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atomic model timeline

  • 460


    (460-370 BC) Democritus’ atomic theory posited that all matter is made up of small indestructible units he called atoms.First proposed the existence of an ultimate particle. Used the word "atomos" to describe this particle.
  • Isaac Newton

    Isaac Newton
    Isaac Newton theorized a mechanical universe with small, solid masses in motion.
  • John Dalton

    John Dalton
    John Dalton proposed that elements consisted of atoms that were identical and had the same mass and that compounds were atoms from different elements combined together.
  • Michael Faraday

    Michael Faraday
    Michael Faraday developed the two laws of electrochemistry.
  • J. Plucker

    J. Plucker
    J. Plucker built one of the first cathode-ray tubes.
  • Dmitri Mendeleev

    Dmitri Mendeleev
    Dmitri Mendeleev created the periodic table.
  • James Clerk Maxwell

    James Clerk Maxwell
    James Clerk Maxwell proposed the theory of electromagnetism and made the connection between light and electromagnetic waves.
  • G.J Stoney

    G.J Stoney
    G.J. Stoney theorized that electricity was comprised of negative particles he called electrons.
  • Sir William Crookes

    Sir William Crookes
    Sir William Crookes’ experiments with cathode-ray tubes led him to confirm the work of earlier scientists by definitively demonstrating that cathode-rays have a negative charge.
  • E. Goldstein

    E. Goldstein
    E. Goldstein discovered canal rays, which have a positive charge equal to an electron.
  • Wilhelm Roentgen

    Wilhelm Roentgen
    Wilhelm Roentgen discovered x-rays.
  • Henri Becquerel

    Henri Becquerel
    Henri Becquerel discovered radiation by studying the effects of x-rays on photographic film.
  • J.J. Thomson

    J.J. Thomson
    J.J. Thomson studied the positive rays in the cathode ray tube and discovered that the charge to mass ratio depended on filling gas in the tube. The largest charge to mass ratio (smallest mass) occurred when hydrogen was the filling gas. This particle was later identified as the proton.
  • Marie Sklodowska Curie

    Marie Sklodowska Curie
    Marie Sklodowska Curie discovered radium and polonium and coined the term radioactivity after studying the decay process of uranium and thorium.
  • Ernest Rutherford

    Ernest Rutherford
    Ernest Rutherford proposed the nuclear atom as the result of the gold-foil experiment in 1911. Rutherford proposed that all of the positive charge and all of the mass of the atom occupied a small volume at the center of the atom and that most of the volume of the atom was empty space occupied by the electrons. This was a very radical proposal that flew in the face of Newtonian Physics. Rutherford discovered alpha, beta, and gamma rays in radiation.
  • Max Planck

    Max Planck
    Max Planck proposed the idea of quantization to explain how a hot, glowing object emitted light.
  • Frederick Soddy

    Frederick Soddy
    Frederick Soddy came up with the term "isotope" to explain the unintentional breakdown of radioactive elements.
  • Hantaro Nagaoka

    Hantaro Nagaoka
    Hantaro Nagaoka proposed an atomic model called the Saturnian Model to describe the structure of an atom.
  • Richard Abegg

    Richard Abegg
    Richard Abegg found that inert gases have a “stable electron configuration.”
  • Hans Geiger

    Hans Geiger
    Hans Geiger invented a device that could detect alpha particles.
  • H.G.J. Moseley

    H.G.J. Moseley
    H.G.J. Moseley discovered that the number of protons in an element determines its atomic number. Henry Moseley discovered that the energy of x-rays emitted by the elements increased in a linear fashion with each successive element in the periodic table. In 1913, he proposed that the relationship was a function of the positive charge on the nucleus. This rearranged the periodic table by using the atomic number instead of atomic mass to represent the progression of the elements.
  • Francis William Aston

    Francis William Aston
    Francis Aston invented the mass spectrograph in 1920. He was the first person to observe isotopes. For example he observed that there were three different kinds of hydrogen atoms. While most of the atoms had a mass number of 1 he also observed hydrogen atoms with mass numbers of 2 and 3. Modern atomic masses are based on mass spectral analysis. His work led Rutherford to predict the existence of the neutron.
  • Niel S. Bohr

    Niel S. Bohr
    Niels Bohr proposed an atomic structure theory that stated the outer orbit of an atom could hold more electrons than the inner orbit.
  • Louis de Broglie

    Louis de Broglie
    1923 Louis de Broglie proposed that electrons have a wave/particle duality.
  • Cockcroft / Walton

    Cockcroft / Walton
    Cockcroft / Walton created the first nuclear reaction, producing alpha particles
  • Paul Dirac

    Paul Dirac
    Paul Dirac proposed the existence of anti-particles.
  • James Chadwick

    James Chadwick
    James Chadwick discovered neutrons, particles whose mass was close to that of a proton. Chadwick was a collaborator of Rutherford's. Interestingly, the discovery of the neutron led directly to the discovery of fission and ultimately to the atomic bomb.
  • Lise Meitner, Hahn, Strassman

    Lise Meitner, Hahn, Strassman
    Lise Meitner, Hahn, Strassman discovered nuclear fission.
  • Enrico Fermi

    Enrico Fermi
    Enrico Fermi created the first man-made nuclear reactor.
  • Glenn Seaborg

    Glenn Seaborg
    Glenn Seaborg discovered eight transuranium elements.
  • Aristotle

    (384-322 BC) Aristotle was a proponent of the continuum. He believed in the four elements of air, earth, water and fire. Aristotle felt that regardless of the number of times you cut a form of matter in half, you would always have a smaller piece of that matter. This view held sway for 2000 years primarily because Aristotle was the tutor of Alexander the Great.