Question

Which is a factor in determining the average atomic mass of an element?
A. Atomic number of the most abundant isotope
B. Relative abundance of each isotope
C. Sum of atomic masses of all isotopes
D. Product of atomic masses of all isotopes
E. Arithmetic mean of atomic masses of the isotopes

Answer 1

B. relative abundance of each isotope   
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Answer 2

Answer: B. Relative abundance of each isotope

Explanation:

Average atomic mass is the average mass of all the isotopes present depending on the relative abundance of each isotope.

$\text{Average atomic mass }=\sum_{i=1}^n\text{(Atomic mass of an isotopes)}_i\times \text{(Fractional abundance})_i$

Example:

Mass of isotope 1 = 203.973 amu

relative abundance of isotope 1 = 0.014

Mass of isotope 2 = 205.9745 amu

relative abundance of isotope 2= 0.241

Mass of isotope 3 = 206.9745 amu

relative abundance of isotope 3 = 0.221

Mass of isotope 4 = 207.9766 amu

relative abundance of isotope 4 = 0.574

Formula used for average atomic mass of an element :

$\text{ Average atomic mass of an element}=\sum(\text{atomic mass of an isotopes}\times {{\text { fractional abundance}})$

$\text{ Average atomic mass of an element}=\sum[(203.973\times0.014)+(205.9745\times 0.241)+(206.9745\times 0.221)+(207.9766\times 0.574)]$

$\text{ Average atomic mass of an element}=209.613amu$