1. 5.1 – Isotopes and average atomic mass
Ms. Munir

2. Weighted Average Mass of (A + B + C) / Number of items
Given three items of mass 1.0 kg, 2.0 kg, 3.0 kg, the weighted average is:
= = kg
Given that you have 2 items of A, 3 items of B, and item of C.
2(1.0) + 3(2.0) + 1(3.0)
= = 1.8 kg

3. Calculating Average Atomic Mass
Example: 1
Lithium exists as 2 isotopes: Li – 6 & Li – 7
Li – 6 = 7.4% Li – 7 = 92.6%
Mass of Li – 7 is 7.015u and Li – 6 is 6.015u
Avg. atomic mass =
= 6.941u

4. Example Isotope Atomic Mass Unit (u) Relative Abundance
Silver has 2 isotopes.
Isotope Atomic Mass Unit (u) Relative Abundance
Ag – %
Ag – %
What is the average atomic mass of Ag?

5. Calculating Isotopic Abundance
Boron exists as 2 isotopes. B – 10 (10.01u) and B – 11 (11.01u). Calculate the isotope abundance.
Solution:
From periodic table the avg. atomic mass of B is u.
Let % abundance of B – 10 be x and B – 11 would be 1 – x.
= x(10.01) + (1 – x)(11.01)
= 10.01x – 11.01x
11.01 – = 11.01x – 10.01x
x =  x = 0.20
Therefore, B -10 is 20% B – 11 is 80%

6. Homework P 167 #1-4 McGraw Hill P 170 # 5 – 8 McGraw Hill
Worksheet for Nelson (see Ms. Munir)