The Development of the Atom

  • Democritus' Theory

    Democritus proposed that there was a limit to how much matter could be subdivided and that there existed an ultimate, indivisible particle called an atom. No experiments were done at this stage.
  • Newtown's Theory

    Isaac Newton proposed a mechanical universe with small solid masses in motion. He also produced the three laws of motion.
  • Dalton's discovery

    Dalton did many experiments and found that elements always combined in fixed ratios. He also proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.
  • G.J Stoney's Theory

    G.J. Stoney proposed that electricity was made of discrete negative particles he called electrons.
  • J.J. Thomson's experiments

    Thomson and other scientists all experimented with passing electricity through gases at low pressures in sealed glass tubes,
  • Plank's explanation

    Max Planck used the idea of quanta (discrete units of energy) to explain hot glowing matter.
  • Nagaoka's theory

    Nagaoka postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.
  • Milikan's experiment

    Milikan's oil drop experiment determined the charge and mass of an electron.
  • E. Rutherford's Theory

    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's theory

    He wrote “The atomic number of an element is equal to the number of protons in the nucleus".
  • James Chadwick's experiment

    The neutron was discovered.
  • Bohr's discovery

    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.