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