How to Calculate an Element's AVG Atomic Mass

Carbon's two most common isotopes help you find the average atomic mass of carbon.

You can tell who is a general by the number of stars on his shoulder; you can tell an atom by the protons in its nucleus. Protons and neutrons squeezed together like seeds in a pomegranate make up an atom's nucleus as electrons (or an electron, in the case of hydrogen) buzz around the outside of the nucleus in a crazy, unpredictable cloud of probabilities. Although the number of protons in the nucleus of an atom of a given element never varies, the number of neutrons can vary. This variability results in what scientists call isotopes.

Things You'll Need

• Chemistry webpage or textbook
• Calculator

Instructions

1. Consult a chemistry webpage or textbook for common isotopes of the element that interests you. For example, the isotope carbon-12, making up 98.89 percent of any given carbon sample, has a mass of 12 atomic mass units (amu), and the isotope carbon-13, making up 1.11 percent of any given carbon sample, has a mass of 13.00335 amu.

2. Multiply the mass of each isotope by the quotient of its percentage in a given sample and 100. For example, 12 x (98.89/100) = 11.8688; 13.003355 x (1.11/100) = 0.14433.

3. Add the answer for each isotope. For example, 11.8688 + 0.14433 = 12.013. The average atomic mass of the element carbon is 12.01 amu.

References

• Westfield State University: Atomic Mass
• University of Illinois, Urbana-Champaign: Atomic Mass
• "Chemistry: The Central Science"; Theodore E. Brown, et al.; 2006

Resources

• Westfield State University: Atomic Mass
• "Chemistry: The Central Science"; Theodore E. Brown, et al.; 2006