Name: _________________________________________________
Period: _____
Average Atomic Mass
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Weighted average of the mass numbers of the naturally occurring isotopes of an element.
Multiply the mass of each isotope by its percentage abundance then add all of the parts!
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Worked out example: Calculate the avg. atomic mass of oxygen if its abundance in nature is 99.76%
16
O, 0.04% 17O, and 0.20% 18O.
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In class example: Two isotopes of copper occur in nature. 69.17% of copper atoms copper-63 and
30.83% are copper-65. What is the average atomic mass?
PRACTICE Show ALL work!!!
1. Calculate the average atomic mass for neon if its abundance in nature is 90.5% neon-20, 0.3% neon21, and 9.2% neon 22.
2. Rubidium has two common isotopes, 85Rb and 87Rb. If the abundance of 85Rb is 72.2% and the
abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?
3. Uranium has three common isotopes. If the abundance of 234U is 0.01%, the abundance of 235U is
0.71%, and the abundance of 238U is 99.28%, what is the average atomic mass of uranium?
4. Titanium has five common isotopes: 46Ti (8.0%), 47Ti (7.8%),
What is the average atomic mass of titanium?
Ti (73.4%),49Ti (5.5%),
48
50
Ti (5.3%).
5. Distinguish between mass number and average atomic mass. Which one is on the periodic table?