Notation
Nuclei are composed of neutrons and protons. A nucleus is specified by its number of neutrons (N) and its number of protons (Z). A third symbol A refers to the total number of nucleons, A = N + Z. A nucleon is either a proton or a neutron. Elements are assigned names (e.g. carbon, lithium, potassium) according to their charge, since the charge of the nucleus determines the number of electrons in the neutral atom that can be formed using the nucleus, and hence determines the chemical properties of that atom. Thus all carbon nuclei have Z = 6, and all gold nuclei have Z = 79. The word isotope refers to a specific combination of N and Z. If two carbon nuclei have different number of neutrons, one would say they are different isotopes of carbon. We illustrate the notation for an isotope by showing the symbol for a sodium (all sodium has Z = 11) nucleus with 12 neutrons.
which is the same as 23Na because the 11 is redundant with the sodium symbol. Since the upper left number refers to A, one must subtract Z from A to find the number of neutrons N. To find the atomic number Z corresponding to any given element, you can use a periodic table. A periodic table does not generally list the various isotopes of each element; instead it gives an effective mass number A that is based on an average according to the relative abundances of the naturally occurring isotopes. For information on individual isotopes, you use a table of the nuclides, rather than a periodic table.