Transcript Average Atomic Mass

Average Atomic Mass
►
Average Atomic Mass – the weighted average of the
masses of the isotopes of an element
Every element is composed of several naturally occurring
isotopes of that element-each with its own atomic mass
► A weighted average of the percentage of each isotope that
exists versus the atomic mass of each isotope is used to
calculate the atomic mass that appears on the periodic table.
►
Step 1) Multiply each atom’s mass by its percent to get
the contribution for each isotope.
Step 2) Add all the contributions together.
Step 3) Divide by 100 to get the average atomic mass.
AAM has units of amu, atomic mass unit.
Average Atomic Mass =
Σ [(isotope mass) * (percent abundance of isotope)]
100
Example 1: The element copper occurs naturally as
69.17% of copper-63 with a mass of 62.9296 amu
and 30.83% of copper-65 with a mass of 64.9278
amu. What is the average atomic mass of copper?
Example 2: The atomic weight of gallium is 69.72 amu. The masses of the
naturally occurring isotopes are 68.9257 amu and 70.908 amu for 69Ga
and 71Ga respectively. Calculate the % abundance of each isotope.
Learning Check
Naturally occurring element consists of 75.78% atoms with a mass of 34.969
amu and 24.22% atoms with a mass of 36.966 amu.
Calculate the average atomic mass.
b) Based on the average atomic mass, what is the element?