Overview
This lecture traces the development of atomic theory, from ancient Greek ideas to the quantum model, highlighting key discoveries and evolving models of atomic structure.
Ancient Beginnings: The Idea of the Atom
- Leucippus and Democritus proposed that matter is made of indivisible particles called "atomos" ("uncuttable").
- They believed substance properties were due to the shapes and connections of their atoms (e.g., iron atoms were hard and hooked).
Foundations of Atomic Theory
- Atomic theory developed slowly, with major advances occurring over 2000 years after the Greeks.
- Antoine Lavoisier proposed the law of conservation of mass: matter's mass remains constant during physical or chemical changes.
- John Dalton determined that elements exist as discrete packets (atoms).
Discovering Subatomic Particles
- Discharge tubes (gas-filled tubes with electrodes) revealed both negatively and positively charged particles in matter.
- J.J. Thompson identified cathode rays as negatively charged particles much lighter than hydrogen, naming them "corpuscles" (now called electrons).
- Eugen Goldstein found rays from the positive electrode, hinting at positive charges.
Early Atomic Models
- Thompson proposed the "plum pudding model": atoms have electrons distributed in a positively charged matrix.
- Ernest Rutherford's 1909 gold foil experiment showed atoms have a small, dense, positively charged nucleus, and are mostly empty space.
- Rutherford later identified protons as fundamental, positively charged particles in the nucleus.
The Planetary and Quantum Models
- Niels Bohr applied physics and math to atomic models, introducing quantized orbits for electrons (planetary model).
- Bohr’s model described electrons in fixed orbits with specific energy levels.
- Werner Heisenberg showed the impossibility of knowing both electron position and momentum exactly (Heisenberg Uncertainty Principle).
- Quantum theory describes electrons as existing in probability-based regions called "orbitals" (s, p, d, f).
- The modern "cloud model" visualizes electron probability, not fixed paths.
The Ongoing Process of Science
- Each model built on previous discoveries, but scientific understanding continues to evolve.
- Confidence in current models is high, but future discoveries may still alter our understanding.
Key Terms & Definitions
- Atomos — Greek term meaning "uncuttable"; early concept of the smallest matter particle.
- Law of Conservation of Mass — Mass remains constant during any physical or chemical process.
- Electron — Negatively charged subatomic particle discovered by J.J. Thompson.
- Proton — Positively charged subatomic particle discovered by Ernest Rutherford.
- Plum Pudding Model — Atom model with electrons embedded in a positive matrix.
- Nucleus — Dense, central part of the atom containing protons (and later discovered, neutrons).
- Planetary Model — Bohr's model describing electrons orbiting the nucleus in fixed energy levels.
- Heisenberg Uncertainty Principle — It is impossible to know both the position and momentum of a particle exactly at the same time.
- Quantum Theory — Modern theory where electrons exist in orbitals defined by probability, not fixed paths.
- Orbital — Region around the nucleus where an electron is most likely to be found.
Action Items / Next Steps
- Review the development of atomic models and their experimental supports.
- Study the definitions and characteristics of subatomic particles and atomic models.
- Prepare for upcoming lessons on nuclear and organic chemistry.