1. Aim: How to Calculate the Average Atomic Mass?
Do Now: (10 min) Complete worksheet from yesterday

2. Isotopes Same element Same chemical properties
Same number of protons, but different number of neutrons and different Masses
Same chemical properties

3. Explore
You have a sample of 100 chlorine atoms. 75 out of 100 chlorine atoms have a mass of 35, and 25 out of 100 chlorine atoms have a mass of 37.
What percentage of the atoms have a mass of 35? Mass of 37?
Calculate what the average mass of all the atoms would be?

4. Most elements occur naturally as mixtures of isotopes.
Average Atomic mass
Most elements occur naturally as mixtures of isotopes.
The mass numbers on the periodic table are the weighted average of the most abundant isotopes’ mass numbers.

6. How to calculate Avg. Atomic Mass
To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products.
(mass of isotope 1× Natural abundance) X(mass of isotope 2× Natural abundance)…..

7. Sample Problem 1
Rubidium has two common isotopes, 85-Rb and 87-Rb. If the abundance of 85-Rb is 72.2% and the abundance of 87Rb is 27.8%, what is the average atomic mass of rubidium?

8. Isotope
% abundance
Fraction of abundance
mass
85-Rb
72.2%
0.722
85 amu
87-Rb
27.8%
0.278
87 amu
61.37
24.186
+______________
amu X = = X
CONVERT TO DECIMAL BY DIVIDING BY 100

9. Sample Problem 2
Uranium is used in nuclear reactors and is a rare element on earth. Uranium has three common isotopes. If the abundance of 234U is 0.01%, the abundance of 235U is 0.71%, and the abundance of 238U is 99.28%, what is the average atomic mass of uranium?

10. Sample Problem 3
Titanium has five common isotopes: 46Ti (8.0%), 47Ti (7.8%), 48Ti (73.4%), 49Ti (5.5%), 50 Ti (5.3%). What is the average atomic mass of titanium?