Atomic number is shown through the symbol Z .
Since protons and neutrons have approximately the same mass ( and the mass of the electrons is negligible for many purposes ) and the mass defect of nucleon binding is always small compared to the nucleon mass , the atomic mass of any atom , when expressed in unified atomic mass units ( making a quantity called the `` relative isotopic mass '' ) , is within 1 % of the whole number A.Atoms with the same atomic number Z but different neutron numbers N , and hence different atomic masses , are known as isotopes .
Atomic number