apr 20, 1909 - Ernest Rutherford discovers the proton and the nucleus in an atom.

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After shooting alpha particles at very thin gold foil, Rutherford discovered that some of the particles bounced back. He explained this by saying that most of the mass in an atom is in a very small center, and this is where the positive part of the atom is. He also discovered that the number of negative particles outside of an atom's nucleus must be the same as the number of positively charged particles inside of the nucleus.

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