English physicist Ernest Rutherford is credited with discovering the nucleus in the early 1900s. Rutherford had a long career in atomic physics, during which he taught many other famous physicists. He studied radioactivity along with Marie and Pierre Curie and Henri Becquerel, and through their experiments, they learned much of what we know today about the atom, including the discovery of the nucleus. Before the nucleus was discovered, it was widely believed that positive and negative charges were distributed evenly throughout the atom. Rutherford, along with Hans Geiger and student Ernest Marsden, conducted some interesting tests that proved otherwise [source: Cambridge].
In one of their most well-known experiments, which took place while Rutherford was a professor at the University of Manchester, the physicists beamed alpha particles at a thin sheet of gold foil. During the experiment, most of the particles passed through the gold foil onto the screen behind it, but some were deflected. Rutherford determined that the particles that made up the gold foil must consist mostly of empty space since most of the alpha particles passed through. However, some regions of the gold must have been too dense to allow the alpha particles through, and this heavy part of the atom was what Rutherford called the nucleus. He concluded that it was the nucleus that held most of the atom's mass [source: ThinkQuest].
After Rutherford's discovery in 1911, studies of the nucleus became known as "nuclear physics" and, eventually, led to the splitting of the atom [source: Cambridge]. Rutherford also used alpha particles to discover the proton, and one of his co-workers and fellow physicists, James Chadwick, discovered the neutron.
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