Calculating
Average Atomic
Mass
Introduction
• The value for atomic mass for a particular element is
the average atomic mass.
• Each sample of an element is composed of a
mixture of different isotopes.
• Therefore, we need to average the masses of the
isotopes to find the average atomic mass.
Calculations
• Copper has two common isotopes.
o
o
63Cu
is 69.15% of copper samples.
65Cu is 30.85% of copper samples.
• We would expect the average atomic mass to be
somewhere in between 63 and 65.
Calculations
• Copper has two common isotopes.
o
o
63Cu
is 69.15% of copper samples.
65Cu is 30.85% of copper samples.
• To find the average atomic mass
o multiply the mass number by the percentages (as a
decimal value)
Calculations
• Copper has two common isotopes.
o
o
63Cu
is 69.15% of copper samples.
65Cu is 30.85% of copper samples.
• To find the average atomic mass
o multiply the mass number by the percentages (as a
decimal value)
• 63 × 0.6915 = 43.56
• 65 × 0.3085 = 20.05
Calculations
• Copper has two common isotopes.
o
o
63Cu
is 69.15% of copper samples.
65Cu is 30.85% of copper samples.
• To find the average atomic mass
o multiply the mass number by the percentages (as a
decimal value)
• 63 × 0.6915 = 43.56
• 65 × 0.3085 = 20.05
Calculations
• Copper has two common isotopes.
o
o
63Cu
is 69.15% of copper samples.
65Cu is 30.85% of copper samples.
• To find the average atomic mass
o multiply the mass number by the percentages (as a
decimal value)
• 63 × 0.6915 = 43.56
• 65 × 0.3085 = 20.05
Calculations
• Copper has two common isotopes.
o
o
63Cu
is 69.15% of copper samples.
65Cu is 30.85% of copper samples.
• To find the average atomic mass
o multiply the mass number by the percentages (as a
decimal value)
• 63 × 0.6915 = 43.56
• 65 × 0.3085 = 20.05
o and add the values
Calculations
• Copper has two common isotopes.
o
o
63Cu
is 69.15% of copper samples.
65Cu is 30.85% of copper samples.
• To find the average atomic mass
o multiply the mass number by the percentages(as a
decimal value)
• 63 × 0.6915 = 43.56
• 65 × 0.3085 = 20.05
o and add the values
• 43.56 + 20.05 =
Calculations
• Copper has two common isotopes.
o
o
63Cu
is 69.15% of copper samples.
65Cu is 30.85% of copper samples.
• To find the average atomic mass
o multiply the mass number by the percentages(as a
decimal value)
• 63 × 0.6915 = 43.56
• 65 × 0.3085 = 20.05
o and add the values
• 43.56 + 20.05 =
Calculations
• Copper has two common isotopes.
o
o
63Cu
is 69.15% of copper samples.
65Cu is 30.85% of copper samples.
• To find the average atomic mass
o multiply the mass number by the percentages(as a
decimal value)
• 63 × 0.6915 = 43.56
• 65 × 0.3085 = 20.05
o and add the values
• 43.56 + 20.05 = 63.61
Calculations
• Iron has three common isotopes.
o
o
o
54Fe
is 5.85% of iron samples.
56Fe is 91.75% of iron samples.
58Fe is 2.82% of iron samples.
• We would expect the average atomic mass to be
somewhere in between 54 and 58.
Calculations
• Iron has three common isotopes.
o
o
o
54Fe
is 5.85% of iron samples.
56Fe is 91.75% of iron samples.
58Fe is 2.82% of iron samples.
• multiply the mass number by the percentages
o 54 × 0.0585 = 3.16
o 56 × 0.9175 = 51.38
o 58 × 0.0282 = 1.61
Calculations
• Iron has three common isotopes.
o
o
o
54Fe
is 5.85% of iron samples.
56Fe is 91.75% of iron samples.
58Fe is 2.82% of iron samples.
• multiply the mass number by the percentages
o 54 × 0.0585 = 3.16
o 56 × 0.9175 = 51.38
o 58 × 0.0282 = 1.61
• and add the values
Calculations
• Iron has three common isotopes.
o
o
o
54Fe
is 5.85% of iron samples.
56Fe is 91.75% of iron samples.
58Fe is 2.82% of iron samples.
• multiply the mass number by the percentages
o 54 × 0.0585 = 3.16
o 56 × 0.9175 = 51.38
o 58 × 0.0282 = 1.61
• and add the values
o 3.16 + 51.38 + 1.61 = 56.15
Try one on your own
• Magnesium has three common isotopes.
o
o
o
24Mg
is 78.99% of magnesium samples.
25Mg is 10.00% of magnesium samples.
26Mg is 11.01% of magnesium samples.
• What would be the average atomic mass of Mg?
Do not move on until you try this problem.
Try one on your own
• Magnesium has three common isotopes.
o
o
o
24Mg
is 78.99% of magnesium samples.
25Mg is 10.00% of magnesium samples.
26Mg is 11.01% of magnesium samples.
• What would be the average atomic mass of Mg?
o 24 × 0.7899 = 18.96
o 25 × 0.1000 = 2.50
o 26 × 0.1101= 2.86
Try one on your own
• Magnesium has three common isotopes.
o
o
o
24Mg
is 78.99% of magnesium samples.
25Mg is 10.00% of magnesium samples.
26Mg is 11.01% of magnesium samples.
• What would be the average atomic mass of Mg?
o 24 × 0.7899 = 18.96
o 25 × 0.1000 = 2.50
o 26 × 0.1101= 2.86
o 18.96 + 2.50 + 2.86 = 24.32
Try one on your own
• Chlorine has two common isotopes.
o
o
35Cl
is 75.76% of chlorine samples.
37Clg is 24.24% of chlorine samples.
• What would be the average atomic mass of Cl?
Do not move on until you try this problem.
Try one on your own
• Chlorine has two common isotopes.
o
o
35Cl
is 75.76% of chlorine samples.
37Clg is 24.24% of chlorine samples.
• What would be the average atomic mass of Cl?
o 35 × 0.7576 = 26.52
o 37 × 0.2424 = 8.97
Try one on your own
• Chlorine has two common isotopes.
o
o
35Cl
is 75.76% of chlorine samples.
37Clg is 24.24% of chlorine samples.
• What would be the average atomic mass of Cl?
o 35 × 0.7576 = 26.52
o 37 × 0.2424 = 8.97
o 26.52 + 8.97 = 35.48
Summary
• To find the average atomic mass of an element, we
need to know the percent abundance of each
isotope.
• We multiply the mass number of each isotope by
the percentages (as a decimal number) and add
the values for each isotope to get the average
atomic mass.