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 (also called the atomic weight) of an element is the weighted average of the masses of all its naturally occurring isotopes. The key factor here is that each isotope contributes to the overall atomic mass in proportion to its natural abundance (percentage).
To explain further:
- Isotopes and Atomic Mass:
- Elements can exist in multiple isotopic forms, which are atoms of the same element but with different numbers of neutrons, leading to different atomic masses.
- The atomic mass of an element is calculated by averaging the masses of its isotopes, weighted by their relative abundances (the percentage of each isotope in a sample of the element).
- Isotopic Masses of Element A and B:
- Element A has isotopes with masses of 21.0, 25.0, and 26.0 amu.
- Element B has isotopes with masses of 22.0, 24.0, and 26.0 amu. However, without knowing the percentages (or natural abundances) of each isotope, it is impossible to determine the actual atomic mass of either element. For example, if element A’s isotope with a mass of 25.0 amu is far more abundant than the other isotopes, it could have a higher atomic mass than element B, even though the isotopic masses of element B are higher.
- Why D is Correct:
- The atomic mass is a weighted average, meaning the abundances of the isotopes must be known to make any accurate comparison.
- While you can compare the isotopic masses themselves, these do not directly give you the atomic mass of the element. The atomic mass will depend on how common each isotope is in nature, which is determined by their relative abundances.
Thus, option D is correct because the atomic mass of each element depends on the specific distribution of isotopes in nature, and this information (percentages of each isotope) is required to determine the true atomic mass.
