Atomic Theory

Timeline created by anna_is_me
  • 460

    Democritus

    Democritus
    Greece
    He hypothesized that all matter (including space and time) is composed of tiny indestructible units, called atoms. He in fact created the first atomic theory which is remarkably similar to modern science's understanding of atomic structure, and avoided many of the errors of their contemporaries.
  • Newtown

    Newtown
    England
    Proposed a mechanical universe with small solid masses in motion.
  • Dalton

    Dalton
    England
    Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass. He also suggested that the sizes of the particles making up gases must be different.
  • G.J. Stoney

    G.J. Stoney
    Irish Midlands
    Proposed that electricity was made of discrete negative particles he called "electrons".
  • J.J. Thomson

    J.J. Thomson
    England
    Used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 108 coulombs/gram.
    He also discovered the electron. Thomson showed that cathode rays were particles with a negative electric charge and much smaller than an atom. He also thought all atoms contained them.
  • Planck

    Planck
    Germany
    Used the idea of quanta (discrete units of energy) to explain hot glowing matter. He found the law of heat radiation.
  • Nagaoka

    Nagaoka
    Japan
    Used the idea of quanta (discrete units of energy) to explain hot glowing matter.
  • Millikan

    Millikan
    U.S.A
    Oil drop experiment determined the charge (e=1.602 x 10-19 coulomb) and the mass (m = 9.11 x 10-28 gram) of an electron.
  • E. Rutherford

    E. Rutherford
    New Zeland
    Using alpha particles as atomic bullets, probed the atoms in a piece of thin (0.00006 cm) gold foil. 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. Moseley

    H.G.J. Moseley
    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

    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

    James Chadwick
    England
    Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron. His discoveries made it possible to create elements heavier than uranium in the lab.