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