In atoms with large nuclei, such as uranium, the repulsive power of lots of positively charged protons can destabilize the nucleus until, eventually, it breaks down spontaneously, emitting high-energy electrons or photons or even whole helium nuclei. As chunks of the nucleus are ejected, the element is transformed into different elements with fewer protons. For example, uranium eventually breaks down to lead. In the first decade of the twentieth century, Ernest Rutherford realized that, even if you could not tell when a particular nucleus was about to break apart, radioactive breakdown was a
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