History of the Atom Notes

History of the Atom: Development of Atomic Theory

Democritus (490 B.C.E)

  • Greek philosopher.
  • Proposed that all matter is composed of tiny, indivisible particles called atoms.
  • The term "Atomos" means "undivided" in Greek.
  • Believed atoms were indestructible units with empty space between them.

John Dalton (1804)

  • English chemist who experimented with gases.
  • Developed a Modern Atomic Theory through experimentation.
  • Theorized that atoms could bond to form new substances.
Dalton’s Modern Atomic Theory
  1. Matter is made up of indivisible atoms.
  2. Atoms of the same element are identical.
  3. Atoms of different elements have different masses and properties.
  4. Atoms of different elements can react and bond into chemical compounds.
  5. Atoms cannot be created or destroyed.
Dalton Model of the Atom
  • A solid sphere, resembling a “billiard ball.”

J. J. Thomson (1898)

  • English chemist who experimented with cathode ray tubes.
  • Discovered negatively charged particles called electrons.
Cathode Ray Tubes
  • The beam in cathode ray tubes is attracted to positively charged plates, indicating that the particles are negatively charged.
Plum Pudding Model of Thomson
  • Thomson proposed that negative electrons are scattered within a positively charged region in the atom.
  • Atoms are neutral overall.

Ernest Rutherford (1911)

  • English chemist famous for the Gold Foil Experiment.
  • Shot a beam of positively charged alpha particles at a thin gold foil.
Gold Foil Experiment
  • Alpha particles were directed at a thin sheet of gold foil.
  • According to the Thomson model, alpha particles were expected to pass through the gold atoms.
  • Some alpha particles deflected, indicating that they encountered something large inside of the gold atoms.
  • Rutherford theorized that each gold atom contains a large, dense area in the center.
  • This dense, positively charged region was called the nucleus, which caused alpha particles to deflect.
Comparison of Thomson Model vs. Rutherford Model

Rutherford Model of the Atom

  • Contains a dense, positively charged nucleus in the center.
  • Electrons orbit around the nucleus.

Niels Bohr (1913)

  • Danish scientist who modified Rutherford’s model.
  • Theorized that electrons orbit the nucleus along specific paths.
Bohr Model of the Atom
  • The Bohr model contains specific regions for electrons called energy levels.
  • The nucleus is in the center of the atom.

James Chadwick (1932)

  • English scientist who experimented with radiation and discovered the neutron.
  • The neutron is a neutral particle located in the nucleus.
Chadwick Model of the Atom
  • Updated the nucleus of the Bohr model.
  • The nucleus contains protons and neutrons.

Erwin Schrödinger (1926)

  • Austrian scientist who modified the Bohr model.
  • Claimed that the exact location of electrons can never be known.
  • His model contains regions (“electron clouds”) where electrons might be located.
Electron Cloud Model of the Atom
  • Updated the electrons of the Bohr model.
  • Electron cloud describes the area where electrons are likely located.