Chapter 17: Radioactivity and Nuclear Chemistry – Study Notes

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Radioactivity and Nuclear Chemistry

Introduction to Radioactivity

Radioactivity is a phenomenon in which unstable atomic nuclei emit energetic particles or electromagnetic radiation. This process is fundamental to nuclear chemistry and has significant applications in medicine, energy, and dating ancient artifacts.

Radioactivity is the emission of tiny, energetic particles by the nuclei of certain unstable atoms.
Atoms that emit these particles are called radioactive.
Radioactivity is used in nuclear medicine for imaging and treatment.

Radioactivity is the emission of particles by the nuclei of certain atoms.

Discovery of Radioactivity

Becquerel and Curie

The discovery of radioactivity began with Antoine-Henri Becquerel and was advanced by Marie Curie.

Becquerel discovered that uranium salts emitted rays that could expose photographic plates, even in the absence of light.
Marie Curie discovered two new radioactive elements: polonium and radium.
Curie coined the term radioactivity and received two Nobel Prizes for her work.

Types of Radioactive Emissions

Alpha, Beta, Gamma, and Positron Emissions

Radioactive decay can occur in several forms, each with distinct properties and effects.

Alpha (α) decay: Emission of an alpha particle (2 protons and 2 neutrons, equivalent to a helium-4 nucleus).
Beta (β) decay: Emission of an electron when a neutron converts to a proton.
Gamma (γ) emission: Emission of high-energy photons (electromagnetic radiation).
Positron emission: Emission of a positron when a proton converts to a neutron.

Alpha decay: emission of a particle composed of two protons and two neutrons (a helium-4 nucleus). Beta decay: a neutron becomes a proton and an electron is emitted from the nucleus. Positron emission: a proton becomes a neutron and a positron is emitted from the nucleus.

Nuclear Equations

Writing and Balancing Nuclear Equations

Nuclear equations represent the changes in the nucleus during radioactive decay. They must be balanced for both atomic number and mass number.

The parent nuclide is the original atom; the daughter nuclide is the product after decay.
In alpha decay, the atomic number decreases by 2 and the mass number by 4.
In beta decay, the atomic number increases by 1; in positron emission, it decreases by 1.

Nuclear equation for the alpha decay of uranium-238.

Example: Alpha Decay of Uranium-238

Alpha decay: parent nuclide emits an alpha particle to form a daughter nuclide.

Example: Beta Decay

Beta decay: neutron becomes a proton and a beta particle is emitted.

Example: Gamma Emission

Gamma emission: excited nuclide emits a photon to become a stable nuclide.

Example: Positron Emission

Positron emission: proton becomes a neutron and a positron is emitted.

Properties and Effects of Radiation

Ionizing and Penetrating Power

Different types of radiation have varying abilities to ionize molecules and penetrate matter.

Alpha particles: High ionizing power, low penetrating power (stopped by paper or skin).
Beta particles: Moderate ionizing and penetrating power (stopped by metal or thick wood).
Gamma rays: Low ionizing power, high penetrating power (requires lead or concrete to stop).
Positrons: Similar to beta particles in ionizing and penetrating power.

Detecting Radioactivity

Detection Methods

Radioactivity can be detected using specialized instruments.

Film-badge dosimeters: Measure cumulative exposure over time.
Geiger-Müller counter: Detects ionizing particles by measuring electrical pulses generated in a gas-filled tube.
Scintillation counter: Measures light flashes produced when radiation interacts with certain materials.

Geiger-Müller counter: diagram and examples of radiation detection devices.

Natural Radioactivity and Half-Life

Background Radiation and Decay Rates

Radioactive elements are present in the environment and decay at characteristic rates, described by their half-lives.

Half-life: The time required for half of the parent nuclides in a sample to decay.
Each nuclide has a unique half-life, unaffected by physical or chemical conditions.

Example: Thorium-232 has a half-life of 14 billion years.

Graph showing the half-life decay of Th-232 over billions of years.

Radioactive Decay Series

Decay Chains

Some radioactive elements decay through a series of steps, producing a sequence of daughter nuclides until a stable isotope is formed.

Uranium-238 decays through multiple steps to form lead-206.
Intermediate products include thorium, protactinium, and radon.

Decay series of uranium-238 showing sequential radioactive decays.

Applications of Radioactivity

Environmental Radon

Radon is a radioactive gas produced in the uranium decay series. It can accumulate in buildings and increase lung cancer risk.

Radon-222 has a half-life of 3.8 days.
High radon levels are found in areas with uranium-rich soil.

EPA map of radon zones in the United States.

Radiocarbon Dating

Radiocarbon dating uses the decay of carbon-14 to estimate the age of formerly living materials.

Carbon-14 is formed in the atmosphere and incorporated into living organisms.
After death, the amount of carbon-14 decreases with a half-life of 5715 years.
The ratio of carbon-14 to carbon-12 in a sample indicates its age.

Nuclear Fission and Fusion

Nuclear Fission

Nuclear fission is the splitting of a heavy nucleus into smaller fragments, releasing energy and neutrons.

Discovered by Meitner, Strassmann, and Hahn in 1939.
Uranium-235 undergoes fission when bombarded with neutrons, producing barium, krypton, and more neutrons.
Fission can lead to a chain reaction, which is the basis for nuclear reactors and atomic bombs.

Nuclear equation for neutron bombardment of uranium. Diagram of nuclear fission: neutron causes nucleus to split, releasing energy and more neutrons. Fission chain reaction: one fission event triggers others.

Nuclear Power Generation

Nuclear power plants use controlled fission reactions to generate electricity.

Fuel rods containing enriched uranium-235 are used in the reactor core.
Control rods absorb neutrons to regulate the chain reaction.
Heat from fission is used to produce steam, which drives turbines to generate electricity.

Diagram of a nuclear reactor showing fuel rods, control rods, and steam generation.

Nuclear Fusion

Nuclear fusion is the combination of two light nuclei to form a heavier nucleus, releasing vast amounts of energy.

Fusion powers stars, including the sun.
Requires extremely high temperatures to overcome repulsion between nuclei.
Fusion is the basis for hydrogen bombs and is being researched for power generation.

Biological Effects and Medical Uses of Radiation

Effects of Radiation on Life

Radiation can damage living cells, leading to acute effects, increased cancer risk, and potential genetic defects.

High doses cause acute radiation sickness and can be fatal.
Lower doses over time increase cancer risk by damaging DNA.
Genetic defects may occur if reproductive cells are affected.

Measuring Radiation Exposure

Curie (Ci): Measures decay events per second.
Roentgen (R): Measures ionization in air.
Rem: Accounts for ionizing power and biological effect.

Radioactivity in Medicine

Radioactive isotopes are used for diagnosis and treatment in medicine.

Technetium-99 is used for bone scans.
Phosphorus-32 and iodine-131 are used for imaging and diagnosing tumors and thyroid disorders.
Gamma rays from cobalt-60 are used to treat cancer by targeting tumors.

Isotope scan using Technetium-99 for bone imaging. Gamma ray treatment for cancer.

Summary Table: Types of Radioactive Decay

Type	Change in Nucleus	Ionizing Power	Penetrating Power
Alpha (α)	−2 protons, −2 neutrons	High	Low
Beta (β)	Neutron → Proton	Intermediate	Intermediate
Gamma (γ)	No change (energy only)	Low	High
Positron	Proton → Neutron	Intermediate	Intermediate

Key Learning Outcomes

Explain the discovery and nature of radioactivity.
Write and balance nuclear equations for various types of decay.
Describe methods for detecting radioactivity.
Use half-life to calculate remaining radioactive material.
Understand applications such as radiocarbon dating and nuclear power.
Compare nuclear fission and fusion.
Describe the effects of radiation on living organisms and its medical uses.