Average Atomic Mass: No atom exists with the exact mass represented on the periodic table of
elements. The masses on the table represent the weighted average of the isotopes known for that
Ex. O-16 15.995 amu 99.759%
O-17 16.995 amu 0.0370%
O-18 17.999 amu 0.204%
O-16 15.995 amu x .99759 = 15.956 amu (rounded to 5 sig.figs)
O-17 16.995 amu x .00037 = 0.00629 amu (rounded to 3 sig.figs)
O-18 17.999 amu x 0.00204 = 0.0367 amu (rounded to 3 sig figs)
Sum: 15.99899 amu 15.999 amu ( rounded to 3 decimal places)
To determine the average atomic mass:
Convert the percent to its decimal form, by moving the decimal over two places to the left.
Multiply the decimal by the mass of that isotope. The answer reflects the mass due to that
particular isotope. Round your answer due to significant figures. Look at the sig. figs. In the
percent as well as the mass.
Add together the masses calculated. Round your answer based on the fewest number of
decimal places because this is addition.
Compare this mass to the periodic table. If you have valid information and if you calculated
properly, the mass you determine should be very close if not identical to the mass found on the
You have determined a weighted average, much in the way that your grade in the course is a
weighted average of assessments ( 60%), coursework(30%), Habits of mind(10%)