Early Atomic Theory: Dalton, Thompson, Rutherford, and Millikan

Introduction

• The evolution of atomic theory from the early 19th to early 20th century.
• Focus on the contributions of John Dalton, J.J. Thompson, Ernest Rutherford, and Robert Millikan.

John Dalton's Atomic Theory

• Dalton's Postulates:
  • Elements are composed of tiny particles called atoms.
  • Atoms of the same element are identical, and atoms of different elements differ.
  • Atoms cannot be subdivided, created, or destroyed.
  • Compounds form when atoms of different elements combine in fixed ratios.
  • Chemical reactions involve the rearrangement of atoms.
• Significance: Provided a scientific approach to understanding matter.

J.J. Thompson's Contribution

• Discovery of the Electron:
  • Conducted experiments with cathode rays leading to the discovery of the electron.
  • Proposed the "plum pudding" model where electrons were embedded in a positively charged sphere.
• Significance: Introduced the concept of subatomic particles.

Ernest Rutherford's Discovery

• Gold Foil Experiment:
  • Rutherford's experiment involved bombarding gold foil with alpha particles.
  • Observed that most particles passed through, but some were deflected.
• Nuclear Model of the Atom:
  • Concluded that atoms have a small, dense, positively charged nucleus.
  • Electrons orbit this nucleus.
• Significance: Overturned Thompson’s model and established the nucleus concept.

Robert Millikan's Experiment

• Oil Drop Experiment:
  • Performed experiments to determine the charge of an electron.
  • Balanced gravitational and electrical forces on oil droplets to measure charge.
• Determined Charge and Mass of Electrons:
  • Calculated the charge of a single electron.
  • Helped establish the quantized nature of electric charge.
• Significance: Provided the foundation for the modern understanding of atomic structure.

Conclusion

• The combined work of these scientists laid the groundwork for today’s atomic theory.
• Each discovery built upon the previous, enhancing the understanding of atomic structure and subatomic particles.

Key Takeaways

• Dalton introduced the idea of indivisible atoms.
• Thompson discovered electrons, suggesting the atom was divisible.
• Rutherford identified the nucleus, reshaping atomic structure understanding.
• Millikan quantified the electron's charge, furthering the development of atomic theory.