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465 BCE
Democritus' Atomic Theory
Democritus created a theory stating that the universe is composed of atoms and the void in which atoms exist.
1. All matter consists of invisible particles called atoms.
2. Atoms are indestructible.
3. Atoms are solid but invisible.
4. Atoms are homogenous.
5. Atoms differ in size, shape, mass, position, and arrangement. https://the-history-of-the-atom.wikispaces.com/Democritus -
400 BCE
Democritus' Atomic Model
Democritus also created an atomic model, which was just a sphere with no electrons, neutrons, or protons. https://the-history-of-the-atom.wikispaces.com/Democritus -
332 BCE
Aristotle's Atomic Theory
Aristotle did not believe in the existence of atoms. Instead, he believed that all materials were made up of five elements: Earth, fire, water, air, and aether. https://the-history-of-the-atom.wikispaces.com/Aristotle -
John Dalton's Atomic Theory
Dalton's theory of the atom states:
1. All matter is made of atoms, which are invisible and indestructible.
2. All atoms of a given element are identical in mass and properties.
3. Compounds are combinations of two or more different types of atoms.
4. A chemical reaction is a rearrangement of atoms. https://www.khanacademy.org/science/chemistry/atomic-structure-and-properties/introduction-to-compounds/a/daltons-atomic-theory-version-2 -
J.J. Thomson Discovers the Electron
He discovered the electron through a series of experiments with electric discharge tubes. https://www.nobelprize.org/educational/physics/vacuum/experiment-1.html -
J.J. Thomson's Plum Pudding Model
Thomson created a model which compared the structure of an atom to that of plum pudding. He stated that electron particles were scattered throughout the pudding-like positive charges. This madel had no nucleus. https://the-history-of-the-atom.wikispaces.com/J.J.+Thomson -
Ernest Rutherford discovers the nucleus and the proton using the gold foil experiment https://en.wikipedia.org/wiki/Atomic_nucleus
This experiment was conducted in order to test Thomson's plum pudding theory. Rutherford decided that if this theory was correct, he could direct alpha particles at a thin sheet of gold foil, and the positively charged alpha particles would easily pass through. On the contrary, the particles were deflected in all sorts of angles and proved that there must be separate positive (protons) and negative charges and a dense, centralized, positively charged mass which was named the nucleus. -
Niels Bohr's Atomic Theory
He proposed that Electrons should move around the nucleus, but only in prescribed orbits, and when jumping to an orbit with lower energy, a light quantum is emitted. https://www.nobelprize.org/nobel_prizes/physics/laureates/1922/bohr-facts.html -
Erwin Schrödinger's Atomic Model
Rather than depicting a nucleus with the predicted paths of the surrounding electrons, this model predicts the odds of a location of an electron. It looks like a nucleus with a surrounding electron cloud. http://www.abcte.org/files/previews/chemistry/s1_p6.html -
Niels Bhor Concept of Complementarity
He suggested that physical properties on an atomic level would be viewed differently depending on experimental parameters. He also applied this concept on a philosophical level, stating that evolving concepts of physics deeply affected human perspectives. http://www.biography.com/people/niels-bohr-21010897#wins-nobel-prize -
James Chadwick Discovers the Neutron
James Chadwick bombarded beryllium with alpha particles, and he observed the emission of an unknown radiation. In 1932, he proved that the radiation consisted of a neutral particle with about the same mass as a proton, and he called it a neutron. https://www.nobelprize.org/nobel_prizes/physics/laureates/1935/chadwick-facts.html