1. Determining Average Atomic Mass
Chemistry
Unit One

2. Atomic Mass Unit (amu) 1/12 of the mass of a Carbon-12 atom
It is used to describe the atomic masses of individual atoms and compounds
Masses of all atoms are determined relative to the Carbon-12 atom

3. Average Atomic Mass
The weighted average of the atomic masses of each of the isotopes of an element
Units are amu
These values can be found on the periodic table

4. Determining Average Atomic Mass
Isotope %abundance Atomic mass 1H % amu 2H 0.015% amu
Step 1:
Obtain the natural abundance and atomic mass of each isotope of an element.

5. Determining Average Atomic Mass
Isotope abundance Atomic mass 1H amu 2H amu
Step 2:
Convert the abundances from percents to decimals.

6. Determining Average Atomic Mass
Isotope abundance Atomic mass 1H X amu = amu 2H X amu = amu
Step 3:
Multiply the abundances and atomic mass for each isotope

7. Determining Average Atomic Mass
Isotope abundance Atomic mass 1H X amu = amu 2H X amu = amu amu
Step 4:
Add the results. This sum is the average atomic mass.

8. Find the average atomic mass of Copper
Isotope %abundance Atomic mass 63Cu 69.17% amu 64Cu 30.83% amu
63.55 amu

9. Find the average atomic mass of Oxygen
Isotope %abundance Atomic mass 16O % amu 17O 0.038% amu 18O 0.200% amu
amu

10. Find the average atomic mass of Uranium
Isotope %abundance Atomic mass 234U 0.005% amu 235U 0.720% amu 238U % amu
amu