Overview
This lecture covers the historical development of atomic theory, highlighting major scientists, their models, and key experiments that shaped our modern understanding of the atom's structure.
Early Atomic Theory
- Ancient Greek philosopher Democritus (c. 500 BC) proposed that matter is made of tiny, indivisible particles called atoms separated by empty space.
- Atomic theory remained largely unchanged until the 1800s.
Dalton’s Atomic Model
- In the early 1800s, John Dalton described atoms as solid spheres.
- Dalton suggested different elements are made up of different types of atoms.
Thomson’s Plum Pudding Model
- In 1897, J.J. Thomson discovered electrons through experiments, proving atoms were not solid spheres.
- Proposed the plum pudding model: atoms as balls of positive charge with embedded electrons.
Rutherford’s Nuclear Model
- In 1909, Ernest Rutherford and his team fired alpha particles at gold foil, expecting most to pass through.
- Some particles were deflected or bounced back, disproving the plum pudding model.
- Rutherford proposed atoms have a dense, positively charged nucleus with electrons around it.
Bohr’s Model and Further Discoveries
- In 1913, Niels Bohr suggested electrons orbit the nucleus in shells, explaining atomic stability.
- Rutherford later found the nucleus is composed of protons (positively charged particles).
- In 1932, James Chadwick discovered neutrons, completing the model of the nucleus.
Key Terms & Definitions
- Atom — the smallest unit of matter, indivisible by chemical means.
- Electron — negatively charged particle found in atoms.
- Nucleus — dense central part of an atom containing protons and neutrons.
- Proton — positively charged particle in the atomic nucleus.
- Neutron — neutrally charged particle in the atomic nucleus.
- Plum pudding model — early model of the atom with electrons in a sphere of positive charge.
- Alpha particle — a positively charged particle used in Rutherford’s gold foil experiment.
Action Items / Next Steps
- Review each atomic model and the experiments that led to their development.
- Study the key definitions and be able to compare different atomic theories.