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?

Question

Wyatt Atkins · Accepted Answer

87.71 amu (I'm assuming here that there are degrees of significance...)

An element's average atomic mass is calculated by adding the weighted masses of all of its isotopes, which is accomplished by taking the weighted mass of each isotope and averaging them.

To calculate the first mass, multiply 0.50% of 84 (atomic mass units) by 0.042 amu. Then, add this amount to 9.9% of 86 amu, which equals 8.51 amu, and so forth.

Your average atomic mass should be closest to the mass of 88 amu, which is the most common isotope of this element. Since the other isotopes are less than this mass, it also makes sense that the average atomic mass will be close to this mass, but slightly less, which it is!

Wyatt Adkins · Answer

undefined About 87.62 atomic mass units (amu) make up strontium's atomic mass.