Answer :
Final answer:
The percentage abundance of isotope X²¹, given an average atomic mass of 20.18 for the element and a 90% abundance of isotope X²⁰, is 10%. This is calculated considering that X²¹ and X²⁰ constitute the substantial contributions to the average atomic mass. So the correct answer is Option C.
Explanation:
To answer the question, which asks for the percentage abundance of the isotope X²¹ given that the average atomic mass of the element is 20.18 and the percentage abundance of the isotope X²⁰ is 90%, we must make use of the concept of isotopes and how they influence the average atomic mass of an element.
Isotopes are different forms of the same element that contain the same number of protons but a different number of neutrons. The collective contribution of these isotopes, proportional to their respective abundances, gives the average atomic mass of an element.
Given that the average atomic mass is 20.18 and X²⁰ represents 90% of this, it can be estimated that the remaining mass contribution should come from isotopes X²¹ and X²². But since no percentages for these isotopes are provided, we must assume that X²² contributes negligibly to the atomic mass.
Hence, the percentage abundance of X²¹ is calculated by considering the remaining contribution needed to account for the average atomic mass of 20.18 not covered by X²⁰. This works out to 10%, meaning the correct answer is c.10%.
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