Nuclei are composed of protons and neutrons. Different nuclei of the same element contain different number of neutrons. These different nuclei are called isotopes. The symbol for an isotope denotes both the atomic number which is the number of protons in the nucleus and the total number of nuclides, the number of protons plus the number of neutrons. For example,

is an isotope with 6 protons and 6 neutrons. From the number of protons the element is determined to be carbon. You can use any table of atomic numbers or a periodic table to identify the element from the atomic number.

Charts are available that list known nuclides, the Chart of the Nuclides. By clicking, this chart reveals three levels. The first click brings in a close up view that permits selecting individual isotopes from the second click. An interesting table of nuclides can be accessed by clicking a graph of atomic numbers.

Table of the Isotopes -- This chart shows a periodic table from which a single click produces an exhaustive list of nuclides for an element.