Timeline of Atomic Theories

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  • 7/29/2019 Timeline of Atomic Theories


    TIMELINE OF ATOMIC THEORIESDemocritus Greeks concept of matter(460 - 370):

    - Matter is discontinuous. It is made up of particlescalled atoms. The properties of the atoms

    determine the properties of matter.

    Daltons atomic theory (1804):

    - Atoms are indivisible and indestructible. They donot change during chemical reaction.

    Cathode ray experiments (1879 - 1897):

    - Cathode rays are made up of negative particlescalled electron.

    Thomsons plum pudding model (1891):

    - An atom is a positive ball of electricity withnegative electrons embedded.

    Albert Einstein photon theory of light (1905):

    - Light wave carries energy equal to h v, consists ofquanta or corpuscles called photon.

    Rutherfords gold fat experiment 1908 - 09):

    - the atom is mostly empty space. Its mass isconcentrated in the nucleus. (Nuclear Model)

    Niels Bohrs atomic model (1913):


    the atom is like a solar system. Its mass isconcentrated in the nucleus in circular orbits.

  • 7/29/2019 Timeline of Atomic Theories


    Moseleys x-ray experiments (1913):

    - all atoms of one elements have the same or fixednumber of protons.

    De Broglies particle-wave concept of the electron

    (1924):- the electron has both particle and wave


    Werner Heisenbergs Uncertainty principle (1927):

    - it is not possible to define accurately the positionand momentum (velocity) of particles of atomic

    dimensions, e.g., electron.

    Clinton Davisson and Lester Germer (1927):- Showed an electron beam impinging on a nickel

    crystal exhibits properties of diffraction and


    - Confirmed de Brogiles particle wave concept ofthe electron.

    Erwin Schroedingers Wave model (1928):

    - Described the wave like motion of the electronwith relevant wave equation.

    James Chadwick (1932):

    - Identified the neutron as the particle producedalong with carbon when beryllium is bombarded

    with alpha particles.

    M. Gell-Man and G, Zweig Quark (1963):

    - Assumed that Quarks are even morefundamental particles than neutrons and


    - The fundamental particles of matter are quarksand leptons.


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