Nuclear Chemistry Study Notes

Introduction to Nuclear Chemistry

• Definition: Nuclear chemistry is the study of the structure of atomic nuclei and the changes they undergo.

Characteristics of Nuclear Reactions

• Isotopes of one element are transformed into isotopes of another element.
• The contents of the nucleus undergo significant changes.
• Large amounts of energy are released during nuclear reactions.
• Comparison with Chemical Reactions:
  • In chemical reactions, bonds are broken and atoms are rearranged.
  • In nuclear reactions, changes concern protons and neutrons, as well as the nucleus itself, with radiation being emitted.
  • Nuclear reactions involve processes that typically release radiation, as the nucleus emits particles (e.g., alpha particles, beta particles, gamma rays).

Discovery of Radioactivity (1895-1898)

• Wilhelm Roentgen: Discovered that invisible rays were emitted when electrons struck a fluorescent screen, leading to the discovery of X-rays.
• Henri Becquerel: Accidentally discovered radioactivity through phosphorescent uranium producing spots on photographic plates.
• Marie and Pierre Curie: Isolated components emitting rays from uranium and identified two new elements, Polonium (Po) and Radium (Ra), challenging Dalton's atomic theory which stated atoms were indivisible.

Review of Atomic Structure

• Nucleus:
  • Contains the majority of the atom's mass (99.9%).
  • Composed of protons (p) and neutrons (n°).
  • Held together by the strong nuclear force.
• Electron Cloud:
  • Contributes to the volume of the atom (0.01% of mass).
  • Contains negatively charged electrons, which are held around the nucleus by weak electrostatic forces.

Nuclear Symbol and Definitions

• Nucleons: Particles found in the nucleus (protons and neutrons).
• Nuclear Symbol: Consists of three parts:
  • Element Symbol (A): Denotes the element (e.g., C for carbon).
  • Atomic Number (Z): Number of protons in the element.
  • Mass Number (A): Sum of protons and neutrons in the nucleus (e.g., carbon-12). Also can be presented as element name-mass number (e.g., carbon-12).
• Ion: An atom with a charge due to loss or gain of electrons.
• Isotopes: Atoms of the same element that have different numbers of neutrons.
• Radioisotopes: Unstable isotopes with nuclei that decay.

Radioactive Decay

• Radioactive Decay: The process through which unstable nuclei lose energy by emitting radiation in order to become more stable. This is a spontaneous reaction.
• Nuclear Stability:
  • Generally, elements with atomic numbers (Z) from 0 to 20 are very stable.
  • Example: Carbon-12 has 6 protons and 6 neutrons.
  • Elements with atomic numbers 21 to 83 are marginally stable; the ratio of protons to neutrons typically approximates 1.5:1.
  • Example: Mercury-200 has 80 protons and 120 neutrons.
  • Elements with atomic numbers greater than 83 are generally unstable and radioactive (e.g., Uranium and Plutonium).

Types of Nuclear Reactions

• Radioactive Decay: Natural transformation where nuclei emit radiation.
• Nuclear Disintegration: Includes alpha and beta particle emission and gamma rays.
• Transmutation: Conversion of one element into another, usually through radioactive decay.
• Nuclear Equation: Represents the radioactive decay of an element.

Alpha Radiation

• Composition: Alpha particles are helium nuclei (He²⁺).
• Charge: +2, deflection towards negatively charged plates.
• Mass: Approximately 4 amu.
• Approximate Energy: 5.0 MeV.
• Penetrating Power: Low (cannot penetrate more than 0.05 mm of body tissue).
• Shielding: Can be stopped by paper or clothing.

Alpha Decay Equations

• Steps for writing alpha decay equations:
  1. Write the nuclear symbol of the element before the decay.
  2. Write the alpha particle (He) as product.
  3. Determine the resulting element using atomic number.
  4. Check that mass numbers are balanced (loss of 4 in mass and 2 in atomic number).
• Example: For the radioactive decay of Polonium-210 by alpha emission:
  ${}^{210}<em>{84} ext{Po} ightarrow {}^{4}</em>{2} ext{He} + {}^{206}<em>{82} ext{Pb}$ For the radioactive decay of Radium-226 by alpha emission: ${}^{226}</em>{88} ext{Ra} <br /> ightarrow {}^{4}<em>{2} ext{He} + {}^{222}</em>{86} ext{Rn}$

Beta Radiation

• Composition: Comprised of beta particles, equivalent to fast-moving electrons.
• Process: A neutron converts into a proton.
• Symbol: $e^-$ (beta particle).
• Charge: -1, deflection towards positively charged plates.
• Mass: Approximately 1/1837 amu.
• Approximate Energy: 0.05-1 MeV.
• Penetrating Power: Moderate (can penetrate around 4 mm of body tissue).
• Shielding: Requires metal foil for protection.

Beta Decay Equations

• Steps for writing beta decay equations:
  • Similar to alpha equations, but utilize a beta particle (e^-):
  • Net effect involves a change in mass number and an increase of 1 in atomic number.
• Example: For the radioactive decay of Carbon-14 by beta emission:
  ${}^{14}<em>{6} ext{C} ightarrow {}^{14}</em>{7} ext{N} + e^-$

Gamma Radiation

• Composition: Gamma rays are high-energy electromagnetic radiation or photons.
• Charge: 0.
• Mass: 0 amu.
• Approximate Energy: 1 MeV.
• Penetrating Power: High (capable of easily penetrating body tissues).
• Shielding: Requires dense materials like lead or concrete.

Gamma Decay Equations

• Example: For the radioactive decay of Uranium-238 accompanied by alpha decay:
  ${}^{238}<em>{92} ext{U} ightarrow {}^{234}</em>{90} ext{Th} + {}^{4}_{2} ext{He} + ext{γ}$

Half-Life (t₁/₂)

• Definition: The time required for half of a radioactive sample to decay into its products.
• Represented as follows: {N(t) = N0 imes (0.5)^{rac{t}{t{1/2}}}} where:
  • $N(t)$ = remaining quantity after time $t$
  • $N_0$ = initial quantity
  • $t_{1/2}$ = half-life
• Carbon-14: Used for radioactive dating due to its long half-life.
• The consistency of remaining isotopes decreases logarithmically with elapsed half-lives.

Examples of Half-Life Calculations

• Example 6: Strontium-90 with a half-life of 29 years:
  • Initial: 10.0 grams. Remaining after 116 years: 0.625 g.
  • Calculation table showing half-lives and remaining amounts:
    | # of 1/2 lives | Time (years) | Amount Remaining (g) |
    |----------------|---------------|---------------------|
    | 0 | 0 | 10.0 |
    | 1 | 29 | 5.00 |
    | 2 | 58 | 2.50 |
    | 3 | 87 | 1.26 |
    | 4 | 116 | 0.625 |
• Examples 7-11: Various scenarios demonstrating calculations of remaining isotopes after specified half-lives, with some calculations displayed for clarity.

Nuclear Fission and Fusion

• Nuclear Fission: The splitting of a nucleus, which releases substantial energy and produces radioactive waste. Typically initiated by bombarding Uranium nuclei with neutrons.
• Example of Fission process:
  • ${^{235}<em>{92} ext{U} + n ightarrow {}^{144}</em>{56} ext{Ba} + {}^{89}_{36} ext{Kr} + 3n}$
• Nuclear Fusion: The process where two light nuclei combine to form a heavier nucleus, releasing energy in the process. This occurs in the core of stars.

Conclusion

• Nuclear chemistry encompasses a wide variety of phenomena regarding atomic nuclei transformations. The principles underlying nuclear reactions lead to practical applications in energy generation, medicine, and understanding the stability of matter.