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A major alteration to John Dalton's atomic theory was discovered in the mid 1800's: atoms were found not to be invisible, they in fact contained smaller particles. In 1898, an English physicist named J.J. Thomson discovered the electron when he found that electrically charged plates and magnets deflected a cathode ray in a certain way, which indicated that the particles making up the ray must be negatively charged. Later, the mass of an electron was found to be 2000 times smaller than that of the smallest atom, a hydrogen atom, which led Thomson to believe that atoms must also be filled with other heavier particles. In addition, atoms were at the time known to be electrically neutral, indicating that there must be some positively charged particles present to balance out the negative charge of the electrons. Based on all of this information, Thomson created the Plum Pudding Model (so called because of its resemblance to an English desert). This Pudding model represented his real atom model: in the shape of a sphere made of physical positively charged matter with electrons distributed throughout evenly due to electrostatic charge. To progress on all of these ideas, Ernest Rutherford, a student of J.J. Thomson, decided that the center of the atom was a nucleus and was surrounded by orbiting electrons. He determined this by performing the famous Gold Foil Experiment, where positively charged alpha particles were beamed through a thin piece of gold foil. Some of these alpha particles were deflected, meaning some type of positively charged matter inside the atoms of the gold foil were repelling the positively charged alpha particles. This would only have been possible if the positive charge was concentrated within the atom; thus leading to the nucleus, Latin for "little nut". The rest of the atom seemed to not be made of any matter, and most of the mass of the atom was labeled as being the nucleus.

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