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5 BCE
Democritus
Democritus's theory was that atoms differed in size and shape and that they were in constant motion. He called the small pieces of matter Atomos. He was was correct with his theory except for that they are different shapes. -
John Dalton - theory that all matter was composed of atoms
Dalton’s atomic theory proposed that everything was composed of atoms, that were invisible to the human eye. While all atoms of an element were identical, different elements had atoms of differing size and mass. Dalton’s atomic theory also showed that all compounds were composed of combinations of these atoms in ratios. Dalton also theorized that chemical reactions resulted in the rearrangement of the reacting atoms. His theory was disapproved by Thompsons theory about atoms can't be split. -
Eugen Goldstein
Goldstein discovered canal rays or what he termed Kanalstrahlen. Canal rays are beams of positively charged ions (cations). He is also credited with the discovery of protons. -
J.J Thomson discovery of electron
Thomson's discovered the electron by experimenting with Crookes or cathode ray tube. Thomson realized that the accepted model of an atom did not account for negatively or positively charged particles. So he created his own model of an atom. His theory disapproved Daltons theory that atoms couldn't be split. -
J.J Thomson model of an atom
Thomson proposed the plum pudding model of the atom in 1904 before the discovery of the atomic nucleus in order to include the electron in the atomic model. -
Ernest Rutherford
Rutherford found that atoms are mostly empty space and its mass is concentrated into small tiny central nucleus. He believed that the nucleus was positively charged and everything around it was negatively charged. He also theorized the nuclear structure of the atom, discovered alpha and beta rays, and proposed the laws of radioactive decay. he won the Nobel prize. -
Robert A. Millikan
Millikan succeeded in precisely determining the magnitude of the electron's charge. By measuring how the various drops of oil moved about, it showed that their charge always was a multiple of a precisely determined charge showing the electron's charge. -
James Chadwick
Chadwick discovered neutrons in 1932. Chadwick's findings were pivotal to the discovery of nuclear fission and led to the making of atomic bombs.