Average atomic mass Transitional Reasoning Download PDF

Questions

1. 1.

   A mass spectrometry analysis of an extraterrestrial sample reveals that element $Z$ consists of only two stable isotopes: $^{35}\text{Z}$ (exact mass 34.97 amu) and $^{37}\text{Z}$ (exact mass 36.97 amu). High-precision spectrometry establishes the average atomic mass of element $Z$ in this sample as 35.47 amu. Based on these analytical findings, what are the relative percent abundances of the two isotopes, and how do they determine this average value?

   • A) $^{35}\text{Z}$ is 50.0% and $^{37}\text{Z}$ is 50.0%, because the average atomic mass represents a simple arithmetic mean of the two mass numbers.
   • B) $^{35}\text{Z}$ is 25.0% and $^{37}\text{Z}$ is 75.0%, because the average atomic mass is shifted toward the isotope with the greater number of neutrons.
   • C) $^{35}\text{Z}$ is 75.0% and $^{37}\text{Z}$ is 25.0%, meaning that every individual atom of element $Z$ in the sample possesses an identical rest mass of 35.47 amu.
   • D) $^{35}\text{Z}$ is 75.0% and $^{37}\text{Z}$ is 25.0%, because the average atomic mass is a weighted average that reflects the higher natural abundance of the lighter isotope.