Nuclear Chemistry Study Notes

Nuclear Chemistry

Definition of a Radioisotope

• A radioisotope is defined as follows:
  • Has an unstable nucleus and emits radiation.
  • Can be one or more isotopes of an element.
  • Includes the mass number in its name.
• Example:
  • Carbon-14
  • Mass Number: 14
  • Atomic Number: 6
  • Application: Used for archeological dating.

Types of Radiation Emitted by Radioisotopes

• Radioisotopes emit various types of radiation:
  • Alpha (α) particles:
  • Identical to a helium nucleus.
  • Beta (β) particles:
  • High-energy electrons.
  • Positrons (β+):
  • Positively charged counterparts of electrons.
  • Gamma (γ) rays:
  • Pure energy emitted as high-energy radiation.

Characteristics of Different Types of Particles

Alpha (α) Particles

• Characteristics include:
  • Mass Number: 4
  • Charge: 2+
  • Contains 2 protons and 2 neutrons.
  • Compared to other radiation, alpha particles have low energy.

Beta (β) Particles

• Characteristics include:
  • Mass Number: 0
  • Charge: 1− (negatively charged electron)
  • Forms in an unstable nucleus when a neutron converts into a proton and an electron.

Positrons (β+)

• Characteristics include:
  • Mass Number: 0
  • Charge: 1+ (positively charged electron)
  • Forms in an unstable nucleus when a proton converts into a neutron and a positron.

Gamma (γ) Rays

• Characteristics include:
  • Mass Number: 0
  • Charge: 0 (pure energy)
  • A form of energy emitted to achieve a more stable, lower-energy nucleus.

Radiation Protection

Properties of Radiation and Shielding Required

• Table 5.3: Properties of Radiation
  

• Radioactive decay is described as follows:
  • An unstable nucleus spontaneously breaks down by emitting radiation.
  • It is represented by a nuclear equation:
  • General form:
    ext{Radioactive nucleus}
    ightarrow ext{new nucleus} + ext{radiation (α, β, β+, γ)}

Balancing Nuclear Equations

• In a balanced nuclear equation:
  • The sum of mass numbers on each side must be equal.
  • The sum of atomic numbers on each side must also be equal.
• Example of balanced nuclear equation:
  • For curium-251:
  • 251 ext{Cf}
    ightarrow 247 ext{Cm} + 4 ext{He}
  • Mass numbers: $251 = 247 + 4$
  • Atomic numbers: $98 = 96 + 2$

Example Problem: Beta Decay of Cobalt-60

• Step 1: Write the incomplete nuclear equation:
  • Given Co-60, beta decay requires balancing mass and atomic numbers.
• Step 2: Determine missing mass number:
  • $60 = ? + 0$
  • $? = 60$
• Step 3: Determine missing atomic number:
  • $27 = ? - 1$
  • $? = 28$
• Step 4: Identify symbol of new nucleus:
  • Atomic number 28 corresponds to Nickel (Ni).
  • Complete nuclear equation:
  • ^{60}{27} ext{Co} ightarrow ^{60}{28} ext{Ni} + eta^-

Radiation Detection and Measurement

Geiger Counter

• The Geiger counter is an instrument designed to:
  • Detect beta and gamma radiation.
  • Use ions produced by radiation to create an electrical current.

Units for Measuring Radiation Activity

• Common units include:
  • Curie (Ci):
  • Defined as the number of disintegrations that occurs in 1 s for 1 g of radium, equal to $3.7 imes 10^{10} ext{ disintegrations/s}$.
  • Becquerel (Bq):
  • The SI unit of radiation activity represented as 1 disintegration/s.
  • Rad (radiation absorbed dose):
  • Measures the amount of radiation absorbed by a gram of material.
  • Rem (radiation equivalent in humans):
  • Quantifies the biological effects of various types of radiation.

Equivalent Dose Calculation

• To determine the equivalent dose in rems:
  • The equation is:
    $ext{Biological damage (rem)} = ext{Absorbed dose (rad)} imes ext{Factor}$
  • Factors for biological damage are:
  • For beta and gamma radiation: 1.
  • For high-energy protons and neutrons: about 10.
  • For alpha particles: 20.

Dosimeters

• Used to measure radiation exposure for individuals working in radiation laboratories.
  • Types of radiation detected include:
  • X-rays
  • Gamma rays
  • Beta particles

Radiation Exposure Statistics

• Average annual radiation exposure for a person in the U.S. is 3.6 mSv.
• Sources of natural radiation exposure include:
  • Natural radioisotopes in:
  • Buildings
  • Food and water
  • Air
  • Notable source: Potassium-40 in potassium-rich foods.
• Medical sources of radiation include X-rays and mammograms.

Effects of Radiation Exposure

• Effects of radiation exposure based on dosage:
  • Less than 0.25 Sv often cannot be detected.
  • Whole body exposure of 1 Sv leads to a temporary reduction in white blood cells.
  • Exposure exceeding 1 Sv may induce radiation sickness:
  • Symptoms include nausea, vomiting, fatigue, and reduced white-cell counts.
  • An exposure of 5 Sv results in a 50% chance of death (LD50).

Half-Life of a Radioisotope

• The half-life is defined as:
  • The time taken for the radiation level (activity) to decay to one-half of its original value.

Decay Curve

• A decay curve graphically illustrates the decay of a radioactive isotope over time.
• Example: For Iodine-131 with a half-life of 8 days, the decay curve shows that half of the sample decays while half remains radioactive.

Half-Life Equations

Example with Strontium-90

• Given:
  • Strontium-90 has a half-life of 38.1 years.
  • Initial amount: 36 mg.
  • Find the remaining mass after 114.3 years.
• Steps include:
  • 1. Analyze the number of half-lives:
  • $ext{Number of half-lives} = \frac{114.3 ext{ years}}{38.1 ext{ years/half-life}} = 3$
  • 2. Calculate remaining mass:
  • Remaining mass after 3 half-lives:
  • $ext{Remaining mass} = \frac{36 ext{ mg}}{2^3} = 4.5 ext{ mg}$ (approximate value)

Radiological Dating Techniques

Carbon Dating

• Carbon-14 is used for dating ancient objects:
  • Produced via cosmic rays bombarding Nitrogen-14.
  • Reacts with oxygen to form radioactive carbon dioxide absorbed by living plants.
  • When the plant dies, carbon-14 uptake ceases; scientists can calculate time since death using its half-life (5730 years).

Medical Applications of Radioisotopes

• Radioisotopes with short half-lives are favored in nuclear medicine because:
  • Body cells do not differentiate between radioactive and non-radioactive atoms, allowing targeted imaging.
• Applications in imaging techniques include:
  • Thyroid scans using Iodine-131.
  • Positron Emission Tomography (PET) which uses short-lived positron emitters to study brain function.
  • Computed Tomography (CT) and MRI scans providing detailed organ images without radiation exposure (in the case of MRI).

Nuclear Reactions

Fission

• In nuclear fission:
  • A large nucleus is bombarded by small particles (e.g., neutrons), causing it to split and release significant energy.
  • Example: Bombarding U-235 causes it to split into smaller nuclei such as Kr-91 and Ba-142.

Fusion

• In nuclear fusion:
  • Smaller nuclei combine at extremely high temperatures (around 100,000,000 °C) to form larger nuclei, releasing large amounts of energy.
  • This process is continuous in stellar bodies like the Sun.