Evolution of Atomic Theory Overview

Sep 4, 2024

Lecture Notes: Evolution of Atomic Theory

Introduction to Atomic Theory

  • Dalton’s postulates: Atoms are small, indivisible particles.
  • Modern representation of atoms includes subatomic particles.

Discovery of Electrons

  • J.J. Thomson's Experiment with Cathode Ray Tubes (CRT):
    • Invented in 1897.
    • CRT: Sealed glass tube with almost all air removed contains cathode and anode.
    • Cathode ray deflected towards positive charge, away from negative charge.
    • Charge-to-mass ratio: (1.759 \times 10^{11}) C/kg
    • Conclusions:
      • Particles are lighter than atoms.
      • They have a negative charge.
      • They are consistent across all elements tested.
    • Known as electrons: negatively charged subatomic particles.

Calculation of Electron Charge and Mass

  • Millikan’s Oil Drop Experiment (1909):
    • Charged microscopic oil droplets with X-rays.
    • Determined charge of individual droplets.
    • Charge of electron: (1.6 \times 10^{-19}) C
    • Mass of electron: (9.107 \times 10^{-31}) kg

Models of the Atom

  • Thomson’s Plum Pudding Model:
    • Electrons (plums) embedded in a positive mass (pudding).
  • Nagaoka’s Planetary Model:
    • Electrons orbit a positive nucleus.

Rutherford's Gold Foil Experiment

  • Alpha particles directed at gold foil.
  • Most particles passed through; some deflected.
  • Conclusions:
    • Nucleus is small, positively charged.
    • Most atom space is empty.
    • Electrons located around the nucleus.
    • Introduction of the proton as a positively charged subatomic particle.

Discovery of Isotopes

  • Analysis of radioactive ores led to discovery of isotopes.
  • Isotopes: Atoms of the same element with different mass.
  • Frederick Soddy: Nobel Prize 1921 for work on isotopes.

Discovery of Neutrons

  • James Chadwick (1932):
    • Proved existence of neutrons.
    • Neutrons: Uncharged subatomic particles with mass similar to protons.
    • Located in the nucleus.
    • Explains existence of isotopes.

Conclusion

  • Understanding of atomic structure evolved to recognize subatomic particles: electrons, protons, and neutrons.
  • Allows distinction between isotopes and understanding of atomic mass.