The identity of an atom is characterized by its atomic number, Z.
Z=1, the element is hydrogen; Z=2, the element is helium;...... Z=26, the element is iron.
And if Z=6, the element is carbon. Of course, a given nucleus may contain various, differing numbers of neutrons, and this gives rise to the phenomenon of isotopes, the same element with a different mass. Most carbon nuclei contain 6 neutrons, i.e. 6 massive nuclear particles with zero charge, and thus the most numerous istope is 12C, a few contain 7 neutrons to give 13C, which is a very important isotope for direct characterization of organic molecules, and an even smaller percentage of carbon nuclei contain 8 neutrons to give the 14C isotope.
In short, the number of protons determine Z, and thus the identity of the element; the number of neutrons determine the mass. Most heavier elements have a range of isotopes, and the average isotopic mass is quoted on the Periodic Table.
