1. Aim: How to Calculate the Average Atomic Mass?
Do Now:1. What are the 3 subatomic particles of an atom? List the three subatomic particles and their charge.
2. If an atom has an atomic mass of 34 and the atom has 16 neutrons, how many protons does the atom have?

2. Isotopes
Atoms of the same element (same number of protons) that have differing numbers of neutrons are called isotopes.
Isotopes of an element have:
Differing Atomic mass #
Differing # of neutrons
Same atomic #
Changing the number of neutrons does not affect the chemical reactivity of the element.

3. Isotopes Same element Same chemical properties
Second one is radioactive

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.

7. 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.

8. 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?

9. 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

10. 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?

11. 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?