Strontium consists of four isotopes with masses of 84 (abundance 0.50%), 86 (abundance of 9.9%), 87 (abundance of 7.0%), and 88 (abundance of 82.6%). What is the atomic mass of strontium?
87.71 amu (I am assuming degrees of significance here...)
In order to determine the average atomic mass of an element, we take the weighted average of all of the isotopes of that element. So, we calculate it by taking the weighted mass of each of the isotopes and adding them together.
So, for the first mass, we will multiply 0.50% of 84 (amu - atomic mass units) = 0.042 amu, and add it to 9.9% of 86 amu = 8.51 amu, and so on.
Since the most abundant isotope of this element is 88 amu, your average atomic mass should be closest to this mass, and since the rest of the isotopes are less than this mass, it should also make sense that the average atomic mass will be close to this mass, but slightly less, which it is!