1. Average Atomic Mass

2. Average of Isotopes
Every element typically exists in several isotope forms
Isotopes are atoms of the same element with different numbers of neutrons
Even though there are several isotopes of the same element, we only see one square given to each element on the periodic table

3. The mass recorded on the periodic table is a weighted average of all the naturally occurring isotopes of the element

4. Example
Chlorine is found in two isotope forms. One has a mass of amu, the other a mass of amu. The first makes up 75.5% of the sample. What is the average atomic mass of the chlorine?

5. The average atomic mass is 35.5 amu
To solve, multiply the mass of each isotope with its percent abundance, then add
(34.97)(.755) + (36.97)(.245) = amu
The average atomic mass is 35.5 amu

6. Example 2 Zinc has 5 stable isotopes as shown 48.89% 63.9291 amu
Find the average atomic mass of Zn.