1. Average Atomic Mass
In nature, most elements are a mixture of different isotopes
The mass of a sample of an element is a weighted average of all the isotopes in the sample

2. If 12.011 is the weighted average of the isotope masses,
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AVERAGE atomic mass
If is the weighted average of the isotope masses,
which isotope is more abundant in nature:
C-12 or C-14?

3. Calculating Average Atomic Mass
The element Chlorine Cl has two naturally occurring isotopes: Cl-35 and Cl-37
Which isotope has more neutrons?
How many more neutrons?

4. Calculating Average Atomic Mass
If 75% of all chlorine atoms in nature are Cl-35 and 25 % are Cl-37, what is the average atomic mass of a sample of chlorine?
*** Remember, this is a weighted average!

5. Calculating Average Atomic Mass
Out of 100 chlorine atoms, 75 will have a mass of 35 amu.
25 will have a mass of 37 amu

6. Calculating Average Atomic Mass
The unweighted average of 35 and 37 is… 36
This is NOT our answer!

7. Calculating Average Atomic Mass
The weighted average is calculated using the following formula (memorize this!)
(mass of isotope A x %) + (mass isotope B x %)

8. Calculating Average Atomic Mass
(mass of isotope A x %) + (mass isotope B x %)
(35 amu x 75) + (37 amu x 25) =
100
35.5 amu

9. (mass of isotope A x %) + (mass isotope B x %) 100
Try this one:
Cu-63 is 69.17% of all copper atoms
Cu-65 is 30.83%
Is the average atomic mass of Cu 64 amu?
NO!
Will the weighted average be closer to 63 amu or 65 amu? WHY?
Now calculate the average atomic mass using the formula

10. (63 amu x 69.17) + (65 amu x 30.83)
100
= amu