1. Average atomic Mass

2. What does the atomic mass tell us on the Periodic table?

3. Atomic Mass Weighted average of element’s natural isotopes
Some isotopes are more abundant than others….
SO atomic mass leans towards more abundant mass

4. How do we calculate atomic mass?
1) Masses of Isotopes
2) Natural abundance of each isotope
usually a percentage
Average atomic mass = mass contributed by all isotopes
Fraction of abundance (isotope mass) = mass from a particular isotope

5. Example 1
Neon has 3 natural isotopes. Ne-20 (90.51%, u) Ne-21 (0.27%, u) Ne-22 (9.22%, u) What is the weighted average atomic mass for Ne?
20.18 u

6. Example 2:
A certain metallic element has two stable isotopes: M-45 and M-48. Given that the atomic weights are and , respectively, what is the percent abundance of the lighter isotope if the average atomic weight of M is ?

7. Example 3:
Calculate the atomic weight of chlorine if % of the atoms have a mass of amu and 24.23% have a mass of amu.
35.45 amu