isotope

Different atoms of a chemical element in the periodic table all have the same number of protons, but may have a different number of neutrons in their nuclei. These different versions of the same element are called isotopes. The number of neutrons is not simply random, however—the nucleus will only be stable for combinations of protons and neutrons. Most chemical elements have several stable isotopes. For example, lithium (A=3) has two stable isotopes, one with three neutrons in the nucleus (6Li) and one with four (7Li). See: atomic number, mass number.