Average Atomic Mass
Masses of Atoms
 A scale designed for atoms gives their small atomic
masses in atomic mass units (amu)
 An atom of 12C was assigned an exact mass of 12.00
amu
 Relative masses of all other atoms was determined
by comparing each to the mass of 12C
 An atom twice as heavy has a mass of 24.00 amu. An
atom half as heavy is 6.00 amu.
Atomic Mass
Na
22.99
 Listed on the periodic table
 Gives the mass of “average” atom of each element
compared to 12C
 Average atom based on all the isotopes and their
abundance %
 Atomic mass is not a whole number
Calculating Atomic Mass
 Weighted average of the masses of the existing
isotopes of an element.
 Percent(%) abundance of isotopes
 Mass of each isotope of that element
 Weighted average =
mass isotope1(%) + mass isotope2(%) + …
100
100
4
Sample Problem
• Calculate the average atomic mass of sulfur if 95.00% of all sulfur
atoms have a mass of 31.972 amu, 0.76% has a mass of
32.971amu and 4.22% have a mass of 33.967amu.
Example #1
• The mass of a Cu-63 atom is 62.94 amu, and that of a Cu-65
atom is 64.93 amu. The percent abundance of Cu-63 is 69.17%
and the percent abundance of Cu-65 is 30.83%. What is the
average atomic mass?
Example #2
• Calculate the average atomic mass of chromium. It is made up of
isotopes with the following percent compositions and atomic masses:
83.79% with a mass of 51.94 amu; 9.50% with a mass of 52.94 amu;
4.35% with a mass of 49.95 amu; 2.36 % with a mass of 53.94 amu.
Example #3
• Calculate the average atomic mass of iron if its abundance in
nature is 15% iron-55 and 85% iron-56.
Example #4
• Chlorine exists as chlorine-35, which has a mass of 34.969 amu
and makes up 75.8% of chlorine atoms. The rest of naturally
occurring chlorine is chlorine-37, with a mass of 36.996 amu.
What is the average atomic mass of chlorine?
Example #5
• Element X has two isotopes. One has a mass of 10.0 amu and an
abundance of 20.0%. The other has a mass of 11.0 amu and an
abundance of 80.0%. Estimate the average atomic mass. What
element is it?