Answer :
Final answer:
The percentage abundance of X21 of an element X having three isotopes X20, X21, and X22 with X20 having an abundance of 90% and X22's abundance as negligible, will be 10%.
Explanation:
The average mass of an element (20.11 in this case) seen on the periodic table reflects a weighted average of the mass of all its isotopes, factoring in their percentage abundance. That's why for calculating the percentage abundance of a certain isotope, you need both the average atomic mass and the percentage abundances of other isotopes.
In this case, the element X has three isotopes: X20, X21, and X22. Given that the percentage abundance of X20 is 90%, and that of X22 is left undefined, it can be assumed that X22 is present in a negligible amount. Hence, X21's percentage abundance will be the remaining percentage, which is 100% - 90% for X20 = 10%.
Remember, if X22 had a significant presence, more information would be needed to resolve the percentage abundance of X21, such as the exact masses of the isotopes and the percentage abundance of X22.
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