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Period: 500 BCE to
500 BC - 2000 AD
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460 BCE
Democritus
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400 BCE
The Four Elements
Aristotle believed all substances were made of the four elements: water, fire, earth, and air. There was also the mixtures of the four including hot, dry, wet, and cold. This was of course, incorrect, but the people at the time believed him, as he was highly renowned for the time and people chose this theory over Democritus'. This was the last anyone would touch on the topic until the 17th century. -
400 BCE
Aristotle
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460
First Model of the Atom
An Greek philosopher by the name of Democritus came up with the concept of a piece of matter so small it cannot be divided into smaller pieces any further. This model however was not adopted in even the slightest, as everyone would later prefer Aristotle's model for the "atom." This concept would later be adopted when researchers started looking closer into the topic. The model appears to be a simple round sphere that cannot likely be broken. -
H2O
Antoine Lavoisier managed to discover the formula containing the two elements for water. He named them Oxygen and Hydrogen. He was not attempting to explain the atom, bur rather was just explaining his discovery of two new elements. His discovery did lead to the further study of atoms though. His model appears that mass is neither created or destroyed, but is rather in a constant state of being. -
Antoine Lavoisier
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John Dalton
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The Atomic Theory
John Dalton believed in Democritus' theory about atoms resembling a marble. Dalton came up with the Atomic Theory, that all matter was made up of atoms. He also thought that atoms of the same element are exactly alike and atoms of other elements are different. Also, that when something goes through a chemical reaction it is their atoms combining, separating, and changing. This developed further upon Lavoisier's research. -
Radiation
Henri Becquerel discovered radiation, and thus that an atom is still dividable and able to change shape. His model shows a radioactive atom splitting into energy and particles due to radiation. This disproves Democritus and John Dalton who both believed that atoms could not be changed or divided. -
Henri Becquerel
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J.J. Thomson
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Plum Pudding
J.J. Thompson performed an experiment that resulted in a magnet bending a light beam towards the positive side of the magnet leading him to believe the ray to have a negative charge. This later led to him creating the Plum Pudding Model in which positively charged neutrons are surrounded be negatively charged ones. Another one of his accomplishments was the discovery of the electron, a crucial part of the atom. -
The Quantum Theory
Max Planck didn't exactly have an atom model, bur rather studied the emission and absorption of radiation. Energy is emitted or absorbed in the form of wave packets (later called photons). The amount of energy is proportion to the frequency of the radiation. His work allowed for Niels Bohr to create his own atomic model, so Planck's work is still relevant to the Atomic Timeline. -
Max Planck
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Marie and Pierre Curie
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Atomic Radiation
Marie and Pierre Curie researched radiation that is emitted from atoms. They discovered two new elements, polonium and radium, as well as that radiation is not from the arrangement of atoms in a molecule, but rather just from the atom itself. They also led towards the work of creating the atomic bomb, and thus atom splitting in general. -
A Single Electron
Robert Millikan is attributed with the discovery of the charge of a single electron. He achieved his goal using the oil-drop experiment, as pictured below. This research proved very useful in further study of the atom. -
Robert Millikan
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Ernest Rutherford
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Golden Foil
Ernest Rutherford, the student of J.J. Thomson performed an experiment with particles going through a piece of gold foil with no resistance. This helped him to create the model of the atom similar to Thomson's but 3D. It is a positive nucleus surrounded by negative electrons. This is similar to the atomic model we use now, but with some minor modifications. -
The Bohr Model
Niels Bohr used Max Planck's research and study of energy to discover that an atom has multiple energy levels with a set amount of electrons on each level and that the electrons had a set path they traveled. While they did have a set path, they could still move between energy levels. -
Niels Bohr
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X-Ray & Atomic Numbers
Henry Moseley managed to use x-ray waves to find the number of protons in the nucleus for each element, and then sorted each element by their number of protons in their nucleus, aka their atomic number. -
Henry Moseley
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Uncertainty Principle
Werner Heisenburg discovered that electrons did not move in unique orbits, but rather that they contain photons and irregularly change their movement patters. -
Werner Heisenburg
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Electron Cloud
Erwin Schrodinger was able to formulate a method of predicting where the electron of an atom is to be at any given time. He also gave a name for the area in which the electron traveled, the "electron cloud." The most dense part of the cloud is where the electron is most likely to be at any given time. This research elaborated on Heisenburg's theory of the electron, both disproving it and reinforcing different elements of it. -
Erwin Schrodinger
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The Neutron
While it may seem surprising that the latest model of the atom was only when James Chadwick finally discovered the neutron. His studies were later used in creating the first atomic bombs. -
James Chadwick
