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400
Democritus
Around 400 B.C, Democritus first proposed the existence of an ultimate particle and that all matter is made up of small indestructable units called atoms. He stated that atoms are the the buildling blocks of all things. Atoms are solid, indestructable, and not infinitely divisable. -
Period: 400 to
Development of Atomic Theory
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John Dalton
Dalton's atomic theory focuses on how matter is made up of atoms that are identical and that compounds are different elements combined together. He also stated that particles could not be divided or destroyed. In a chemical reaction, atoms can be separated, combined or rearanged. -
J.J Thomson
In 1897, scientist J.J Thomson determined the charge to mass ratio of electrons. Thomson used a cathode ray tube to create a tube with a positively charged anode side and a negatively charged cathode side which made a ray then he placed a magnet in the middle and the beam bended toward the positive end of the magnet meaning the particles in the beam were negatively charged electrons.Thomson proposed the Plum Pudding Model and inferred that atoms must be negatively charged. -
Robert Millikan
In 1909, Robert Millikan determined the per unit chagre of the electron. With his oil-drop experiment he figured out that the mass of an electron was very small and that the overall charge of an atom was neutral. This allowed calculations of mass and charge of the electron and the positively charged atoms. -
Ernest Rutherford
In 1907, Rutherford did his famous gold foil experiment. He shot alpha particles at gold foil, most of them went right through the foil but some were reflected meaning there was a small, dense, positive charged area he called it a nucleus of an atom. In 1911 he proposed the nuclear atom as a result. 8 years after his experiment he stated that the nucleus contained positively charged particles called protons. In 1920 Rutherford proposed the exsistence of the thrid atomic particle, the neutron.
