How do you calculate the atomic mass of carbon?

Jan 4, 2015

The atomic mass of any element actually represents an atomic weight average, the sum of the atomic masses of its naturally-occuring isotopes, each multiplied by their respective abundance.

Carbon has only two naturally-occuring isotopes, Carbon-12 and Carbon-13, so both of their atomic masses will contribute to the atomic mass of carbon.

Notice that the famous ${\text{_}}^{14} C$ isotope does not contribute to the atomic mass of carbon, since it is not naturally-occuring (it is formed in the upper atmosphere from the bombardment of Nitrogen-14 atoms with neutrons).

${\text{_}}^{12} C$ is listed as having a fractional abundance of 0.989 (or a percent abundance of 98.9%) and an atomic weight of 12.000000 u (u represents atomic mass units).

${\text{_}}^{13} C$ is listed as having a fractional abundance of 0.011 (or a percent abundance of 1.1%) and an atomic weight of 13.003355 u.

The atomic mass of carbon is then determined to be

$\text{atomic mass} = 0.989 \cdot 12.000000 + 0.011 \cdot 13.003355$

$\text{atomic mass} = 12.011$ $\text{u}$

This is the value for carbon's atomic mass listed in the periodic table.

More on atomic weight average here:

http://socratic.org/questions/what-is-average-atomic-mass