Chapter 5: Nuclear Chemistry – Study Notes for GOB Chemistry

Chapter 5: Nuclear Chemistry

5.1 Natural Radioactivity

Nuclear chemistry explores the behavior of unstable atomic nuclei, which emit radiation to become more stable. This process is fundamental to understanding radioisotopes and their applications in medicine and industry.

• Radioactivity: The spontaneous emission of particles or energy from unstable nuclei.

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

• Types of Radiation:

  • Alpha particles (\( ^4_2He \)): Identical to helium nuclei; low penetration.

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

  • Positrons (\( ^0_{+1}e \)): Antiparticles of electrons; used in PET scans.

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

• Biological Effects: Ionizing radiation can damage rapidly dividing cells, leading to cancer.

• Radiation Protection: Different types of shielding are required:

  • Paper and clothing: Block alpha particles.

  • Lab coat/gloves: Block beta particles.

  • Lead/concrete: Block gamma rays.

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 mass and atomic numbers.

• Radioactive Decay: The process by which unstable nuclei emit radiation.

• Balancing Nuclear Equations: The sum of mass numbers and atomic numbers must be equal on both sides.

  • Alpha Decay: Mass number decreases by 4, atomic number decreases by 2. $^{241}_{95}Am \rightarrow ^{4}_{2}He + ^{237}_{93}Np$

  • Beta Decay: Mass number unchanged, atomic number increases by 1. $^{60}_{27}Co \rightarrow ^{0}_{-1}e + ^{60}_{28}Ni$

  • Positron Emission: Mass number unchanged, atomic number decreases by 1. $^{11}_{6}C \rightarrow ^{0}_{+1}e + ^{11}_{5}B$

  • Gamma Radiation: Only energy is emitted; mass and atomic numbers unchanged.

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

5.3 Radiation Measurement

Radiation is measured using specialized instruments and units that quantify its activity, absorbed dose, and biological effects.

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

• Units of Measurement:

  • 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: Measures biological effects; adjusted by a factor for radiation type.

  • Sievert (Sv): SI unit for biological damage; $1 Sv = 100 rem$.

• Dosimeters: Devices worn to monitor radiation exposure.

• Average Annual Exposure: Includes natural sources (food, cosmic rays) and medical sources.

• Lethal Dose (LD50): Dose expected to cause death in 50% of exposed individuals.

5.4 Half-Life of a Radioisotope

The half-life is the time required for half of a radioactive sample to decay. This concept is used in dating artifacts and calculating remaining radioisotope amounts.

• Half-Life (t1/2): Time for radiation level to decrease to half its original value.

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

• Calculation Example:

  • After n half-lives, remaining fraction = $\frac{1}{2^n}$

  • Example: If Sr-90 has a half-life of 38.1 years, after 152.4 years (4 half-lives), $\frac{1}{2^4} = \frac{1}{16}$ of the original sample remains.

• Applications: Used in radiometric dating (e.g., carbon-14 for Dead Sea Scrolls).

5.5 Medical Applications Using Radioactivity

Radioisotopes are widely used in medicine for diagnosis and treatment. Their short half-lives make them ideal for imaging and therapy.

• Nuclear Medicine: Radioisotopes are incorporated into cells and detected via emitted radiation.

• Scans: Gamma rays from radioisotopes produce images of organs (e.g., thyroid scans with I-131).

• Positron Emission Tomography (PET): Uses positron emitters to study brain function and metabolism.

• Computed Tomography (CT): Uses X-rays to create layered images of organs.

• Magnetic Resonance Imaging (MRI): Uses magnetic fields and radio waves; does not involve nuclear radiation.

• Brachytherapy: Internal radiation therapy for cancer; uses 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. Fission splits large nuclei, while fusion combines small nuclei.

• Nuclear Fission: Splitting of a large nucleus (e.g., U-235) into smaller nuclei and neutrons; releases energy. $^{235}_{92}U + ^{1}_{0}n \rightarrow ^{140}_{56}Ba + ^{93}_{36}Kr + 3 ^{1}_{0}n$

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

• Nuclear Fusion: Combining small nuclei (e.g., hydrogen isotopes) to form larger nuclei (e.g., helium); occurs in stars. $^{2}_{1}H + ^{3}_{1}H \rightarrow ^{4}_{2}He + ^{1}_{0}n$

• Energy Release: Both fission and fusion release large amounts of energy, but fusion produces less radioactive waste.

Summary Table: Types of Radiation and Shielding

Type of Radiation	Symbol	Penetration	Shielding Required
Alpha	$^{4}_{2}He$	Low	Paper, clothing
Beta	$^{0}_{-1}e$	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)