Question

why must we use percent abundance when calculating average atomic mass?

to eliminate isotopes with low mass

to determine which isotope is radioactive

to give more influence to isotopes that occur more frequently

Explanation:

• The first option is incorrect because we don't want to eliminate any isotopes; we need to account for all to calculate the average.
• The second option is incorrect as percent abundance has nothing to do with determining radioactivity of an isotope.
• The third option is correct. When calculating average atomic mass, we use the formula \( \text{Average Atomic Mass}=\sum(\text{isotope mass} \times \text{percent abundance (in decimal)}) \). So, isotopes that are more abundant (occur more frequently) have a larger weight (due to their higher percent abundance) in the calculation, thus giving them more influence on the final average atomic mass.

Answer:

C. To give more influence to isotopes that occur more frequently