The document summarizes the historical development of atomic theory from ancient Greece to the early 20th century. It describes the early ideas of Democritus and Aristotle, followed by John Dalton's atomic theory in the early 1800s which proposed that all matter is made up of atoms. It then discusses J.J. Thomson's discoveries of the electron using Crookes tubes and his "plum pudding" model of the atom. Finally, it outlines Ernest Rutherford's refinement of the atomic model through his gold foil experiment and discovery of the nucleus, and the later quantum mechanical model including the discovery of the neutron by James Chadwick.