Ernest Rutherford was a New Zealand-born British physicist who was able to shed light on the important features of an atom through his gold foil experiment. In his experiment Rutherford bombarded a very thin sheet of gold foil with alpha particles. Let's make a note here that alpha particles are positively charged.
Rutherford observed two things. First, most of the alpha particles were able to pass through the sheet. Second, some alpha particles were deflected back. What did these observations signify? The fact that most alpha particles went through signified that most of the atom is empty space. The observation that some particles deflected signified that they encountered something dense on the their path. What they encountered was the nucleus. Therefore, nucleus is very small and dense, and contains most of the mass of an atom. Since alpha particles are positively charged, he concluded that nucleus was positively charged as well, because they deflected back (opposite charges repel).
Rutherford's experiment and conclusions gave great insight into the structure of an atom. This was perhaps the most prominent of Rutherford's experiments but not the only one. He came up with alpha and beta naming for different kind of radiation, as well as being able to induce a nuclear reaction from a stable element.
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