To calculate the atomic mass of an element based on its isotopic abundances, we use a weighted average. The atomic mass reported on the periodic table is a reflection of the weighted averages of all the naturally occuring isotopes of that element. The formula to calculate the average atomic mass $ M $ of an element when given the abundance and atomic mass of each isotope is:

$M = \sum (f_i \times m_i)$

where:

• $ f_i $ is the fractional percent abundance (as a decimal) of the $ i $-th isotope.


• $ m_i $ is the mass number of the $ i $-th isotope.

For iron (Fe), we have three isotopes $ ^{54}Fe, ^{56}Fe, ^{57}Fe $ with abundances 5%, 90%, and 5%, respectively. To convert these percentages into decimals, we divide each by 100.

Let's calculate the weighted average:

$M = (0.05 \times 54) + (0.90 \times 56) + (0.05 \times 57)$

Multiplying each fractional abundance by its respective mass we get:

$M = (0.05 \times 54) + (0.90 \times 56) + (0.05 \times 57)$

$M = (2.7) + (50.4) + (2.85)$

$M = 55.95$

Therefore, the calculated atomic mass of Fe is 55.95, which corresponds to Option B.