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Honors Chemistry Atomic Theory Timeline

  • Period: 460 BCE to

    Atomic Theory

  • 450 BCE

    Leucippus of Abdera

    Leucippus of Abdera
    Very little is known about he fifth century BC born Greek philosopher. Though he was credited to be the first to theorize the existence of atoms. Though very little is known about him, it may have been possible he had relations with Democritus.
  • 400 BCE


    Though Leucippus may have been the first person to believe in atomic theory, Democritus was still credited as the first coin the term "atomos" which is what the modern word for atom is derived from. He has stated that “The universe is composed of two elements: the atoms and the void in which they exist and move.”
  • 340 BCE


    Aristotle believed that there were 4 elements which were water, earth, fire, and air (5 elements actually if you count Aether which he considered to be the element that made up stars and celestial bodies) that made up everything, each which had different physical properties. Unlike Democritus, Aristotle believed that there was no singular smallest unit of matter and that the elements will continue to decrease in size the more they are cut.
  • 100


    Due to technicalities with the website's format, dates before BC are not shown on the desktop version of the site. To view the BC events, go to the bottom of the page and select "Mobile site". Or simply use this link:
  • Apr 24, 600


    These early "pseudo-chemists" theorized that all metals are made up kinds of sulfur and mercury and that could be changed into gold.
  • Isaac Newton

    Isaac Newton
    Though he did not focus his work on developing an atomic theory, Newton did propose that in this universe there are small particles of solid mass that were always constantly in motion in his book "Optics" in 1704. He contributed the idea that these tiny solid "massy" particles are held together with different tangible forces.
  • Benjamin Franklin

    Benjamin Franklin
    While he did not focus his work on the development of the atomic theory, his studies in electricity did help contribute to it. What Franklin discovered was through the rubbing of fur on glass and rubber rods he observed the two charges of electricity which later helped formulate the science behind protons and electrons.
  • Antoine Lavoisier

    Antoine Lavoisier
    He conducted experiments turning HgO into Hg+O. Through this he developed the Law of Conservation. The law states that matter cannot be made or destroyed. He supported the rearrangement of matter in reactions.
  • Joseph Proust

    Joseph Proust
    Proust stated the Law of Definite Proportions. The law states that the ratio of elements in a compound will always be the same and constant. His proposal helped Dalton to create the Law of Multiple Proportions and ultimately his atomic theory.
  • John Dalton

    John Dalton
    Dalton proposed significant ideas in the atomic theory and was the first to do so in a long time. His rules are still applied today. They were:
    1) All matter is made of atoms. Atoms are indivisible.
    2) All atoms of an element are identical in mass and properties
    3) Compounds are formed by a combination of two or more different kinds of atoms.
    4) A chemical reaction is a rearrangement of atoms.
    His model for the atom was still simplistic as subatomic particles had not been discovered yet.
  • Michael Faraday

    Michael Faraday
    Faraday studied the effect of electricity on solutions. He was the one who coined term "electrolysis" as a splitting of molecules with electricity. Through his discovery in the electromagnetic field we were able to perform more accurate studies of the atom,
  • Dmitri Mendeleev

    Dmitri Mendeleev
    Mendeleev published the periodic table of elements which included the all the known elements at the time. It was arranged into 7 groups with similar properties. He developed the Periodic Law.
  • James Clerk Maxwell

    James Clerk Maxwell
    Maxwell published physical and mathematical theories of the electromagnetic field. He postulated that electric and magnetic fields filled the empty space within an atom.
  • Sir William Crookes

    Sir William Crookes
    Crookes conducted experiments with cathode rays. He found that they travel in straight lines from the cathode but bend in path when different magnetic poles are applied to the tube. He concluded that the rays had a negative charge and that it had mass because it caused pinwheels to spin.
  • Eugen Goldstein

    Eugen Goldstein
    Goldstein noted that cathode-ray tubes have a glow at the end of the tube near the cathode. He concluded that in addition to the negative particles (electrons), that traveled from the cathode toward the anode, there is another ray that travels in the opposite direction. Goldstein called them canal rays. which must have positive charges.
  • Werner Heisenberg

    Werner Heisenberg
    He proposed Principle of Uncertainty- you can not know both the position and velocity of a particle. This contributed to both quantum mechanics and atomic theory.
  • Henri Becquerel

    Henri Becquerel
    While studying the effect of x-rays on photographic film, Becquerel discovered some chemicals spontaneously decompose and give off very penetrating rays. This led to the discovery of radiation. This was significant to the atomic theory because it led to explaining how atoms decay.
  • Joseph John Thomson

    Joseph John Thomson
    Thomson discovered and named the electron as well as isotopes. He also invented the mass spectrometer. This helped develop atomic theory further. He also developed the plum pudding model which is now outdated.
  • Marie Sklodowska Curie & Pierre Curie

    Marie Sklodowska Curie & Pierre Curie
    The couple studied uranium and thorium and observed spontaneous decay. They called the process "radioactivity". The two contributed to atomic theory by discovered the radioactive elements radium and polonium.
  • Hantaro Nagaoka

     Hantaro Nagaoka
    Nagaoka came up with a "Saturnian" model of the atom with flat rings of electrons revolving around a positively charged center. This was almost 20 years before Bohr came up with his planetary model. Unlike Bohr, Nagaoka thought that the atomic spectra should directly correlate to the electron's orbit in terms of frequency.
  • Albert Einstein

    Albert Einstein
    Einstein mathematically proved the existence of atoms. Through his work modern physics was born and further development of the atomic theory was enabled.
  • Robert A. Millikan

    Robert A. Millikan
    Millikan conducted the oil drop experiment. He determined the unit charge of the electron in his oil drop experiment at the University of Chicago. This allowed him to calculate the mass of the electron and the positively charged atoms.
  • Ernest Rutherford

    Ernest Rutherford
    Rutherford conducted the gold foil experiment by using alpha particles as projectiles to shoot at the atoms in a piece of thin (0.00006 cm) gold foil . He established that the nucleus was a very dense, small, and positively charged particle in the center of the cell. He also concluded that the electrons were located outside the nucleus and that majority of the atom was empty space.
  • Henry Moseley

    Henry Moseley
    Moseley re-organized Mendeleev's periodic table by number of protons. He did this by measuring the wavelengths of the x-rays given off by certain metals. This way he was able to find the number of positive charges in the nucleus of each atom.
  • Niels Bohr

    Niels Bohr
    Bohr's greatly contributed to the atomic theory with the Bohr model. It resembled a solar system with a positively charged nucleus and electrons in orbits. Bohr discovered that electrons traveled in different orbits and that the number of electrons in the outer orbit determines the properties of the atom.
  • Louis-Victor de Broglie

    Louis-Victor de Broglie
    De Broglie devised wave-particle duality which stated that all matter had wave like properties to a certain extent. It helped to further establish Bohr's model.
  • Erwin Schrodinger

    Erwin Schrodinger
    Schrodinger utilized quantum mechanics to create his interpretation of an atom, the cloud model. This is what modern scientists use as well. He explained the movement of an electron as a wave.
  • James Chadwick

    James Chadwick
    James Chadwick experimented with beryllium atoms by bombarding them with alpha particles. An unknown radiation with that was neutral in charge was produced. this led to the discovery of the proton, a neutrally charged particle with the approximate mass of a proton. After this discovery, more adequate models of the atom became available to chemists.
  • Lisa Meitner, Otto Hahn, & Fritz Straussman

    Lisa Meitner, Otto Hahn, & Fritz Straussman
    These three scientists performed experiments and managed to split Uranium atoms in a process called fission. This lead to further studies in the structure of atoms.
  • Enrico Fermi

    Enrico Fermi
    Fermi conducted the first controlled chain reaction experiment by releasing energy from the atoms nucleus. He developed a self sustaining control of nuclear energy.
  • Max Planck

    Max Planck
    used the idea of quanta (discrete units of energy) to explain hot glowing matter. He also was the founder of quantum theory which is still used today to help develop atomic theory.