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?
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%