Atomic Theory Timeline

  • 340

    BCE Aristotle

    BCE Aristotle
    Aristotle believed that there were only five elements in the world: air which was cold, earth which was dry, water which was wet, fire which was hot, and Aether which he viewed as a divine substance which made up the stars and planets.He believed that all these elements made up the earth. 340 BCE
  • 465

    BCE Democritus of Adbera

    BCE Democritus of Adbera
    Democritus' theory is the following:
    All matter consists of invisible particles called atoms.
    Atoms are indestructible.
    Atoms are solid but invisible.
    Atoms are homogenous.
    Atoms differ in size, shape, mass, position, and arrangement.
    Solids are made of small, pointy atoms.
    Liquids are made of large, round atoms.
    Oils are made of very fine, small atoms that can easily slip past each other.
    465 BCE
  • 500

    BCE Alchemists

    BCE Alchemists
    The Alchemists developed the theory that all metals are composed of mercury and sulfur and that it is possible to change base metals into gold. 500 BCE
  • Benjamin Franklin

    Benjamin Franklin
    Benjamin Franklin experimentally verified that the flow of electricity was from a positive charge to a negative charge.
  • Antoine Lavoisier

    Antoine Lavoisier
    Antoine Lavoisier came up with the Law of Conservation, which stated that matter cannot be created or destroyed.
  • Joseph Proust

    Joseph Proust
    Joseph Proust etablished the Law of Definite Proportions - a compound is composed of exact proportions of elements by mass regardless of how the compound was created.
  • John Dalton

    John Dalton
    John Dalton's theory was the following:
    Elements are made of extremely small particles called atoms.
    Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
    Atoms cannot be subdivided, created, or destroyed.
    Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
    In chemical reactions, atoms are combined, separated, or rearranged.
  • Michael Faraday

    Michael Faraday
    Michael Faraday studied the effect of electricity on solutions. He called the splitting of molecules with electricity electrolysis. He also developed rules for electrolysis.
  • Dmitri Mendeleev

    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.
  • William Crookes

    William Crookes
    Sir William Crookes discovered the following properties of cathode rays:
    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.
  • Henry Becquerel

    Henry Becquerel
    While examining the effects of x-rays on film, Henry Becquerel realized that some chemicals spontaneously decompose and give off rays.
  • Joseph John Thomson

    Joseph John Thomson
    Joseph John Thomson used a cathode ray tube to experimentally determine the charge to mass ratio of an electron (1.759 x 10^8 coulombs/gram).
  • Marie & Pierre Curie

    Marie & Pierre Curie
    Marie & Pierre Curie studied the radioactivity of elements such as uranium, thorium, polonium, and radium.
  • Max Planck

    Max Planck
    Max Planck used the idea of quanta (discrete units of energy) to explain hot glowing matter.
  • Albert Einstein

    Albert Einstein
    Albert Einstein created the equation e=mc^2
  • Robert Millikan

    Robert Millikan
    Robert Millikan used his oil drop experiment to determine the charge
    (1.602 x 10^-19 coulombs) and the mass (9.11 x 10^-28 grams) of an electron.
  • Ernest Rutherford

    Ernest Rutherford
    Ernest Rutherford fired alpha particles through a piece of gold foil, and he determined that the nucleus was very dense, very small in comparison to the atom, and it was positively charged. He also determined that their was an electron cloud around the nucleus.
  • Niels Bohr

    Niels Bohr
    Niels Bohr created a new model of the atom that consisted of electrons in orbital shells.
  • Louis de Broglie

    Louis de Broglie
    Louis de Broglie discovered that electrons had particle/wave duality.
  • Werner Heisenberg

    Werner Heisenberg
    Werner Heisenberg described atoms using a formula that was connected to the frequencies of spectral lines. He proposed the Principle of Indeterminancy - you can not know both the position and velocity of a particle.
  • Erwin Schrodinger

    Erwin Schrodinger
    Erwin Schrodinger viewed electrons as continuous clouds and introduced wave mechanics as a mathematical model of the atom.
  • James Chadwick

    James Chadwick
    James Chadwick used alpha particles to find an atomic particle that was neutral with a similar mass to protons. This was the neutron.