Overview
This lecture traces the development of atomic theory from ancient Greek philosophers through key scientific experiments to the modern quantum model, highlighting major contributors and their discoveries.
Early Ideas of the Atom
- Leucippus and Democritus (5th century BCE) proposed matter is composed of indivisible particles called "atomos."
- They thought atoms' properties explained material differences (e.g., iron atoms were hard and hook-shaped).
- Their ideas were guesses, not based on experimentation.
18th–19th Century Advances
- Antoine Lavoisier formulated the Law of Conservation of Mass: mass remains constant in chemical reactions.
- John Dalton theorized that elements are made up of discrete packets of matter (atoms).
Discovery of Subatomic Particles
- In the 1870s, discharge tubes revealed cathode rays (later identified as electrons) with negative charge.
- J.J. Thomson measured cathode rays' mass, finding them about 1000x lighter than hydrogen and identified them as negatively charged particles (electrons).
- Thomson's Plum Pudding Model described atoms as electrons scattered in a positively charged "pudding."
Rutherford's Gold Foil Experiment
- Ernest Rutherford (1909) used gold foil and alpha particles, finding most passed through but some deflected sharply.
- Concluded atoms are mostly empty space, with a small dense, positively charged nucleus.
- Identified protons as positively charged, fundamental particles in the nucleus.
The Bohr Model
- Niels Bohr proposed electrons move in specific orbits (energy levels) around the nucleus (Planetary Model).
- Bohr's model explained electron arrangements and energy levels but was only accurate for hydrogen atoms.
Quantum Theory and the Modern Model
- Werner Heisenberg showed you cannot simultaneously know an electron's position and momentum (Uncertainty Principle).
- Quantum Theory describes electrons as existing in "orbitals," regions of probability, not fixed orbits.
- The current Quantum (Cloud) Model illustrates atoms as clouds of electron probability density.
Key Terms & Definitions
- Atomos — Greek term meaning "uncuttable," origin of "atom."
- Law of Conservation of Mass — Mass is neither created nor destroyed in chemical reactions.
- Electron — Negatively charged subatomic particle.
- Plum Pudding Model — Thomson’s model with electrons in a positive matrix.
- Nucleus — Small, dense, positively charged center of the atom.
- Proton — Positively charged particle in the atomic nucleus.
- Bohr Model — Atomic model with electrons in fixed orbits around the nucleus.
- Quantum Model — Modern model describing electron positions as probability clouds.
- Uncertainty Principle — It is impossible to know both the exact position and momentum of a subatomic particle.
Action Items / Next Steps
- Review the main scientists and their contributions to atomic theory.
- Study the strengths and limitations of each atomic model.
- Prepare for further study on nuclear and organic chemistry as indicated.