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400
Democritus
He discovered the atom around 400 B.C. He was the one that believed the whole universe was made up of atoms. He also came up with the Atomic theory somewhere about 400 B.C also. -
John Dalton
In 1803 he came up with the Dalton Law. In the 1800s he discovered the behaviors of atoms -
Ernest Rutherford
Consummate experimentalist, 1871 Ernest Rutherford was responsible for a remarkable series of discoveries in the fields of radioactivity and nuclear physics. He discovered alpha and beta rays, set forth the laws of radioactive decay, and identified alpha particles as helium nuclei. -
JJ Thomson
1897 the British physicist Joseph John Thomson discovered the electron in a series of experiments designed to study the nature of electric discharge in a high-vacuum cathode-ray tube, an area being investigated by numerous scientists at the time. -
Rober Millikan
In 1909 Robert did the oil drop experiment that measured the size of an electron. He also discovered the that there was a smaller “Unit” Charge. -
James Chadwick
The discovery of neutron quickly changed scientists' view of the atom, and Chadwick was awarded the Nobel Prize in 1935 for the discovery. Scientists soon realized that the newly discovered neutron, as an uncharged but fairly massive particle, could be used to probe other nuclei. -
1885
Bohr developed the Bohr model of the atom, in which he proposed that energy levels of electrons are discrete and that the electrons revolve in stable orbits around the atomic nucleus but can jump from one energy level to another.
