Question

A hypothetical element has two main isotopes with mass numbers of 62 and 65. If 66.00% of the isotopes have a mass number of 62 amu, what atomic weight should be listed on the periodic table for this element? Answer in units of amu.

Answer 1

Answer is 63.04amu.

Explanation: We are given in the question that an element has two main isotopes 1 and 2.

Mass Number of isotope 1 = 62 amu

Mass Number of 2 isotope = 65 amu

% abundance of isotope 1 = 66%

Fractional abundance of isotope 1 = 0.66

Total fractional abundance = 1

Fractional abundance of isotope 2 = (1-0.66)

                                                          = 0.34

For calculating he atomic mass of element, we use

$\text{atomic mass of element}=\left [(\text{fractional abundance of isotope 1})\times (\text {atomic mass of isotope 1)} \right ]+\left [(\text{fractional abundance of isotope 2})\times (\text {atomic mass of isotope 2}) \right ]+...$

We have only 2 isotopes for an element, so the formula stops at isotope 2 only.

By putting the values in the above equation, we get

$\text{atomic mass of element}=(62amu\times 0.66)+(65amu}\times 0.34)$

Atomic mass of element = 63.04amu