Timeline of Atomic Theory

  • 450 BCE


    Empedocles proposed that all matter is made up of the four elements(fire, water, earth, air). These four building blocks come together to create everything we see around us.
  • 442 BCE


    Democritus used the word "atomos" to define the microscopic building blocks of matter. He proposed that these particles could not be divided further and were specific to the material they made up. The only difference between these atomos is their size, shape, mass, and position which dictate their properties.
  • Period: 300 BCE to

    Dark Ages

    The idea that at certain point materials cannot be divided further was rejected by the general populous for around 2000 years due to the idea that God is all-powerful and could divide materials as much as he pleases.
  • Period: AD 1 to


    Alchemy is the predecessor of chemistry laying down foundations upon which modern chemistry is built. In an attempt to purify, alter, and manipulate materials for a wide array of goals, alchemists began isolating and classifying elements. Alchemists also created methods for isolating elements slowly chipping away at the subatomic mystery that is atoms.
  • John Dalton

    John Dalton
    Dalton built upon past concepts when he created the "billiard ball" model. The model is an updated version of Aristotle's, except the four basic elements are replaced by the evergrowing list of modern-day elements. Basics ideas that atoms are indivisible and atoms of the same element are identical and share the same properties remained present.
  • Dmitri Mendeleev

    Dmitri Mendeleev
    Although not directly affecting the development of the atomic theory, he built the first periodic table which became a crucial tool allowing for future discoveries to be made. Mendeleev played a significant role in the history of chemistry and deserves a spot on the timeline.
  • Max Planck

    Max Planck
    Max Planck's most famous discovery was that energy comes specific discrete amounts instead of on a spectrum. He proposed that the energy of light can only exist as a multiple of Plack's constant. Although not directly related to atomic theory, this discovery would play an important role in the future.
  • Elizabeth Laird

    Elizabeth Laird
    I actually share a family connection to Elizabeth Laird through my father's side of the family. Elizabeth Laird was born in Owen Sound, Ontario before studying at the University of Toronto. Afterward, she worked with Max Planck at the Humboldt University of Berlin between 1898 to 1899. In 1901 she was hired as the first woman to work for J.J. Thompson at Cambridge University's Cavendish Laboratory. Her doctorate thesis was on the absorption spectrum of chlorine. Can you see the resemblance?
  • Marie Sklodowska Curie

    Marie Sklodowska Curie
    Researching how certain elements made air conduct electricity more effectively, Marie Curie researched the radiation coming off of these elements coining the term "radioactivity". She discovered that the rays came from the atomic structure creation the branch of atomic physics. These discoveries helped her to work towards creating portable x-ray devices to be used in WW1.
  • J.J. Thompson

    J.J. Thompson
    Thompson built upon Dalton's model by theorizing that the atom is a positively charged sphere with negatively charged electrons scattered throughout. This creates a neutrally charged atom and was dubbed the "Plum-Pudding" model. The creation of this model was made possible by Thompson's discovery of negatively charged particles in the atom called electrons. Using a cathode ray tube and magnets, Thompson observed that negatively charged particles exist and have a very small mass.
  • Hantaro Nagaoka

    Hantaro Nagaoka
    Nagaoka created the Saturnian model in which electrons orbit around a dense nucleus similar to how planets orbit the sun. This model was mentioned by Ernest Rutherford when he proposed the atomic nucleus.
  • Albert Einstein

    Albert Einstein
    Einstein impacted atomic theory in multiple ways. He devised a way of calculating the size of atoms and molecules even before the existence of atoms had been confirmed. Einstein also devised and predicted the results of an experiment that would later prove the existence of atoms three years later.
  • Ernest Rutherford

    Ernest Rutherford
    Rutherford continued to develop the atomic model by altering Thompson's model. Instead of electrons being fixed throughout a positively charged atom, Rutherford suggested that an atom is composed of a small positively charged nucleus around which electrons orbit. This model of the atom is mainly empty space and remains neutral with charges balancing. This model was made after Rutherford conducted his gold foil experiment prooving atoms are mostly made up of empty space.
  • Niels Bohr

    Niels Bohr
    Building upon his teacher Rutherford's model where electrons orbit around a small positive nucleus, Bohr placed these electrons in specific energy shells. Electrons in rings closer to the nucleus have lower levels of energy that increase as multiples of Planck's constant as they move away from the nucleus. When electrons jump up or drop down energy levels, they emit/absorb that discrete amount of energy.
  • Louis de Broglie

    Louis de Broglie
    Louis de Broglie is most famous for his proposal that particles can hold properties of waves. He created an equation that relates wavelength and particle with speed. Regarding atomic theory, this applies the most to electrons which contain properties of both a particle and wave.
  • Wolfgang Pauli

    Wolfgang Pauli
    The Pauli exclusion principle states that two or more fermions(particles with a half-integer spin) such as electrons cannot share the exact four quantum numbers. This means two electrons found in the same orbital must have opposite spins. It should be noted that Pauli was primarily a theoretical physicist and created this principle by observing the papers of others as well as their findings.
  • Erwin Schrodinger

    Erwin Schrodinger
    Instead of being found in specific rings, Shrodinger found that electrons instead occupy clouds of probability. Because electrons behave both as particles and waves, it's impossible to know the electrons' position and velocity. There are certain places electrons will be more likely to be located but their exact location is unsure.
  • Werner Heisenberg

    Werner Heisenberg
    Heisenburg is famous for his uncertainty principle which states that no two conjugate physical quantities can be measured simultaneously with 100%. Position and momentum are two conjugate physical properties and so only one can be known for sure, this explains electron probability clouds.
  • James Chadwick

    James Chadwick
    Shortly after Schrodinger creates the quantum mechanical model, Chadwick proved the existence of the neutron. Initially suggested by Rutherford, the existence of the neutron was long speculated and if proven would help explain the inner workings of the atom more extensively. Chadwick discovered the proton by bombarding numerous materials with alpha radiation determining that the resulting radiation that was produced was a neutral particle roughly equal in mass to a proton, now called a neutron.
  • Irene Joliot Curie

    Irene Joliot Curie
    Discovering that radioactive elements can be artificially produced through exposure to radiation, Irene Curie followed in the footsteps of her parents. This discovery led to advancements regarding radioisotopes as well as various uses for radiochemistry. Her discovery that radioactive elements could be made artificially significantly reduced the costs of future research on radioactivity.
  • Robert J. LeRoy

    Robert J. LeRoy
    When asked to explain his profession to the general public, LeRoy said: "I study the sex life of molecules.". He is the father of the LeRoy radius, a technique to determine the radius of a molecule allowing for greater understanding of both internal and external forces acting on the molecule. Unsure of when he created the LeRoy radius, I used his birthday as the date.
  • Dorothy Crowfoot Hodgkin

    Dorothy Crowfoot Hodgkin
    Born in Egypt, she discovered three-dimensional biomolecular structures. She continued to decipher the structures of steroids, penicillin, vitamin B12, and insulin. The method of x-ray crystallography which she helped to develop was used to discover the structures of many more molecules later on. She received a Nobel prize in chemistry in 1964 for her work.
  • Ronald J. Gillespie

    Ronald J. Gillespie
    Ronald J. Gillespie is best known for his work developing valence shell electron repulsion (VSEPR) theory. This theory allows for the shape of molecules to be predicted from their formula given atoms in a molecule will arrange themselves to reduce the repulsion between electrons in different valence shells.
  • Richard F.W. Bader

    Richard F.W. Bader
    Richard F.W. Bader (Canadian Chemist) is best known for his work to create the QTAIM (Quantum Theory of Atoms in Molecules). This theory emphasizes the importance of electron density and it's relationship to physical properties. Bader devoted his life to promoting the importance of electron density but had a difficult time convincing the scientific community. Nearing the end of his life he finally saw his theory published in a scientific paper.