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Average Atomic Mass

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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) Change each percent abundance into a decimal weight. Step 2) Multiply each atom’s mass by its decimal weight to get the contribution for each isotope. Step 3) Add all the contributions together to get the average atomic mass. Has units of amu, atomic mass unit.

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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? 100% * Average Atomic Mass = Σ [(isotope mass) * (percent abundance of isotope)]

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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 69 Ga and 71 Ga respectively. Calculate the % abundance of each isotope.

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