1. Average Atomic Mass

2. Atomic Mass and Isotopes
Every isotope of an element has a different atomic weight.
Carbon – 12
Carbon -13
Carbon – 14

3. Atomic Mass and Isotopes(con’t)
Some isotopes are very common, while others are rare.
Each isotope can be given percent value for how common it is.
Carbon-12 = 98.92
Carbon-13 = 1.07
Carbon-14= 0.01

4. Atomic Mass and Isotopes(con’t)
A sample of any element will be a mix of all isotopes.
We use a weighted average to determine the atomic mass.

5. Practice Problem
Determine the average atomic mass for the following element.
12C(98.92), 13C(1.07), 14C(0.01)

6. Solution
Multiply each atomic mass by the percent as a decimal, then add the answers.
12(.9892) + 13(.0107) + 14(.0001) =
= 12.01