History of Atomic Structure

Early Theories

• Greek Philosophers: Proposed matter is made up of indivisible particles (atoms).

John Dalton (Early 19th Century)

• Theory: Matter is composed of tiny, indivisible atoms.

William Crookes (Late 19th Century)

• Experiment 1: Using a Maltese cross inside a glass tube with electric current.
  • Observation: Shadow of the Maltese cross formed, indicating that cathode rays travel in straight lines and produce fluorescence.
• Experiment 2: Using a paddle wheel.
  • Observation: Cathode rays pushed the paddle wheel, suggesting they contain particles with enough energy to move it.
• Conclusion: Beginning of discovering electrons.

J.J. Thomson

• Expanded on Crookes' experiments.
• Experiment: Passed cathode rays between charged plates.
  • Observation: Deflection towards positive plates, indicating cathode rays are negatively charged.
• Discoveries:
  • Cathode rays (electrons) have a negative charge.
  • Determined charge-to-mass ratio of the electron.

Robert Millikan (1909)

• Oil-Drop Experiment: Measured the charge on an electron.
  • Process: Observed oil droplets between electrically charged plates with and without voltage.
  • Finding: Charges were multiples of 1.6 x 10^-19 coulombs, the charge of a single electron.

Ernest Rutherford (1909)

• Gold Foil Experiment: Passed alpha particles through gold foil.
  • Observations:
    • Most particles went through undeflected.
    • Some were deflected at large angles.
    • A few were reflected straight back.
  • Conclusions:
    • Atom mostly empty space.
    • Positive charge concentrated in the nucleus (protons).
    • Rutherford's model disproved Thomson's plum-pudding model.

James Chadwick

• Discovery of Neutron: Used similar experiment to Rutherford with beryllium and paraffin wax.
  • Process: Neutrons knocked out from beryllium, causing protons to be knocked from paraffin block.

Summary of Subatomic Particles

• Proton: Positive charge, located in nucleus, relative mass = 1.
• Neutron: No charge, located in nucleus, relative mass = 1.
• Electron: Negative charge, located in electron cloud, relative mass = ~0.