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Democritus' Atomic Theory
Democritus was the first person to come up with the idea of the atom. Democritus was a Greek philosopher around the time period of 440 B.C. In his theory, if you kept cutting a piece of silver in half, eventually you would get an atom. He said that the atoms are small, hard, and uncuttable. He also said that atoms are made of a single material and are formed into different shapes and sizes. He believed that atoms are always moving and can make different materials when combined. -
John Dalton's Atomic Theory
John Dalton was a British Chemist in the late 1700s. He made new discoveries about the ato that Democtitus did not find. He discovered that all substances are made of atoms, atoms of the same element are the same and atoms of different elements are different, and atoms join with other atoms to create new substances. -
J.J. Thomson's Atomic Theory
In 1897, J.J. Thomson made a discovery on atoms. He made a discovery based on his experiment with a cathrode-rode tube. He discovered that a positively charged plate attracted an invisible beam, which he concluded had to be negatively charged. Thomson knew that atoms have no overall charge, so he figured that the negatively charges must be balanced by positive charges on the particle. -
Ernest Rutherford's Atomic Theory
In 1909, Ernest Rutherford conducted an experiment. He aimed positively charged particles at a thin piece of foil and was surprised by the results. This experiment elped him create the idea of the nucleus. Three terms were used in Rutherford's model: every atom has a nucleus, atoms are mostly empty space through which electrons travel, and electrons travel around the nucleus like the earth orbits the sun, but their exact arrangement could not be described. -
Niels Bohr's Atomic Theory
In 1913, Niels Bohr made a great discovery. He discovered that electrons travel in definite paths around the nucleus. He found out that these paths are certain distances from the nucleus. He believed that electrons could jump from one path to another path. -
Modern Atomic Theory
In today's atomic theory, we know that Bohr was wrong with his theory. Electrons do not travel in definite paths as he suggested. We now know that electrons are likely to be found in the electron cloud, as shown in this picture.
