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Average Atomic Mass

Average Atomic Mass. Average Atomic Mass – the weighted average of the masses of the isotopes of an element Every element is composed of several naturally occurring isotopes of that element-each with its own atomic mass

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Average Atomic Mass

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  1. Average Atomic Mass • Average Atomic Mass – the weighted average of the masses of the isotopes of an element • Every element is composed of several naturally occurring isotopes of that element-each with its own atomic mass • A weighted average of the percentage of each isotope that exists versus the atomic mass of each isotope is used to calculate the atomic mass that appears on the periodic table. Step 1) Multiply each atom’s mass by its percent to get the contribution for each isotope. Step 2) Add all the contributions together. Step 3) Divide by 100 to get the average atomic mass. AAM has units of amu, atomic mass unit.

  2. Average Atomic Mass = Σ [(isotope mass) * (percent abundance of isotope)] 100 Example 1: The element copper occurs naturally as 69.17% of copper-63 with a mass of 62.9296 amu and 30.83% of copper-65 with a mass of 64.9278 amu. What is the average atomic mass of copper?

  3. Example 2: The atomic weight of gallium is 69.72 amu. The masses of the naturally occurring isotopes are 68.9257 amu and 70.908 amu for 69Ga and 71Ga respectively. Calculate the % abundance of each isotope.

  4. Learning Check Naturally occurring element consists of 75.78% atoms with a mass of 34.969 amu and 24.22% atoms with a mass of 36.966 amu.  Calculate the average atomic mass. b) Based on the average atomic mass, what is the element?

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