The Nucleus Ernest Rutherford’s proposal that the atom consisted of negatively charged electrons orbiting a positively charged nucleus triggered a great deal of curiosity about the structure of that nucleus. It is important to understand how small it really is. If a hydrogen atom were the size of San Diego, the nucleus at the center would be smaller than a baseball. The atom as we understand it consists almost entirely of empty space. The phenomenon of radiation emitted from certain rare minerals had been studied long enough to conclude that it came chiefly in three particle-like forms named alpha ( α ), beta ( β ), and gamma ( γ ). Alpha particles were heavy, comparatively slow, and positively charged, judging by they way they deflected in a magnetic field. They could be blocked effectively by a sheet of paper. Beta particles were much faster and lighter, negatively charged, and required something denser, like wood or skin, to stop them. Gamma particles appeared to move at the speed of light. They were uncharged and could
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