The History of Atomic Theory

  • 500

    Democritus (Greece)

    Said that something cannot be made from nothing; that there are ‘atoms’ of which everything is composed of.
  • Newton (England)

    Proposed a mechanical universe with small solid masses in motion
  • Dalton (England)

    Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.
  • G.J Stoney (Ireland)

    Proposed that electricity was made of discrete negative particles he called electrons
  • J.J. Thomson (England)

    Used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron.
  • Planck (Germany)

    Introduced quantum theory
  • Nagoaka (Japan)

    Postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.
  • Milikan (America)

    Oil drop experiment determined the charge and the mass of an electron
  • E.Rutherford (Britain)

    He established that the nucleus was: very dense, very small and positively charged. He also assumed that the electrons were located outside the nucleus.
  • H.G.J Mosely (England)

    Using x-ray tubes, determined the charges on the nuclei of most atoms. He wrote "The atomic number of an element is equal to the number of protons in the nucleus". This work was used to reorganize the periodic table based upon atomic number instead of atomic mass.
  • Bohr (Denmark)

    Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of successive orbital shells of electrons.
  • James Chadwick (England)

    Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron.