Determining Average Atomic Mass
Chemistry
Unit One
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
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
Determining Average Atomic Mass
Isotope %abundance Atomic mass1H 99.985% 1.007825 amu2H 0.015% 2.014102 amu
Step 1:Obtain the natural abundance and atomic mass of each isotope of an element.
Determining Average Atomic Mass
Isotope abundance Atomic mass1H 0.99985 1.007825 amu2H 0.00015 2.014102 amu
Step 2:Convert the abundances from percents to decimals.
Determining Average Atomic Mass
Isotope abundance Atomic mass1H 0.99985 X 1.007825 amu = 1.00767 amu2H 0.00015 X 2.014102 amu = 0.00030 amu
Step 3:Multiply the abundances and atomic mass for each isotope
Determining Average Atomic Mass
Isotope abundance Atomic mass1H 0.99985 X 1.007825 amu = 1.00767 amu2H 0.00015 X 2.014102 amu = 0.00030 amu
1.00797 amu
Step 4:Add the results. This sum is the average atomic mass.
Find the average atomic mass of Copper
Isotope %abundance Atomic mass63Cu 69.17% 62.929599 amu64Cu 30.83% 64.927793 amu
63.55 amu
Find the average atomic mass of Oxygen
Isotope %abundance Atomic mass16O 99.762% 15.994915 amu17O 0.038% 16.999131 amu18O 0.200% 17.999160 amu
15.999 amu
Find the average atomic mass of Uranium
Isotope %abundance Atomic mass234U 0.005% 234.040947 amu235U 0.720% 235.043924 amu238U 99.275% 238.050784 amu
238.03 amu