A nucleus may be represented conveniently by the chemical symbol for the element together with a subscript and superscript for the atomic number and mass number. (The subscript is often omitted, since the element symbol fixes the atomic number.) The nucleus of ordinary hydrogen, i.e., the proton, is represented by 1H1, an alpha particle (a helium nucleus) is 2He4, the most common isotope of chlorine is 17Cl35, and the uranium isotope used in the atomic bomb is 92U235.
Nuclear reactions involving changes in atomic number or mass number can be expressed easily using this notation. For example, when Ernest Rutherford produced the first artificial nuclear reaction (1919), it involved bombarding a nitrogen nucleus with alpha particles and resulted in an isotope of oxygen with the release of a proton: 2He4+7N14→8O17+1H1. Note that the total of the atomic numbers on the left is equal to the total on the right (i.e., 2+7 = 8+1), and similarly for the mass numbers (4+14 = 17+1).
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.