The emission of an a particle, or 4He nucleus, is a process called a decay. Since a particles contain protons and neutrons, they must come from the nucleus of an atom. The nucleus that results from a decay will have a mass and charge different from those of the original nucleus. A change in nuclear charge means that the element has been changed into a different element. Only through such radioactive decays or nuclear reactions can transmutation, the age-old dream of the alchemists, actually occur. The mass number, A, of an a particle is four, so the mass number, A, of the decaying nucleus is reduced by four. The atomic number, Z, of 4He is two, and therefore the atomic number of the nucleus, the number of protons, is reduced by two. This can be written as an equation analogous to a chemical reaction. For example, for the decay of an isotope of the element seaborgium, 263Sg:
| 263Sg
----> 259Rf + 4He
|
The atomic number of the nucleus changes from 106 to 104, giving rutherfordium an atomic mass of 263-4=259. a decay typically occurs in heavy nuclei where the electrostatic repulsion between the protons in the nucleus is large. Energy is released in the process of a decay. Careful measurements show that the sum of the masses of the daughter nucleus and the a particle is a bit less than the mass of the parent isotope. Einstein's famous equation, E=m•c2, which says that mass is proportional to energy, explains this fact by saying that the mass that is lost in such decay is converted into the kinetic energy carried away by the decay products.
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