Overview
This lecture traces the evolution of atomic theory from ancient Greece to modern quantum physics, highlighting key contributors and discoveries shaping our understanding of what matter is made of.
Early Atomic Theory
- Around 440 BCE, Democritus proposed that all matter consists of indivisible particles called "atomos."
- He believed atoms differed in size and shape for different substances and existed in empty space.
- Aristotle opposed this, claiming matter consisted of four elements: earth, wind, water, and fire.
- The atomic idea faded until revived centuries later.
Dalton’s Atomic Model
- In 1808, John Dalton demonstrated that substances break down into consistent elements in fixed proportions.
- Dalton concluded compounds are combinations of unique atoms, each with a particular size and mass.
- He stated atoms could not be created or destroyed.
Discovery of the Electron
- In 1897, J.J. Thompson discovered the electron, showing atoms as positive spheres with embedded negative electrons (the "chocolate chip cookie" model).
- Thompson received the Nobel Prize for identifying the electron.
Rutherford’s Nuclear Model
- Ernest Rutherford, a student of Thompson, conducted gold foil experiments that revealed atoms are mostly empty space.
- He discovered a dense, positively charged nucleus at the atom's center.
- Electrons orbited far from this nucleus.
Bohr’s Planetary Model
- In 1913, Niels Bohr refined the model: electrons orbit the nucleus at specific energies and cannot exist between levels.
- His model drew on Planck's and Einstein's work on energy and photons.
Quantum Model and Heisenberg
- Experiments showed electrons behave as both particles and waves.
- Werner Heisenberg’s uncertainty principle established that an electron’s exact position and speed cannot both be known.
- This led to the quantum model: electrons exist in probable regions around the nucleus, not fixed paths.
Atoms and Light
- Electrons moving between energy levels absorb or emit light at specific wavelengths, producing visible colors (like fireworks).
Key Terms & Definitions
- Atomos — The indivisible particles proposed by Democritus as the building blocks of matter.
- Electron — A negatively charged subatomic particle discovered by Thompson.
- Nucleus — The dense, positively charged center of the atom discovered by Rutherford.
- Uncertainty Principle — Heisenberg’s idea that one cannot simultaneously know an electron’s exact position and speed.
- Quantum Model — The current atomic model where electrons exist within probability zones rather than defined orbits.
Action Items / Next Steps
- Review key atomic models and their differences.
- Study the roles of Democritus, Dalton, Thompson, Rutherford, Bohr, and Heisenberg in atomic theory development.