-
425 BCE
Democritus' Theory of Atoms
Reasoned that, since you can divide something in half and both halves would have similar properties to the whole, that if you kept dividing the object it would reach a point where there would be an indivisible piece. This was the first suggestion of the existence of atoms. He also stated that the interactions between these atoms where what caused states of matter to differ. This theory was one of the first suggestions of what was on a smaller level. According to this, an atom was a small sphere. -
350 BCE
Aristotle's Theory of 5 Elements
Ignored Democritus’ theory. A very influential Greek philosopher that thought there were five elements--Earth, Air, Fire, Water, and Aether-- that made up everything. Aristotle’s theory directly contradicted Democritus’ theory, which meant a lot because of how big Aristotle’s influence was on the people. Although this theory was never officially adopted, it caused many people to believe it. -
Period: 1000 to 1001
Sources 1
Information Sources:
http://www.abcte.org/files/previews/chemistry/s1_p2.html
https://atomicmodelhistory.wordpress.com/2010/11/27/aristotles-contribution/
https://www.britannica.com/biography/Antoine-Laurent-Lavoisier
https://www.atomicheritage.org/profile/werner-heisenberg
https://micro.magnet.fsu.edu/optics/timeline/people/planck.html
https://www.britannica.com/biography/Henry-Moseley
https://www.nobelprize.org/prizes/themes/marie-and-pierre-curie-and-the-discovery-of-polonium-and-radium/ -
Period: 1002 to 1003
Sources 2
https://www.britannica.com/biography/Robert-Millikan
Picture Sources:
https://en.wikiquote.org/wiki/Democritus
https://www.timetoast.com/timelines/development-of-the-periodic-table-7859d12d-52eb-4a1f-9138-62e6127cad74
https://www.allposters.com.au/-sp/Antoine-Lavoisier-French-Chemist-posters_i10023686_.htm
https://www.timetoast.com/timelines/atomic-theory-project-5a529abc-5b5e-49d3-9502-0c6ff54b3b4b
https://www.timetoast.com/timelines/atomic-theory-project-5a529abc-5b5e-49d3-9502-0c6ff54b3b4b -
Period: 1004 to 1005
Sources 3
https://www.timetoast.com/timelines/the-history-of-the-atom--4
https://www.timetoast.com/timelines/the-history-of-the-atom-2cbdfda7-50c2-4e43-b3a2-ce1dd2f5aa35
https://en.wikipedia.org/wiki/History_of_quantum_mechanics
https://slideplayer.com/slide/8621109/
http://www.saburchill.com/chemistry/visual/atoms/084.html
https://byjus.com/chemistry/rutherford-atomic-model-drawbacks/
https://prestonstimeline.weebly.com/erwin-schroumldinger.html
https://timelineatomictheory.weebly.com/henry-moseley.html -
-
Antoine Lavoisier's Theories of Conservation of Mass and Disproving of the Phlogiston theory
Enforced the conservation of mass. An 18th century french chemist who set foundations for atomic investigations. He proved that matter was neither created nor destroyed during chemical reactions, also disproving the Phlogiston theory. Lavoisier suggested that elements could be more than just spheres-- they could be compounds, enhancing the model of the atom. -
John Dalton's Five Assumptions
John Dalton was the creator of a generally accepted theory that included assumptions such as the existence of atoms, the indestructibility of atoms, and the ratios of molecules. John agreed with Democritus’ theory of atoms and Lavoisier’s idea of compounds and conservation of mass. Dalton set forth assumptions that used previous theories and were used by many future theorists. -
Henri Becquerel and Radiation
Henri Becquerel is the discoverer of radiation. Henri placed a piece of radioactive material on a plate wrapped in paper, and was surprised to find that the paper had a hole in the shape of uranium. Further studies showed that the radiation shared properties with light and x-rays. Henri Becquerel’s discovery contradicted previous theories that atoms could not be divided or broken apart. This changed the picture of an atom from one sphere to multiple. -
Thomson and the Discovery of Electrons
Thomson suggested the existence of electrons--tiny particles with the atom that had a negative charge. Thomson thought that the electrons were floating around inside the atom, which he compared to bits of fruit inside a plum pudding. This added to the previous theories of atoms being indestructible bits, but took away from the theories that atoms were the smallest particles in the universe. -
Marie and Pierre's Studies of Radiation
Two students of Henri Becquerel. These two shared the nobel prize with Henri Becquerel for the discovery of radiation. They helped people understand radioactivity. They also added to studies of elements, as when they discovered radium and polonium. They discovered that the radioactivity of molecules was not due to the molecule, but from the atoms. -
Robert Millikan's Experiments
Proved photoelectric patterns introduced around a decade earlier by Albert Einstein. He then used this to prove Planck’s constant. Robert Millikan enhanced Planck’s theory with an exact value and enhanced Einstein’s photoelectric theory. Millikan’s work did not greatly affect the model of an atom. An important result of Millikan's experiments was the charge of an electron, which he measured using his oil-drop experiment. -
Ernest Rutherford's Atom Volume Experiments
Studied radiation and discovered three forms that were emitted-- Alpha, Beta, and Gamma. Rutherford was a former student of J.J. Thomson. Rutherford proved Thomson’s theory that electrons were inside the nucleus incorrect. He put particles through a foil and saw that while most of the particles passed through, some bounced off, proving that most of the atom was empty space. This in part proved the existence of the nucleus, changing the way people saw the atom. -
Henry Moseley's Atomic Number Definitions
Moseley was the first person to declare that the atomic number was what determined properties that elements had. Prior to this, most thought that the atomic weight was what gave elements their properties. Moseley also gave an equation for how to find the frequencies of waves in the X-ray spectra. -
Niels Bohr's Levels of Electrons
Niels Bohr theorized that electrons had their own orbits around the nucleus. He said that electrons could have different levels of orbit around the nucleus, farther ones having higher levels of energy, which allowed for energy to be absorbed or emitted. This enhanced the idea that electrons float around the nucleus in an atom, and helped people understand potential transfers of energy between atoms. -
Max Planck's Quantum Theory of Energy
Max Planck is was the original creator of quantum theory of energy. Planck told that energy emitted from atoms was not unbroken waves, but minute packets. He gave equations to find the energy of a quantum, also creating what is now an often used constant of physics known as Planck’s constant. Planck greatly enhanced atomic theories by adding a whole new section to them -
Werner Heisenberg's Uncertainty Principle
Credited with the creation of new quantum theories, which allowed for studies to find more forms of elements and molecules, including hydrogen allotropes. Werner Heisenberg is also credited with the creation of the “uncertainty principle”, which states that it is impossible to know the exact position and exact velocity of an object at any given time. This principle states that the amount you can know is limited. You can either know more about its position and less about velocity, or vice versa. -
Erwin Schrodinger's "Electron Cloud" Model
Created a new model for the atom using mechanical mathematical equations that would predict the probability of an atom being in a certain location, not the path of an electron. This resulted in a model of a nucleus with an “electron cloud” around it, with the most dense areas of the cloud being where electrons would most likely to be. -
James Chadwick's Discovery of Neutrons
James Chadwick used alpha particles and beryllium atoms to produce a new form of radiation. In this new form Chadwick found that there were objects with the mass of a proton and a neutral charge. This was the discovery of neutrons. This discovery gave an explanation for radiation, enhancing the theories and studies of people before who studied radiation. Chadwick’s model introduced neutrons to the nucleus of the atom, an addition from previous models.
