Determining the Average Atomic Mass of Carbon

Many of the chemical elements we find around us have many varieties known as isotopes. All atoms of carbon contain six protons within its nucleus. What makes one isotope of carbon different from another isotope is the number of neutrons in their nucleus. The atoms of each isotope have a unique mass.

Below is a mass spectrum showing the atomic mass and relative abundance of 2 isotopes of carbon.

The mass of individual atoms can be accurately measured by the use of a mass spectrometer. The mass spectrum for carbon is shown above.

To determine the average atomic mass of carbon:

• Multiply the Atomic Mass and % Natural Abundance for each isotope.
• Add the above products together.
• Divide the resultant sum by 100.

Following the procedure described above, what do you get for the value of the average atomic mass for carbon?

Record this number for later use in estimating the value of Avogadro's Number.

Return to list of procedures to continue determination of Avogadro's Number.