History of Atomic Theory

Introduction

• Atomic theory is crucial for understanding chemistry.
• Atoms make up the things around us, but their theory was once controversial.

Democritus and Early Concepts

• Democritus (Greek philosopher):
  • Proposed matter is made of indivisible particles called "atomos."
  • This was more philosophical than scientific, lacking evidence.

John Dalton and Empirical Evidence

• John Dalton:
  • Provided first empirical evidence of atoms.
  • Proposed that:
    • Compounds consist of atoms in whole number ratios.
    • Chemical reactions involve rearrangement of atoms.
  • Explained observations like water composition (1/9 hydrogen, 8/9 oxygen by mass).
  • Concept of atoms explained the formation of compounds (e.g., carbon reacting with oxygen).

J.J. Thomson's Discovery of Electrons

• J.J. Thomson:
  • Discovered electrons via the cathode ray experiment.
  • Found particles with negative charge, indicating atoms have smaller charged components.
  • Proposed the "Plum Pudding Model" of atom structure.

Ernest Rutherford and the Nucleus

• Ernest Rutherford:
  • Conducted the gold foil experiment.
  • Discovered the nucleus:
    • Alpha particles mostly passed through gold foil, but some scattered.
    • Suggested a dense positive center (nucleus) with electrons far away.
    • Atoms mostly empty space.

Summary

• Demonstrated that atoms exist and are not indivisible.
• Revised understanding of atomic structure was foundational for chemistry.
• Further revisions and discoveries to follow in the study of atomic structure.

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