different nuclides, expressed through nuclear equations.

Nuclear Equations

• Balance of atomic number and mass number is crucial in nuclear equations, akin to balancing charge and mass in chemical reactions.

Types of Radiation

• The common types of radiation emitted by radionuclides include:
  • Alpha radiation (Charge: 2+, Mass: 6.64 imes 10^{-24} ext{ g})
  • Beta radiation (Charge: 1−, Mass: 9.11 imes 10^{-28} ext{ g})
  • Gamma radiation (Charge: 0, Mass: 0)
  • Relative penetrating powers: Alpha (1), Beta (100), Gamma (10,000)

Types of Radioactive Decay

• Alpha decay: Loss of an alpha particle (He nucleus)
• Beta decay: Loss of a beta particle (high-speed electron)
• Gamma emission: Release of gamma rays usually accompanying particle loss
• Positron emission: Emission of positron ((^{0}_{+1}e))
• Electron capture: Absorption of an electron from surrounding electron cloud

Balancing Nuclear Reactions

• Alpha decay: Example:

  • Equation: (^{238}{92} ext{U} \rightarrow ^{234}{90} ext{Th} + ^{4}_{2} ext{He})
  • Balance check: Atomic number: 92 = 90 + 2; Mass number: 238 = 234 + 4

• Beta decay: Example:

  • Equation: (^{131}{53} ext{I} \rightarrow ^{131}{54} ext{Xe} +^{0}_{-1} ext{e})
  • Balance check: Atomic number: 53 = 54 + (−1)

Nuclear Stability

• Repulsion between protons necessitates a balance provided by neutrons (i.e., neutron-to-proton ratios).
• For small nuclei, a ratio close to 1:1 is stable.
• Larger nuclei require more neutrons for stability.

Unstable Nuclei

• Nuclei with too many neutrons undergo beta decay; those with excess protons may emit positrons or capture electrons.
• Very large nuclei typically decay via alpha emission.

Radioactive Decay Chains

• Some nuclei require multiple decays to achieve stability, often forming stable isotopes like lead.

Stable Nuclei

• Certain magic numbers (2, 8, 20, 28, 50, 82) and even numbers of protons/neutrons produce stable nuclides.

Nuclear Transmutations

• Induced by particle collisions using particle accelerators.
• Types:
  • Linear accelerators: Accelerate particles in straight lines using varying lengths.
  • Cyclotrons: Use magnets to spiral particles.
  • Synchrotrons: Accelerate particles in circular paths.

Kinetics of Radioactive Decay

• The decay process follows first-order kinetics.
• The half-life (time for half of the sample to decay) can be used for dating objects (e.g., Carbon-14, Uranium-238).

Measuring Radioactivity

• Units of activity: Becquerel (Bq), Curie (Ci)
• Instruments: Film badges, Geiger counters, scintillation counters.

Radiotracers in Medicine

• Use of radioisotopes for tracking chemical reactions in biological systems.
• Certain radionuclides are employed for diagnostic imaging (e.g., Iodine-131 for thyroid studies).

Medical Applications of Radiotracers

• Iodine-131: Thyroid studies, half-life 8.04 days
• Iron-59: Red blood cells, half-life 44.5 days
• Technetium-99m: Heart, bones, half-life 6 hours.

Radiation Effects

• Ionizing radiation is more harmful, especially when it interacts with water (forming reactive OH radicals).
• Types of exposure factors affect damage levels in cells, with external gamma radiation being particularly dangerous.

Radiation Dose Measurement

• Common measures: Gray (Gy) and Rad (for absorbed dose).
• Effective dose is expressed in rem (1 Sv = 100 rem).

Short-Term Exposure Effects

• Different radiation doses produce varying effects, with higher doses correlating to increased danger, including potential death thresholds in extreme cases.