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.

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Presentation transcript:

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 h as units of amu, atomic mass unit.

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

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

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