Average Atomic Mass Weighted Averages

When calculating the average mass of atoms -- we have to take into account all of the naturally occurring isotopes of that atom. Because remember- different isotopes have different masses!

This means we need to take a weighted average. This means multiplying each mass by its abundance (in decimal form) and then adding them up!

I highly recommend this video for a little refresher on these calculations:

You can also find the percent abundance of an isotope by replacing the % abundance with a variable (say, y) in your calculations. You know that both of your % abundances need to add up to 1 (because they need to add up to 100% ... all of the element). So one of the percentages is y, and one is the rest of the stuff (1-y).

Try these practice problems to make sure you're good with this topic:

1) What is the average atomic mass of Boron if it exists as 19.90% Boron-10 (10.013 amu) and 80.10% Boron-11 (11.009 amu)?

2) Out of 500 silicon atoms, 460 are Si-28 (27.98 amu), 25 are Si-29 (28.98 amu) and 15 are Si-30 (29.97amu). What is the average atomic mass?

3) Without doing any math, are there more Bromine-79 atoms or more Bromine-80 atoms on earth? (Hint: look at the periodic table.)

4) The average atomic mass of Lithium is 6.941 amu. The two isotopes are Lithium-6 (6.015 amu) and Lithium-7 (7.016). What are the percentage abundances of these two isotopes?


1) 10.811amu

2) 28.09 amu

3) There are more Bromine-80 atoms because the atomic mass on the periodic table (79.904)is closer to 80.

4) 7.42% and 92.58%

Made with Adobe Slate

Make your words and images move.

Get Slate

Report Abuse

If you feel that this video content violates the Adobe Terms of Use, you may report this content by filling out this quick form.

To report a Copyright Violation, please follow Section 17 in the Terms of Use.