The atomic mass unit or amu relates to an element's average mass.
Before calculating the amu it is vital to understand that an element can have several different masses. This is because an element may have atoms that have different numbers of neutrons which are called isotopes. Each element has its own number of isotopes. The amu represents an average of an element and its different isotopes.
To calculate the amu of an element you must first identify the mass of the different isotopes and the abundance in which they naturally occur in nature. Next, take each isotope's mass and multiply it by the extent that it occurs in nature. Make sure that you move the decimal two places to the left to turn your percentage into a decimal before multiplying. Once you have multiplied each isotope by its percentage in which it occurs in nature, you add these numbers together to get the amu.
For example, oxygen has three naturally occurring isotopes. These are O16, O17, and O18.
O16's mass is 15.99491463 and occurs at 99.757%. (15.99491463 x .99757 = 15.9560469874)
O17's mass is 16.9991312 and occurs at 0.038%. (16.9991312 x .00038= 0.00645966985)
O18's mass is 17.9991603 and occurs at 0.205%. (17.9991603 x .00205= 0.03689827861)
15.9560469874 + 0.00645966985 + 0.03689827861 = 15.9994049359
15.9994 is the amu of oxygen.
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