The Evolution of Atomic Theory

Ancient Greek Philosophy

• Democritus (c. 440 BCE)
  • Proposed that everything is made up of tiny particles called "atomos" (indivisible).
  • Believed these particles vary in size and shape depending on the substance.
  • Idea opposed by philosophers such as Aristotle, who believed matter was made of four elements: earth, wind, water, and fire.

Revival in the 19th Century

• John Dalton (1808)
  • Challenged Aristotelian theory with evidence that substances break down into same elements in same proportions.
  • Proposed that compounds are combinations of atoms of different elements, each with specific size and mass.
  • Lived modestly as a Quaker despite receiving many honors.

Early 20th Century Discoveries

• J.J. Thompson (1897)

  • Discovered the electron, proposing the "chocolate chip cookie model" where atoms are packed spheres of positive matter with electrons.
  • Received a Nobel Prize in 1906.

• Ernest Rutherford

  • Investigated atoms using alpha particles and gold foil, leading to the conclusion that atoms consist of empty space with a central nucleus.
  • This experiment suggested that most mass is concentrated in the nucleus.

Quantum Model Development

• Niels Bohr (1913)

  • Proposed a model where electrons orbit the nucleus at fixed energies and distances.
  • Introduced the idea that electrons can jump between energy levels.

• Werner Heisenberg

  • Developed the uncertainty principle, stating the impossibility of determining both position and speed of electrons.
  • Led to the development of the quantum model where electrons exist in a range of possible locations.

Conclusion

• Atomic theory has evolved significantly, from Democritus to modern quantum theory.
• Despite changes, the basic concept of atoms is celebrated, illustrated by the colors in fireworks as electrons shift energy levels.
• Democritus' early ideas are validated, showing the enduring nature of his hypothesis.