<P> Each chemical element has a unique atomic number ( Z ) representing the number of protons in its nucleus . Most elements have differing numbers of neutrons among different atoms , with these variants being referred to as isotopes . For example , carbon has three naturally occurring isotopes : all of its atoms have six protons and most have six neutrons as well , but about one per cent have seven neutrons , and a very small fraction have eight neutrons . Isotopes are never separated in the periodic table ; they are always grouped together under a single element . Elements with no stable isotopes have the atomic masses of their most stable isotopes , where such masses are shown , listed in parentheses . </P>
what 's the atomic number on the periodic table
the number of protons in its nucleus