Since atoms are very small their mass is not expressed in gram (e.g. the mass of a carbon atom having 6 protons, 6 neutrons and 6 electrons is 1.992x10-23 g . A relative scale is used where the carbon 12 isotope (6 protons, 6 neutrons, 6 electrons) has been assigned the value of 12.000 atomic mass units (amu). Thus, 1 amu = 1.6605x10-24 g.
Protons and neutrons each weigh about one atomic mass unit. The atomic mass unit is defined as exactly 1/12th the mass of a carbon-12 atom, or 1.66057 x 10 -24 g. Atomic mass units allow for determination of relative atomic masses of atoms for different elements.