How to Calculate the Average of Isotopes
- 1). Write down the percent abundances and masses for each isotope of the element you are working with. The percent abundance is the percentage of a random sample of an element that exists as a particular isotope. The values for percent abundances should be given in a chemistry problem or reference table. For example, carbon naturally exists as two different isotopes: carbon-12 and carbon-13. In a random sample of elemental carbon, 98.93 percent exists as carbon-12, which has a mass of 12.000 atomic mass units (amu) and 1.07 percent exists as carbon-13, with a mass of 13.003 amu.
- 2). Multiply each percent abundance by the mass of each isotope. Convert the percentages into decimals by moving the decimal point to the left two spaces. For carbon's isotopes, you have 0.9893 x 12 = 11.871 amu for carbon-12 and 0.0107 x 13.003 = 0.1391 amu for carbon-13.
- 3). Add the values calculated in Step 2 for each isotope to get the average atomic mass for your element. For carbon, you have 11.871 amu + 0.1391 amu = 12.0103 amu. As you might predict, the average atomic mass is close to 12, since almost 99 percent of naturally occurring carbon has a mass of 12 amu.
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