☢️Radiochemistry Unit 2 – Atomic Structure and Nuclear Stability

Fundamental Concepts

• Atoms consist of protons, neutrons, and electrons which determine an element's properties
• Protons have a positive charge, neutrons are neutral, and electrons have a negative charge
• Atomic number (Z) represents the number of protons in an atom's nucleus
• Mass number (A) is the sum of protons and neutrons in an atom's nucleus
• Isotopes are atoms of the same element with different numbers of neutrons
• Atomic mass is the average mass of an element's isotopes weighted by their natural abundances
• Radioactivity is the spontaneous emission of radiation from an unstable atomic nucleus
• Radioactive decay occurs when an unstable nucleus releases energy in the form of particles or electromagnetic radiation

Atomic Structure Basics

• Electrons occupy discrete energy levels or shells (K, L, M, etc.) around the nucleus
  • The K shell is closest to the nucleus and has the lowest energy
  • Subsequent shells (L, M, N, etc.) have increasing energy levels and are farther from the nucleus
• Electron configuration describes the arrangement of electrons in an atom's shells and subshells
• Valence electrons in the outermost shell determine an element's chemical properties and reactivity
• The Bohr model depicts electrons orbiting the nucleus in fixed circular paths
• The quantum mechanical model describes electrons as probability waves in orbitals
  • Orbitals are regions in space where electrons are most likely to be found
  • Orbitals are characterized by quantum numbers (principal, angular momentum, magnetic, and spin)
• The Aufbau principle states that electrons fill orbitals in order of increasing energy
• Hund's rule specifies that electrons occupy orbitals of the same energy singly before pairing up

Nuclear Stability and Instability

• Nuclear stability depends on the ratio of protons to neutrons in the nucleus
• The band of stability represents the range of proton and neutron numbers that result in stable nuclei
• Nuclei with too many or too few neutrons relative to protons are unstable and prone to radioactive decay
• The strong nuclear force holds protons and neutrons together in the nucleus
• The Coulomb force causes electrostatic repulsion between positively charged protons in the nucleus
• Nuclear binding energy is the energy required to break apart a nucleus into its constituent protons and neutrons
• Mass defect is the difference between the mass of a nucleus and the sum of its individual proton and neutron masses
• The liquid drop model compares the nucleus to a drop of liquid held together by surface tension
• Magic numbers (2, 8, 20, 28, 50, 82, 126) correspond to particularly stable nuclear configurations

Radioactive Decay Processes

• Alpha decay involves the emission of an alpha particle (two protons and two neutrons) from the nucleus
  • Alpha particles have a positive charge and are relatively heavy and slow-moving
  • Alpha decay typically occurs in heavy nuclei with high atomic numbers (uranium, radium)
• Beta decay involves the emission of a beta particle (electron or positron) and an antineutrino or neutrino
  • Beta minus ($\beta^-$) decay occurs when a neutron converts into a proton, emitting an electron and an antineutrino
  • Beta plus ($\beta^+$) decay occurs when a proton converts into a neutron, emitting a positron and a neutrino
• Gamma decay involves the emission of high-energy photons (gamma rays) from an excited nucleus
  • Gamma decay often accompanies alpha or beta decay as the nucleus transitions to a lower energy state
• Spontaneous fission is the splitting of a heavy nucleus into two smaller nuclei and neutrons
• Decay chains are series of radioactive decays that occur until a stable nucleus is reached
• Half-life is the time required for half of a given quantity of a radioactive substance to decay

Nuclear Reactions and Energy

• Nuclear reactions involve changes in the composition of atomic nuclei
• Fusion reactions combine light nuclei to form heavier nuclei, releasing energy (stars, hydrogen bombs)
• Fission reactions split heavy nuclei into lighter nuclei, releasing energy (nuclear reactors, atomic bombs)
• Nuclear transmutation is the conversion of one element into another through nuclear reactions
• Nuclear cross-section is a measure of the probability that a particular nuclear reaction will occur
• Q-value is the energy released or absorbed in a nuclear reaction
• Mass-energy equivalence, expressed by Einstein's equation $E=mc^2$, relates mass and energy
• Binding energy per nucleon is the average energy required to remove a nucleon from a nucleus
• Nuclear power plants generate electricity through controlled fission reactions in nuclear reactors

Isotopes and Their Applications

• Isotopes have the same number of protons but different numbers of neutrons
• Radioisotopes are unstable isotopes that undergo radioactive decay
• Stable isotopes do not undergo radioactive decay and have a constant abundance in nature
• Isotope notation is written as $^A_Z\text{X}$, where X is the element symbol, A is the mass number, and Z is the atomic number
• Radioisotopes are used in nuclear medicine for diagnostic imaging (technetium-99m) and targeted therapy (iodine-131)
• Stable isotopes are used as tracers in environmental studies (carbon-13) and metabolic research (deuterium)
• Isotope ratios can be used for dating geological samples (carbon-14) and determining the origin of materials (oxygen-18)
• Isotope enrichment techniques (gas centrifugation, laser separation) increase the abundance of a specific isotope
• Isotope labeling involves incorporating a specific isotope into a compound for tracking or analysis

Measurement and Detection Techniques

• Geiger-Müller counters detect ionizing radiation by measuring electrical pulses in a gas-filled tube
• Scintillation detectors use materials that emit light when exposed to radiation (sodium iodide, plastic)
  • Photomultiplier tubes amplify the light signals from scintillation detectors into measurable electrical signals
• Semiconductor detectors (germanium, silicon) measure radiation through the creation of electron-hole pairs
• Neutron activation analysis identifies elements in a sample by measuring gamma rays emitted after neutron irradiation
• Autoradiography uses the exposure of photographic film or imaging plates to detect the distribution of radioactivity in a sample
• Liquid scintillation counting measures the radioactivity of liquid samples mixed with a scintillator
• Alpha spectrometry measures the energy and intensity of alpha particles emitted by a sample
• Gamma spectrometry measures the energy and intensity of gamma rays emitted by a sample
• Radiation dosimetry quantifies the absorbed dose of ionizing radiation using devices like thermoluminescent dosimeters (TLDs) or optically stimulated luminescence (OSL) dosimeters

Safety and Environmental Considerations

• Radiation exposure can cause biological damage, including DNA mutations and cell death
• The principles of radiation protection are time, distance, and shielding
  • Minimize time spent near radiation sources
  • Maximize distance from radiation sources
  • Use appropriate shielding materials (lead, concrete) to reduce exposure
• The ALARA principle (As Low As Reasonably Achievable) guides radiation safety practices
• Radiation dose limits are set by regulatory agencies to protect workers and the public
• Contamination occurs when radioactive materials are deposited on surfaces or incorporated into objects
• Decontamination involves the removal of radioactive contamination from surfaces or equipment
• Radioactive waste management involves the safe handling, storage, and disposal of radioactive materials
• Environmental monitoring assesses the levels of radioactivity in air, water, soil, and biota
• Nuclear accidents (Chernobyl, Fukushima) can release radioactive materials into the environment
• Radon is a naturally occurring radioactive gas that can accumulate in buildings and pose health risks