1. Isotopic Abundance Pages 23-27

2. Thinking question Why are there decimal places for atomic masses on the periodic table if protons and neutrons have amu values of 1?

3. Isotopic Abundance Elements have multiple isotopes which exist in different percentages. For example: 1.3 % isotope one 18.7% isotope two 80.0% isotope three The values on the periodic table represent the “average” mass

4. Isotopic Abundance Atomic masses are a “weighted average” of the isotopes Weighted average ◦ ◦

5. Isotopic Abundance Questions Two main types: 1. Find the “average” mass given the percentages. 2. Find the percentages given the “average” mass.

6. Question #1 Given: mass of 7 Li = 7.015 u mass of 6 Li = 6.015 u Of all existing Li atoms, 92.58% are 7 Li, while the remaining is 6 Li. What is the “average” mass of an Li atom?

7. Question #2 In nature, silicon is composed of three isotopes. Calculate the average atomic mass of silicon given: 28 Si (92.23%, 27.98 u) 29 Si (4.67%, 28.97 u) 30 Si (3.10%, 29.97 u)

8. Question #3 Given: mass of 10 B = 10.01 u mass of 11 B = 11.01 u If the “average” mass of a B atom is 10.81 u, what is the percent abundance of each of the isotopes given?

9. Homework Pg. 27, #1 Pg. 29 #7,9