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Up until the early 1900s, the structure of the atom was not understood. It
was thought that the particles carrying the + charges and those carrying the -
charges were mixed up in a atom-sized ball. (The "plum-pudding" model.)
Ernest Rutherford carried out a classic experiment in 1911 that showed the
above model to be incorrect. Instead, virtually all the mass of the atom is
centered in a tiny nucleus, consisting of protons and
neutrons. The
lighter electrons
surround the nucleus in a thin cloud: almost all of the atom is actually empty
space
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