1. Average Atomic Mass & % Abundance

2. Average Atomic Mass
The weighted average of the atomic masses of the naturally occurring isotopes of an element
Most elements occur naturally as mixtures of isotopes

3. Average Atomic Mass
Dependent upon both mass and the relative abundance of each of the elements isotopes

4. Example
Naturally occurring copper exists with the following abundances:
69.17% is Cu-63 w/ atomic mass amu
30.83% is Cu-65 w/ atomic mass 64.93
(.6917)x(62.93) + (.3083)x(64.93)= amu

5. Problem 1 3 Isotopes of Ar occur in nature 0.337% as Ar-36, 35.97 amu
Calculate the Average Atomic Mass

6. Answer Check
(.00337)x(35.97) + (.00063)x(37.96) + (.996)x(39.96)= 39.95amu

7. Problem 2
2 Naturally occurring Isotopes of Boron occur with the following abundances:
80.20% B-11, amu
19.80% B-10, amu
What is the Average Atomic Mass

8. Answer Check
(.8020)x(11.01) + (.1980)x(10.81) = amu

9. Calculating & Abundance
Chlorine has two isotopes: chlorine-35 (mass amu) and chlorine-37 (mass amu).
What is the percent abundance of these two isotopes if chlorine's atomic mass is ?

10. Answer Check Part 1
if 2 isotopes, then the total is 100%. assume one is x% (x), the other is automatically 100-x%, (1-x)
x(34.97) + (1-x)(36.97) =

11. Answer Check Part 2 x(34.97) + (1-x)(36.97)=35.453 Solve for x

12. Answer Check Part 3
Therefore Cl-35 has a % abundance of 75.85% and Cl-37 has a % abundance of 24.15%

13. Problem 1
The two naturally occurring isotopes of nitrogen are nitrogen-14, with an atomic mass of amu, and nitrogen-15, with an atomic mass of amu. What are the percent natural abundances of these isotopes?
The atomic mass of nitrogen is amu

14. Answer Check The atomic mass of nitrogen is 14.00674amu
= p( ) + (1 -p)( ) = p p p =
p = = 99.63% (N14) 1 - p = = 0.37% (N15)