Chem1- Unit 4.2 Nuclear Radioactivity

Unit Overview

• Unit 4: Going Nuclear!

• Topics covered: sub-atomic particles, radioactivity, nuclear chemistry, and large-scale nuclear weapons.

Part I: Subatomic Particles

• Definition and types of subatomic particles (protons, neutrons, electrons).

• Structure of the atom discussed in context of nuclear chemistry.

Part II: Nuclear Radioactivity

Introduction

• Discovery of nuclear radiation while studying ionizing radiation.

• Ionizing Radiation: Radiation that causes ionization in the medium; damages cells by breaking DNA.

• Key discoveries:

  • Wilhelm Röntgen discovered X-rays.

  • Henri Becquerel discovered radioactivity in uranium.

Discovery of Radioactivity

• First ionizing radiation: X-rays.

• Röntgen's experiments using a cathode ray tube confirmed radiation's ability to penetrate materials.

• Becquerel’s findings highlighted that uranium spontaneously emits rays, later named 'radioactivity' by Marie Curie.

Types of Ionizing Radiation

• Rutherford categorized three main types of ionizing radiation:

  • Alpha Radiation: Positively charged; blocked by paper.

  • Beta Radiation: Negatively charged; blocked by aluminum foil.

  • Gamma Radiation: Neutral; penetrates materials, requires lead shielding.

Decay Types

• 9 types of radioactive decay modes exist, distinguished by the type of particles emitted or absorbed during decay.

• Decay modes discussed include:

  • Alpha decay

  • Beta decay (β- and β+)

  • Electron capture (EC)

  • Gamma decay

  • Neutron emission

  • Cluster decay

  • Spontaneous fission

Decay Mechanisms

Alpha Decay

• Ejects an alpha particle: 2 protons, 2 neutrons (helium nucleus).

• Causes a decrease in atomic number by 2, mass by 4.

Beta Decay

• Beta- decay: Ejects high-energy electron, increases atomic number by 1; no mass change.

• Beta+ decay: Ejects a positron (antimatter); decreases atomic number by 1.

• Electron Capture: Absorbs an orbiting electron, also decreases atomic number by 1.

Gamma Decay

• Emission of high-energy photon; no change in atomic or mass number.

• Nucleons become more stable.

Cluster Decay and Spontaneous Fission

• Cluster decay: Emission of nuclear fragment larger than alpha; occurs in large nuclei.

• Spontaneous fission: Rare decay mode in large, heavy nuclei.

Factors Affecting Stability

Magic Numbers

• Nuclei are often stable with even numbers of protons/neutrons.

• Stable isotopes: Sn (Tin) has the most stable isotopes (10).

Size and Composition

• Larger atoms are generally more unstable due to increased electrostatic repulsion.

• Stable isotopes exist for elements up to atomic number 83 (bismuth).

Nucleon Interactions

• Binding dynamics: Strong nuclear force binds protons and neutrons; however, it weakens at larger distances, leading to instability.

Unstable Nuclei

• Nuclei that are too neutron-heavy will experience beta decay or neutron ejection.

• Proton-heavy nuclei may beta+ decay or undergo electron capture.

• Large nuclei typically eject alpha particles to gain stability.

Measurement of Radioactivity

Half-Life

• Defined as the time for half of a sample of radioactive material to decay.

• Some isotopes have extremely long half-lives (e.g., Thorium-232: 14 billion years) while others have very short ones (e.g., Radon-220: 1 minute).

Evaluating Decay Events

• While individual decay events are random, predictable decay behavior emerges at large sample sizes over extended periods.

Applications of Nuclear Radioactivity

Medical Uses

• Used in diagnosis (scintigraphy, PET scans) and treatment (targeting tumors with gamma rays).

Other Uses

• Radiocarbon dating to determine the age of organic materials.

Summary of Learning Goals

• Assess dangers associated with ionizing radiation.

• Differentiate types and causes of radioactive decay.

• Predict daughter nuclei post-decay.

• Calculate half-lives and discuss stable arrangements.

• Explore beneficial uses of radioactivity in medicine.