Chapter 5: Nuclear Chemistry – Study Notes for GOB Chemistry

Chapter 5: Nuclear Chemistry

5.1 Natural Radioactivity

Nuclear chemistry explores the behavior of atomic nuclei, especially those that are unstable and emit radiation. Radioactivity is a property of certain isotopes whose nuclei cannot offset the repulsions between protons, resulting in the emission of particles or energy to achieve stability.

• Radioisotope: An isotope of an element that emits radiation. The mass number is included in its name (e.g., iodine-131).

• Types of Radiation:

  • Alpha particles (\( \alpha \)): Identical to a helium nucleus (\( ^4_2He \)).

  • Beta particles (\( \beta \)): High-energy electrons (\( ^0_{-1}e \)).

  • Positrons: Positive electrons (\( ^0_{+1}e \)).

  • Gamma rays: Pure energy (\( ^0_0\gamma \)).

• Biological Effects: Ionizing radiation can damage rapidly dividing cells, causing cancer. Different types of radiation penetrate the body to different depths.

• Radiation Protection:

  • Alpha: blocked by paper or clothing

  • Beta: blocked by lab coat or gloves

  • Gamma: blocked by lead or thick concrete

5.2 Nuclear Reactions

Nuclear reactions involve changes in the nucleus, often resulting in the emission of radiation and the formation of new elements. These reactions are represented by nuclear equations, which must be balanced for both mass number and atomic number.

• Alpha Decay: The nucleus emits an alpha particle, decreasing mass number by 4 and atomic number by 2. Equation: $^{241}_{95}Am \rightarrow ^{4}_{2}He + ^{237}_{93}Np$

• Beta Decay: The nucleus emits a beta particle, mass number remains the same, atomic number increases by 1. Equation: $^{14}_{6}C \rightarrow ^{0}_{-1}e + ^{14}_{7}N$

• Positron Emission: A proton is converted to a neutron and a positron, atomic number decreases by 1. Equation: $^{11}_{6}C \rightarrow ^{0}_{+1}e + ^{11}_{5}B$

• Gamma Radiation: Energy is emitted, mass number and atomic number remain unchanged. Equation: $^{60}_{27}Co^m \rightarrow ^{60}_{27}Co + ^{0}_{0}\gamma$

• Transmutation: Stable nuclei can be made radioactive by bombardment with particles. Example: $^{10}_{5}B + ^{4}_{2}He \rightarrow ^{13}_{7}N + ^{1}_{0}n$

5.3 Radiation Measurement

Radiation is measured using specialized instruments and units. The biological effects depend on the type and amount of radiation absorbed.

• Geiger Counter: Detects alpha, beta, and gamma radiation by measuring ions produced.

• Units:

  • Curie (Ci): Number of disintegrations per second for 1 g of radium.

  • Becquerel (Bq): SI unit, 1 disintegration per second.

  • Rad: Amount of radiation absorbed by a gram of material.

  • Rem: Biological effect of radiation; $1 \text{ rem} = 1000 \text{ mrem}$

  • Sievert (Sv): SI unit for biological effect; $1 \text{ Sv} = 100 \text{ rem}$

• Dosimeters: Devices worn to measure exposure to X-rays, gamma rays, and beta particles.

• Radiation Sickness: Exposure to high doses can be lethal; $5 \text{ Sv}$ is the lethal dose for 50% of humans.

5.4 Half-Life of a Radioisotope

The half-life is the time required for half of a radioactive sample to decay. This property is used to date ancient objects and calculate remaining radioisotope amounts.

• Decay Curve: Shows the fraction of a sample remaining after each half-life.

• Calculation Example: For Sr-90 (half-life 38.1 years), after 152.4 years (4 half-lives), $36.0 \text{ mg} \times (1/2)^4 = 2.25 \text{ mg}$ remains.

• Application: Carbon-14 dating is used to determine the age of archaeological samples.

5.5 Medical Applications Using Radioactivity

Radioisotopes are used in medicine for diagnosis and treatment. Short half-life isotopes are preferred to minimize long-term exposure.

• Imaging: Radioisotopes are incorporated into organs and detected by scanners, producing images.

• PET Scans: Use positron emitters to study brain function and metabolism.

• CT Scans: Use X-rays to image organs based on tissue density.

• MRI: Uses magnetic fields and radio waves, not nuclear radiation, to image soft tissues.

• Brachytherapy: Internal radiation therapy for cancer, using implanted radioactive seeds or needles.

5.6 Nuclear Fission and Fusion

Nuclear fission and fusion are processes that release large amounts of energy by altering atomic nuclei.

• Nuclear Fission: A large nucleus (e.g., U-235) splits into smaller nuclei and neutrons, releasing energy. Equation: $^{235}_{92}U + ^{1}_{0}n \rightarrow ^{140}_{56}Ba + ^{94}_{36}Kr + 3 ^{1}_{0}n$

• Chain Reaction: Neutrons produced in fission cause further fission reactions.

• Nuclear Fusion: Small nuclei (e.g., hydrogen isotopes) combine to form larger nuclei (e.g., helium), releasing energy. Equation: $^{2}_{1}H + ^{3}_{1}H \rightarrow ^{4}_{2}He + ^{1}_{0}n$

• Fusion in Stars: Fusion occurs at high temperatures in stars, producing energy and less waste than fission.

Summary Table: Types of Radiation and Shielding

Type of Radiation	Symbol	Penetration	Shielding Required
Alpha	$^{4}_{2}He$	Low	Paper, clothing
Beta		Moderate	Lab coat, gloves
Gamma	$^{0}_{0}\gamma$	High	Lead, concrete

Summary Table: Radiation Measurement Units

Property	Unit
Activity	Becquerel (Bq), Curie (Ci)
Absorbed Dose	Rad, Gray (Gy)
Biological Damage	Rem, Sievert (Sv)

Summary Table: Half-Lives of Selected Radioisotopes

Radioisotope	Half-Life	Application
Carbon-14	5730 years	Archaeological dating
Iodine-131	8 days	Thyroid imaging
Strontium-90	38.1 years	Environmental studies
Technetium-99m	6 hours	Medical imaging

Summary Table: Medical Applications of Radioisotopes

Radioisotope	Application	Half-Life
Iodine-131	Thyroid imaging/treatment	8 days
Technetium-99m	Imaging organs	6 hours
Fluorine-18	PET scans	110 minutes
Palladium-103	Brachytherapy	17 days

Concept Map: Nuclear Chemistry

• Natural Radioactivity → Types of Radiation → Biological Effects → Radiation Protection

• Nuclear Reactions → Decay Equations → Transmutation

• Radiation Measurement → Units → Dosimeters → Radiation Sickness

• Half-Life → Decay Curves → Calculations → Applications

• Medical Applications → Imaging → Therapy

• Nuclear Fission and Fusion → Energy Production → Chain Reactions → Fusion in Stars