⚛️Nuclear Physics Unit 1 – Nuclear Physics: Atoms and Their Structure

Atomic Basics

• Atoms are the fundamental building blocks of matter consisting of a dense nucleus surrounded by electrons
• Atomic theory developed over centuries through contributions from scientists like Dalton, Thomson, Rutherford, and Bohr
• Atoms are electrically neutral with an equal number of protons and electrons
• Elements are defined by the number of protons in their nucleus (atomic number)
• Atomic mass is determined by the total number of protons and neutrons in the nucleus
• Atoms can form chemical bonds by sharing, gaining, or losing electrons to achieve a stable electronic configuration
• Atomic size decreases across a period due to increasing effective nuclear charge and increases down a group due to additional electron shells

Structure of the Atom

• Atoms consist of a positively charged nucleus containing protons and neutrons surrounded by negatively charged electrons
• Electrons occupy discrete energy levels or shells around the nucleus (K, L, M, etc.)
• Electron shells have different subshells (s, p, d, f) with specific shapes and orientations
• Electrons fill shells and subshells in a specific order following the Aufbau principle, Hund's rule, and the Pauli exclusion principle
  • Aufbau principle states that electrons fill orbitals in order of increasing energy
  • Hund's rule states that electrons fill orbitals of equal energy singly before pairing up
  • Pauli exclusion principle states that no two electrons in an atom can have the same set of quantum numbers
• Valence electrons in the outermost shell determine an atom's chemical properties and bonding behavior
• Atomic radius, ionization energy, and electronegativity are periodic trends related to the electronic structure of atoms

Subatomic Particles

• Protons are positively charged particles located in the nucleus with a mass of approximately 1 atomic mass unit (amu)
• Neutrons are electrically neutral particles located in the nucleus with a mass slightly greater than protons
• Electrons are negatively charged particles that orbit the nucleus with a mass approximately 1/1836 that of a proton
• Quarks are fundamental particles that make up protons and neutrons
  • Protons consist of two up quarks and one down quark
  • Neutrons consist of one up quark and two down quarks
• Leptons are elementary particles not composed of quarks, including electrons, muons, and neutrinos
• Photons are massless particles that carry electromagnetic radiation, such as light and X-rays
• Antiparticles are counterparts to particles with opposite charge and other properties (positrons, antiprotons)

Nuclear Forces and Stability

• Strong nuclear force is the fundamental force that binds protons and neutrons together in the nucleus
  • Mediated by the exchange of gluons between quarks
  • Strongest of the four fundamental forces but has a very short range
• Electrostatic repulsion between positively charged protons in the nucleus is overcome by the strong nuclear force
• Weak nuclear force is responsible for radioactive decay and neutrino interactions
• Nuclear binding energy is the energy required to break apart a nucleus into its constituent protons and neutrons
• Nuclear stability depends on the ratio of protons to neutrons and the overall number of nucleons
  • Most stable nuclei have a 1:1 ratio of protons to neutrons for light elements and a slightly higher ratio of neutrons for heavier elements
  • Magic numbers of protons or neutrons (2, 8, 20, 28, 50, 82, 126) confer extra stability to nuclei

Isotopes and Nuclear Notation

• Isotopes are atoms of the same element with different numbers of neutrons
• Atomic notation represents an isotope as $^A_Z X$, where X is the chemical symbol, Z is the atomic number (number of protons), and A is the mass number (total number of protons and neutrons)
• Examples of isotope notation include $^{12}_6 C$ (carbon-12), $^{14}_6 C$ (carbon-14), and $^{235}_{92} U$ (uranium-235)
• Isotopes have the same chemical properties but may have different physical properties (radioactivity, half-life)
• Relative atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes
• Isotope abundance is the percentage of an element's atoms that are a specific isotope
• Radioisotopes are unstable isotopes that undergo radioactive decay, emitting particles and energy

Radioactivity and Decay

• Radioactivity is the spontaneous emission of particles or energy from an unstable atomic nucleus
• Types of radioactive decay include alpha decay (emission of $^4_2 He$ nucleus), beta decay (emission of electron or positron), and gamma decay (emission of high-energy photons)
  • Alpha decay decreases the mass number by 4 and the atomic number by 2
  • Beta minus decay increases the atomic number by 1 while keeping the mass number constant
  • Beta plus decay decreases the atomic number by 1 while keeping the mass number constant
  • Gamma decay does not change the mass number or atomic number
• Half-life is the time required for half of a given quantity of a radioactive isotope to decay
• Decay rate is the number of decay events per unit time, proportional to the number of radioactive atoms present
• Radioactive dating uses the predictable decay of radioisotopes to determine the age of materials (carbon dating, uranium-lead dating)
• Radiation exposure can cause biological damage, with effects depending on the type and dose of radiation

Nuclear Reactions and Energy

• Nuclear reactions involve changes in the composition of atomic nuclei, often accompanied by the release or absorption of energy
• Fusion reactions combine light nuclei to form heavier nuclei, releasing large amounts of energy
  • Fusion powers the Sun and other stars, converting hydrogen into helium
  • Controlled fusion is a potential source of clean, abundant energy but faces technical challenges
• Fission reactions split heavy nuclei into lighter fragments, also releasing energy
  • Nuclear power plants use controlled fission of uranium or plutonium to generate electricity
  • Nuclear weapons rely on uncontrolled fission reactions to create explosive yields
• Mass-energy equivalence, expressed by Einstein's equation $E=mc^2$, relates the mass defect in nuclear reactions to the energy released
• Binding energy per nucleon is the energy required to disassemble a nucleus divided by the number of nucleons, indicating nuclear stability
• Transmutation is the conversion of one element into another through nuclear reactions (bombardment with particles, radioactive decay)

Applications in Nuclear Physics

• Nuclear power generation uses heat from fission reactions to produce steam and drive turbines
• Nuclear medicine uses radioisotopes for diagnostic imaging (PET scans, SPECT) and targeted radiation therapy for cancer treatment
• Radioisotope thermoelectric generators (RTGs) convert heat from radioactive decay into electricity for spacecraft and remote applications
• Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) use nuclear spin properties for chemical analysis and medical imaging
• Radiocarbon dating and other radiometric dating methods determine the age of organic materials and geological samples
• Nuclear weapons, while destructive, have also led to advances in nuclear physics and international treaties on non-proliferation
• Particle accelerators, such as the Large Hadron Collider, enable the study of high-energy nuclear reactions and the search for new particles
• Radiation detectors, including Geiger counters and scintillators, are used for safety monitoring, scientific research, and industrial applications