Introduction to Atomic Theory

Early Theories of Matter

• Democritus
  • Coined the term "Atomos"
  • Proposed that all matter is made up of tiny, indivisible particles
  • Each type of particle retains the properties of the substance
  • Prior belief: matter made up of four elements (earth, water, air, fire)

John Dalton's Atomic Theory

• Built on Democritus' ideas with scientific basis
• Atoms are tiny, indivisible spheres
• Atoms of a given element are identical; different elements have different atoms
• Law of definite proportions: elements combine in fixed ratios

J.J. Thomson and the Electron

• Proposed atoms are divisible
• Discovered electrons through cathode ray experiments
• Proposed "Raisin Bun" or "Plum Pudding" model
  • Atoms have negatively charged electrons within a positively charged matrix

Ernest Rutherford's Nuclear Model

• Conducted the Gold Foil Experiment
• Discovery: atoms mostly empty space with a small, dense, positively charged nucleus
• Revised model: electrons orbit around nucleus

Niels Bohr's Atomic Model

• Electrons occupy fixed orbits or quantized energy levels
• Electrons do not spiral into nucleus
• Model explained behavior of hydrogen atoms and other single-electron systems

James Chadwick and the Neutron

• Discovered neutrons in the nucleus
• Neutrons prevent protons from repelling each other

Evolution of Atomic Theory

• Understanding of atoms evolved over thousands of years
• Future discoveries will likely further alter our understanding

Conclusion

• The lecture provided an overview of the historical development of atomic theory
• Current models still subject to change with future scientific advancements