12C is assigned the a mass of 12 atomic mass units (u) . One atomic mass unit is one twelfth of 12C mass . The masses of all other atoms are given relative to this standard.
For example: When 13C and 12C are analyzed in a mass spectrometer, the ratio of their masses is found to be
Since the atomic mass unit is defined such that the mass of 12C is exactly 12 atomic mass units, then on this same scale, Mass of 13C
13C = (11.0836129) (12 u) = 13.003355 u
sometimes the atomic mass called atomic weight.
Average atomic mass is an average of masses of the isotopes compositing the natural element.
Note: The atomic masses in the periodic table is an average atomic masses
Example: The carbon have isotopic mixture: 12C (98.89%), 13C (1.11 %), 14C (negligible), then the atomic mass (average atomic mass)=
atomic mass = 98.89% of 12u + 1.11% of 13.0034u= 12.01u