After what year was the " Z " number known as a physical characteristic of atoms ?
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 roughly ( to within 1 %) equal to the whole number " A " .