Element A exists in three isotopic forms with masses of 21.0, 25.0 and 26.0 amu respectively. Element B also exists in three isotopic forms with masses of 22.0, 24.0 and 26.0 amu respectively. It is true that __.
A) element A has a higher atomic mass than B
B) element B has a higher atomic mass than A
C) A and B have identical atomic masses since the sums of their isotopic masses are equal
D) you need the percentages of each isotope to determine their atomic masses
The correct answer and explanation is :
The correct answer is D) you need the percentages of each isotope to determine their atomic masses.
Explanation:
The atomic mass of an element is not solely determined by the masses of its isotopes but also by their relative abundances in nature. This is because the atomic mass is a weighted average of the masses of all the isotopes of that element, with each isotope’s mass weighted by its natural abundance.
Understanding Isotopes and Atomic Mass:
Isotopes are variants of the same element that have the same number of protons but differ in the number of neutrons, resulting in different masses. For example, chlorine has two main isotopes:
- Chlorine-35 (^35Cl) with a mass of approximately 34.97 amu.
- Chlorine-37 (^37Cl) with a mass of approximately 36.97 amu.
The average atomic mass of chlorine is calculated by considering both the masses of these isotopes and their relative abundances. If chlorine-35 constitutes about 75.77% of chlorine atoms and chlorine-37 about 24.23%, the average atomic mass is:
[ \text{Average Atomic Mass} = (0.7577 \times 34.97\, \text{amu}) + (0.2423 \times 36.97\, \text{amu}) \approx 35.45\, \text{amu} ]
Applying This to Elements A and B:
For elements A and B, each with three isotopes having masses of 21.0, 25.0, and 26.0 amu, the average atomic mass cannot be determined without knowing the relative abundances (percentages) of these isotopes. Without this information, it’s impossible to calculate a weighted average, which is essential for determining the elements’ atomic masses.
Conclusion:
Therefore, to accurately determine and compare the atomic masses of elements A and B, the percentages of each isotope must be known. This information allows for the calculation of the weighted average atomic mass, making option D the correct choice.