In each of the following pairs, which isotope would you expect to be more common in nature? Briefly explain your answer.

Question

In each of the following pairs, which isotope would you expect to be more common in nature? Briefly explain your answer.

a. ³⁹₁₉K or ⁴⁰₁₉K

b. ³⁰₁₃Al or ²⁷₁₃Al

Answer 1

The average atomic mass found in the periodic table is a weighted average of the atomic masses of each isotope.

The average atomic mass for K is 39.10. This value is closer to 39 than it is to 40, so the ³⁹₁₉K would be the more abundant isotope.

The average atomic mass for Al is 26.98. This value is closer to 27 than it is to 30, so the ²⁷₁₃Al would be the more abundant isotope.