Overview
This lecture covers key historical models of the atom, types of radiation, nuclear decay processes, and introduces concepts such as half-life, mass-energy equivalence, nuclear fission, and nuclear fusion.
Historical Atomic Models
- Democritus proposed that matter consists of indivisible particles called atoms.
- Aristotle believed matter is infinitely divisible and composed of five elements.
- Dalton's Billiard Ball Model described atoms as solid, combinable masses.
- Thomson's Plum Pudding Model theorized atoms as positive mass with embedded negative electrons.
- Rutherford's Gold Foil Experiment showed atoms have a small, dense, positively charged nucleus with orbiting electrons.
- The Rutherford Model could not explain electron stability, atomic spectra, or the photoelectric effect.
- The Bohr Model introduced electrons in distinct orbits with quantized energies.
Periodic Table and Atomic Structure
- Atomic number equals number of protons; atomic mass is protons plus neutrons.
- Atomic mass is often a decimal due to averaging isotopic abundance.
- Protons = atomic number, electrons = atomic number (if neutral), neutrons = atomic mass - atomic number.
Isotopes
- Isotopes are atoms of the same element with different numbers of neutrons.
- Example: Hydrogen-1, Hydrogen-2 (Deuterium), Hydrogen-3 (Tritium).
Types of Radiation
- Alpha radiation: 2 protons and 2 neutrons (He nucleus), large, slow, and positively charged.
- Beta radiation: fast-moving electron, negatively charged.
- Gamma radiation: high-energy electromagnetic wave, no charge.
Radiation in Electric and Magnetic Fields
- Beta particles deflect strongly due to low mass and negative charge.
- Alpha particles deflect weakly due to high mass and positive charge.
- Gamma rays are unaffected as they're uncharged.
- The right-hand rule determines direction of force on charges in magnetic fields.
Radiation Penetration and Ionization
- Alpha stopped by paper; beta by aluminium; gamma mostly by lead.
- Alpha has high ionization, beta moderate, gamma low.
- Direct ionization: kinetic ejection of electrons (alpha/beta).
- Indirect ionization: via secondary processes (gamma).
Radioactive Decay
- Alpha decay: atomic mass decreases by 4, atomic number by 2.
- Beta decay: neutron becomes proton (atomic number increases by 1), electron emitted.
- Gamma decay: nucleus releases excess energy, no change in mass or atomic number.
Detection and Visualization of Radiation
- Cloud chambers visualize charged particle tracks from radiation.
- Alpha leaves thick tracks; beta leaves thin tracks; neutrinos leave no track.
Half-Life
- Half-life: time for half of a radioactive sample to decay; it's statistical, not exact.
- Radioactive decay follows an exponential decrease on a graph.
Mass-Energy Equivalence
- Energy and mass are related by E = mc².
- 1 kg of mass releases a huge amount of energy.
Nuclear Fission and Fusion
- Fission: splitting a nucleus into smaller nuclei, releases energy (spontaneous or induced).
- Chain reactions in fission sustain energy production; uncontrolled in weapons, controlled in reactors.
- Fusion: combining light nuclei to form a heavier nucleus, releases energy for elements lighter than iron.
Key Terms & Definitions
- Atom — Smallest unit of matter, indivisible in chemical processes.
- Isotope — Atoms of the same element with different neutron numbers.
- Alpha Radiation — Helium nucleus (2 protons, 2 neutrons), highly ionizing.
- Beta Radiation — High-energy electron emitted from a neutron.
- Gamma Radiation — Electromagnetic wave, high penetration, low ionization.
- Half-life — Time for half of the unstable nuclei in a sample to decay.
- Fission — Splitting of a heavy nucleus into lighter nuclei.
- Fusion — Combining lighter nuclei to form a heavier nucleus.
- E = mc² — Equation for mass-energy equivalence.
Action Items / Next Steps
- Review provided PDF notes and corrections.
- Prepare for Level 3 topics: atomic spectra and the photoelectric effect.