Overview
This lecture covers the historical development of atomic theory, from ancient Greek ideas to the modern quantum model, highlighting key experiments and scientists involved.
Early Atomic Theory
- Leucippus and Democritus (around 400 BCE) proposed matter is composed of indivisible particles called "atomos."
- They believed different materials had different types of atoms with properties matching the substance (e.g., iron atoms are hard and hooked).
18th–19th Century Advances
- Antoine Lavoisier introduced the law of conservation of mass: mass remains constant during chemical changes.
- John Dalton stated that elements exist as discrete packets of matter, supporting atomic theory.
Discovery of Subatomic Particles
- In the late 1800s, discharge tubes showed matter produced rays of negative (cathode rays) and positive charge.
- J.J. Thomson identified cathode rays as negatively charged particles (electrons) much lighter than hydrogen atoms.
- Thomson's "plum pudding model" depicted electrons embedded in a positively charged matrix.
Rutherford's Nuclear Model
- Ernest Rutherford's gold foil experiment (1909) showed that atoms have a small, dense, positively charged nucleus.
- Most of the atom is empty space; positive charge is concentrated in the nucleus.
- Rutherford later identified protons as fundamental positive particles in atoms.
Bohr Model and Energy Levels
- Niels Bohr applied mathematical models to propose that electrons orbit the nucleus in discrete energy levels.
- Bohr's "planetary model" introduced the idea of quantized orbits for electrons.
Quantum Model and Modern Understanding
- Werner Heisenberg showed it is impossible to know both an electron’s position and momentum simultaneously (Heisenberg uncertainty principle).
- Modern quantum theory describes electrons as existing in probabilistic regions called orbitals, forming the "cloud model" of the atom.
- Orbitals are regions with high probability of finding electrons; named s, p, d, and f.
Key Terms & Definitions
- Atomos — Greek term meaning indivisible; first concept of atoms.
- Law of Conservation of Mass — Mass remains unchanged during chemical reactions.
- Cathode Ray — Stream of electrons emitted from the negative electrode in a discharge tube.
- Electron — Negatively charged subatomic particle.
- Plum Pudding Model — Atomic model with electrons in a positive matrix.
- Nucleus — Small, dense, positively charged center of an atom.
- Proton — Positively charged subatomic particle in the nucleus.
- Bohr Model — Atomic model with electrons in fixed energy orbits.
- Heisenberg Uncertainty Principle — Cannot know both the position and momentum of a particle exactly.
- Orbital — Region around the nucleus where electrons are likely to be found.
- Quantum Model (Cloud Model) — Modern atomic model based on probabilities.
Action Items / Next Steps
- Review orbital types (s, p, d, f) and their characteristics.
- Prepare for upcoming lessons on nuclear and organic chemistry.