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300
Aristotle 300 BC
Aristotle didn't like Democritus' idea of the atom being the smallest, indivisible component of matter. He claimed that there was no smallest part of matter and that different substances were made up of proportions of fire, air, earth, and water. There were no experimental means available to test this. Aristotle's prevailed mainly because people liked his philosophy better (300 BC) -
430
Democritus 430 BC
His atomic theory stated that the universe is composed of 2 things: the atoms and the void in which they exist and move. He believed that all matter is made up of indestructable units called atoms. He stated that atoms are the building blocks for all things and that atoms are tiny, indivisible, and only differ by shape and arrangement. (430 BC) -
Period: 500 to
Development in Atomic Theory
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John Dalton
Dalton's atomic theory was that all elements are made of atoms. He said that all atoms of an element are identical and all atosm of different elementss are different. Dalton also said that atom of one element can combine with another element to produce a compound. He also believed that atoms can combine in chemical reactions but can't be created or destroyed, only changed. Dalton did experiments with different temperatures, gasses, vapours, and pressures. -
Henri Becqerel
Henri worked with the properties of the atom, like radioactivity. His biggest achievements were in the field of radioactivity. He experimented with light and the absorption of light by crystals. When experimenting with an Uranium ore he accidently found that leaving the ore on a photographic place without any light that the ore left an image. Henri discovered that this occured because Uranium is radioactive. This discovery of radioactivity allowed later scientists to perfect the atomic model. -
J.J. Thomson
Thomson is mostly known for his discoveries of the electron and the mass to charge ratio of the electron. Thomson performed the Cathode Ray Tube experiment to prove this. This experiment showed that the atom had a negative "piece" in it which he called the electron. Also it found the mass to charge ratio of the electron -
Marie and Pierre Curie
Marie and Pierre became interested with Becquerel's discovery. They bagan expirementing with their own Uranium ore. While comparing the activity of the pure Uranimum and the Uranimum ore sample, they noticed that the ore was a lot more radioactive that the pure Uranimum. They came to the conclusion that the ore contained additional radioactive materials other than the Uranium. This observation led to the discovery of two new radioactive elements which were named Polonium and Radium. -
Ernest Rutherford
Rutherford shot alpha particles at gold foil, thinking they were going to go right through the foil. Most of them did go right through the but some others were reflected meaning that there was a small, dense positive charged area which he called the nucleus of an atom. This experiment was called the goil foil experiment which was done in 1908 -
Robert Millikan
Millikan discovered a way to accurately determine the charge carried by an electron. He proved this by the "Oil Drop Experiment" in 1909, This also proved that this quantity was a constant for all electrons. -
Niels Bohr
Niels Bohr is credited with the planetary model of the atom. Bohrs experiment with the hydrogen spectrum. He also stated that electrons traveled in specified energy levels and that spectrum lines are produced when electrons move. -
James Chadwick
Chadwick proved the existence of nuetrons. This resulted in the solution for the hidden mass in the atoms which led to the investigation of the nature of the nucleus and its forces. This made Chadwick recieve the Nobel Prize for Physics in 1935.
