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Democritus 440 BC in his theory states if you keep cutting something in half you eventually will get to the individual particles of a single material John Dalton 1803 his atomic theory states that atoms are the smallest particles cannot be created, divided, or destroyed. Atom of the same substance are exactly alike; atoms of different elements are different. JJ Thompson 1897 He discovers that there are smaller particles inside the atom; which means atoms can be divided into smaller parts. One of these particles has a negative charge (the electron) Ernest Rutherford 1911 Developes a model of the atom with an extremely dense center called the nucleus with a postitive charge and electrons. Niels Bohr 1913 Develops a newer model of the atom showing the electrons moving around the nucleus in specific paths of energy levels Modern Theory 1920 Ernet Rutherford & Werner Heisenber propose a new model of the atom showing the electrons still moving around the nucleus on levels but instead of specific paths they surround the nucleus like clouds