The Evolution of the Atomic Theory

  • Issac Newton

    Issac Newton
    Proposed a mechanical universe with small solid masses in motion.
  • John Dalton

    John Dalton
    Proposed an "atomic theory" with spherical solid atoms based upon measurable properties of mass.
  • Michael Faraday

    Michael Faraday
    Studied the effect of electricity on solutions, coined term "electrolysis" as a splitting of molecules with electricity, developed laws of electrolysis. Faraday himself was not a proponent of atomism.
  • J. Plucker

    J. Plucker
    Built one of the first gas discharge tubes ("cathode ray tube").
  • Dmitri Mendeleev

    Dmitri Mendeleev
    Arranged elements into 7 groups with similar properties. He discovered that the properties of elements "were periodic functions of the their atomic weights". This became known as the Periodic Law.
  • James Clerk Maxwell

    James Clerk Maxwell
    Proposed electric and magnetic fields filled the void.
  • Sir William Clerkes

    Sir William Clerkes
    Discovered cathode rays had the following properties: travel in straight lines from the cathode; cause glass to fluoresce; impart a negative charge to objects they strike; are deflected by electric fields and magnets to suggest a negative charge; cause pinwheels in their path to spin indicating they have mass.
  • E. Goldstein

    E. Goldstein
    Used a CRT to study "canal rays" which had electrical and magnetic properties opposite of an electron.
  • G.J. Stoney

    G.J. Stoney
    Proposed that electricity was made of discrete negative particles he called "electrons ".
  • Wilhelm Roentgen

    Wilhelm Roentgen
    Using a CRT he observed that nearby chemicals glowed. Further experiments found very penetrating rays coming from the CRT that were not deflected by a magnetic field. He named them "X-rays".
  • Henri Becquerel

    Henri Becquerel
    While studying the effect of x-rays on photographic film, he discovered some chemicals spontaneously decompose and give off very pentrating rays.
  • J.J. Thomson

    J.J. Thomson
    Used a CRT to experimentally determine the charge to mass ratio (e/m) of an electron =1.759 x 10 8 coulombs/gram. Studied "canal rays" and found they were associated with the proton H + .
  • Rutherford

    Studied radiations emitted from uranium and thorium and named them alpha and beta.
  • Marie Sklodowska Curie

    Marie Sklodowska Curie
    Studied uranium and thorium and called their spontaneous decay process "radioactivity". She and her husband Pierre also discovered the radioactive elements polonium and radium.
  • Soddy

    Observed spontaneous disintegration of radioactive elements into variants he called "isotopes" or totally new elements, discovered "half-life", made initial calculations on energy released during decay.
  • Max Planck

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

    Postulated a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged particle.
  • Abegg

    Discovered that inert gases had a stable electron configuration which lead to their chemical inactivity.
  • Albert Einstein

    Albert Einstein
    Published the famous equation E=mc 2
  • Hans Geiger

    Hans Geiger
    Developed an electrical device to "click" when hit with alpha particles.
  • R.A. Millikan

    R.A. Millikan
    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.
  • Ernest Rutherford

    Ernest Rutherford
    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
    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.
  • Aston

    Discovered the existence of isotopes through the use of a mass spectrograph.
  • Niels Bohr

    Niels Bohr
    Developed an explanation of atomic structure that underlies regularities of the periodic table of elements. His atomic model had atoms built up of sucessive orbital shells of electrons.
  • de Broglie

    de Broglie
    Discovered that electrons had a dual nature-similar to both particles and waves. Particle/wave duality. Supported Einstein.
  • Heisenberg

    Described atoms by means of formula connected to the frequencies of spectral lines. Proposed Principle of Indeterminancy - you can not know both the position and velocity of a particle.
  • Cockcroft/Walton

    Built an early linear accelerator and bombarded lithium with protons to produce alpha particles
  • Shrodinger

    Viewed electrons as continuous clouds and introduced "wave mechanics" as a mathematical model of the atom.
  • Paul Dirac

    Paul Dirac
    Proposed anti-particles . Anderson discovered the anti-electron (positron) in 1932 and Segre/Chamberlain detected the anti-proton in 1955..
  • James Chadwick

    James Chadwick
    Using alpha particles discovered a neutral atomic particle with a mass close to a proton. Thus was discovered the neutron.
  • Lise Meitner,Hahn , Strassman

    Lise Meitner,Hahn , Strassman
    Conducted experiments verifying that heavy elements capture neutrons and form unstable products which undergo fission. This process ejects more neutrons continuing the fission chain reaction.
  • Glen Seaborg

    Glen Seaborg
    Synthesized 6 transuranium elements and suggested a change in the layout of the periodic table.
  • Enrico Fermi

    Enrico Fermi
    Conducted the first controlled chain reaction releasing energy from the atoms nucleus.