Alpha Decay is a radioactive decay of an atomic nucleus that is accompanied by the emission of an alpha particle. An atom that undergoes alpha decay will reduce its atomic mass by 4 and become the element two atomic numbers less. Since an atom loses two protons during alpha decay, it changes from one element to another. For example, after undergoing alpha decay, an atom of uranium (with 92 protons) becomes an atom of thorium (with 90 protons). In the alpha decay of a nucleus, the change in binding energy appears as the kinetic energy of the alpha particle and the daughter nucleus.