Nuclear Physics Alpha decay is essentially limited to the radioactive elements more massive than lead. It is as though they are trying to lose as much mass as possible to move downward in atomic number toward elements that have stable forms. We already know that an alpha particle is a helium nucleus, but it is notsimply helium; it is entirely ionized and moving at a significant fraction of the speed of light. Still, it has two protons and four total nucleons, so we could symbolize it as 24!. Since the protons and the nucleons in general are each forms of mass, they tend to be conserved during a decay process. Suppose you know that uranium-238 undergoes alpha decay and you would like to know what isotope it transmutes into. Looking uranium up on the periodic table reveals that it has Z= 92protons. We already know it has A = 238total nucleons. Two of those protons must be lost to the alpha particle as well as four total nucleons. In other words, we can solve this as a double subtraction problem: 92238U!
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