Overview
This lecture traces the historical development of atomic theory, highlighting key experiments and models that shaped our current understanding of atomic structure.
Early Atomic Concepts
- Greek philosophers Leucippus and Democritus proposed matter is made of indivisible particles called "atomos."
- They believed the properties of substances were due to the forms of their atoms (e.g., hard iron atoms, flexible clay atoms).
Advances in Atomic Theory (1700s-1800s)
- Antoine Lavoisier established the law of conservation of mass: matter changes form but mass stays the same.
- John Dalton proposed elements exist as discrete packets of matter, advancing atomic theory.
Discovery of Subatomic Particles
- Discharge tubes (gas-filled tubes with electrodes) revealed negatively charged cathode rays and, later, positive rays.
- J.J. Thompson identified cathode rays as negatively charged, extremely light particles, now known as electrons.
- Thompson’s "plum pudding model" depicted electrons embedded randomly in a positive matrix.
- Eugen Goldstein discovered positive rays, implying positive charges in matter.
The Nucleus and Structure of the Atom
- Ernest Rutherford’s gold foil experiment showed atoms are mostly empty space with a tiny, dense, positively charged nucleus.
- Rutherford identified protons as the fundamental positive particles in the nucleus.
Models of Electron Arrangement
- Niels Bohr built on Rutherford’s nuclear model and quantum theory, proposing electrons move in specific orbits with particular energy levels (planetary model).
- Each electron orbit corresponds to a specific energy state.
Quantum Model and Modern Understanding
- Werner Heisenberg’s uncertainty principle states it is impossible to know both the exact position and momentum of an electron.
- Electrons are described by probability regions called orbitals (e.g., s, p, d, f), not definite paths.
- The "cloud model" or quantum model visualizes electrons as probability clouds rather than orbits.
Key Terms & Definitions
- Atomos — Greek term meaning "uncuttable," basis for the word "atom."
- Electron — Negatively charged subatomic particle discovered by J.J. Thompson.
- Proton — Positively charged subatomic particle located in the nucleus.
- Nucleus — Dense, positively charged center of the atom.
- Plum pudding model — Thompson’s model with electrons in a positive matrix.
- Planetary model — Bohr’s model with electrons in orbits around the nucleus.
- Quantum model — Current atomic model, describes electron positions as probability clouds.
- Orbital — Region around a nucleus where an electron is likely to be found.
Action Items / Next Steps
- Review the models of atomic structure and their key differences.
- Prepare for lessons on nuclear chemistry and basics of organic chemistry.
- Keep questioning and exploring to further understand atomic theory.