Overview
This lecture traces the evolution of atomic models from ancient Greek philosophy to modern quantum mechanics, highlighting key scientists and discoveries that changed our understanding of atoms.
Early Concepts of the Atom
- Democritus and Leucippus (ancient Greece) proposed that matter is made of tiny, uncuttable particles called "atomos."
- Democritus thought atoms differed in size and shape, e.g., iron atoms had hooks and salt atoms had spikes.
- Aristotle rejected atomism, instead suggesting everything was made from earth, water, air, fire, and ether.
- The Greek atomic theory was largely ignored for 2,000 years as it lacked experimental evidence.
Dalton’s Atomic Theory
- In 1808, John Dalton used experiments to propose that matter consists of indivisible atoms arranged in different combinations.
- Dalton viewed atoms as solid, indivisible spheres.
Discovery of Subatomic Particles
- Late 1800s: J.J. Thomson discovered electrons, showing atoms contain smaller, negatively charged particles.
- Thomson’s Plum Pudding Model depicted electrons within a positively charged "dough," making the atom neutral overall.
The Nuclear Model
- Early 1900s: Ernest Rutherford’s gold foil experiment revealed that positive charge is concentrated in a small nucleus at the center of the atom.
- Most of the atom is empty space with electrons surrounding the nucleus.
Bohr Model
- In 1913, Niels Bohr proposed that electrons orbit the nucleus in fixed, circular paths (orbits), like planets around the sun.
- This model explained some atomic behaviors but was later found incomplete.
Quantum Mechanical Model
- 1920s: Erwin Schrödinger and others showed electrons move in regions called "orbitals" with complex shapes, not fixed orbits.
- Electrons "buzz" around the nucleus, forming shapes like spheres and dumbbells.
Structure of the Nucleus
- 1919: Rutherford discovered protons in the nucleus.
- 1932: James Chadwick discovered neutrons, completing the basic picture of atomic structure.
Using Atomic Models
- The quantum mechanical model is the most accurate but complicated for simple explanations.
- For basic discussions, the Bohr model is still commonly used, showing electrons in rings around the nucleus.
Key Terms & Definitions
- Atomos (Atoms) — Ancient Greek term meaning "uncuttable"; smallest particle of matter.
- Electron — Negatively charged subatomic particle discovered by J.J. Thomson.
- Plum Pudding Model — Early atomic model with electrons in a positive matrix.
- Nucleus — Dense center of the atom containing protons and neutrons.
- Proton — Positively charged subatomic particle in the nucleus.
- Neutron — Subatomic particle with no charge found in the nucleus.
- Bohr Model — Atomic model with electrons in circular orbits.
- Quantum Mechanical Model — Modern model where electrons occupy orbitals with complex shapes.
Action Items / Next Steps
- Review diagrams comparing atomic models (Dalton, Thomson, Rutherford, Bohr, quantum mechanical).
- Practice drawing both the Bohr and quantum mechanical (orbital) models for simple atoms.
- Read more about the gold foil experiment and its impact on atomic theory.