A scientist named Rutherford bombarded alpha particles on a thin metal foil (gold, silver), from which he made the following conclusions.
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Most of the alpha particle escapes straight away. From this, it was concluded that most of the atom is hollow.
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When alpha particles are bombarded near the atomic center, the alpha particle deviate from their path.
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He clarified that there is a positively charged part in the center of the atom.
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When the alpha particle is bombarded exactly at the midpoint, one or two particles return to their path.
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All the mass of the atom and all the positive charges are concentrated or contained in the center of the atom. Which is called the nucleus of the atom.