Hoyo-McCauley-Per 3 By Mike_McCauley 500 Aristotle Aristotle develops his theory stating that no matter how many times you divide matter there will always be a piece of matter that is smaller. 500 Democritus States that all matter is made up of tiny, invisible, indivisible particles called atoms. Period: 500 to Oct 11, 2011 The History of The Atom Oct 11, 1803 James Dalton Dalton formed the theory that all matter is composed of small particles called atoms. These atoms were said to all be the same Oct 11, 1896 Henri Becquerel Henri Becquerel discovers some chemicals desintigrate with the effects of x-rays. He discovered radioactivity. Oct 11, 1898 Marie and Pierre Curie The duo started experimenting with radioactive substances and discovered radium and polonium. Oct 11, 1898 J.J Thomson Discovered the electron and put together the plum pudding model. Oct 11, 1908 Robert Millikan Millikan discovered the electric charge in an electron. Oct 11, 1909 Ernest Rutherford After Rutherfords gold foil experiment he discovered the possitive charge at the center of an atom. Oct 11, 1922 Niels Bohr Niels Nohr develops an explaination of the atomic structure that became the basis of the periodic table. Oct 11, 1932 James Chadwick Chadwick used alpha particles to discover the neutron.