Lecture Notes: Evolution of Atomic Theory

Introduction

• Exploration of the question: What is stuff made of?
• Connection between ancient philosophers and modern scientists.

Democritus (c. 440 BCE)

• Proposed that everything in the world is composed of tiny particles surrounded by empty space.
• Named these particles "atomos" meaning indivisible.
• Suggested particles vary in size and shape based on the substance.
• His ideas were opposed by Aristotle.

Aristotle's Counterpoint

• Believed matter was composed of four elements:
  • Earth
  • Wind
  • Water
  • Fire
• His theory dominated until the early 19th century.

John Dalton (1808)

• Quaker teacher who challenged Aristotelian theory.
• Demonstrated that substances break down into the same elements in fixed proportions.
• Concluded that compounds are combinations of atoms with specific sizes and masses.
• His work led to the acceptance of atomic theory in the scientific community.

J.J. Thomson (1897)

• Discovered the electron and proposed the "chocolate chip cookie model" of the atom.
• Suggested atoms are uniformly packed spheres of positive matter with negatively charged electrons.
• Won the Nobel Prize in 1906 for his discovery.

Ernest Rutherford

• A student of Thomson, known as the father of the nuclear age.
• Conducted experiments with alpha particles and gold foil.
• Concluded atoms consist largely of empty space and that mass is concentrated in a dense, positively charged nucleus.

Niels Bohr (1913)

• Expanded on Rutherford's model by proposing that electrons orbit the nucleus at fixed energies and distances.
• Introduced the concept of energy levels where electrons can jump but not exist between levels.

Quantum Mechanics

• Electrons behave like waves, complicating atomic theory.
• Werner Heisenberg's uncertainty principle: Impossible to determine both the exact position and speed of electrons simultaneously.
• Led to the current quantum model of the atom.

Conclusion

• Continuous evolution of atomic theory reflects ongoing discoveries.
• Electrons shifting energy levels contribute to the emission of light in various wavelengths, resulting in observable colors.
• Celebration of atomic theory's triumph, recognizing Democritus' early contributions.