Question

A certain element consists of two stable isotopes. The first has an atomic mass of 14.0031 amu and a percent natural abundance of 99.63 %. The second has an atomic mass of 15.0001 amu and a percent natural abundance of 0.37 %. What is the atomic mass of the element

Answer 1

Considering the definition of atomic mass, isotopes and atomic mass of an element, the atomic mass of the element is 14.0068 amu.

Definition of atomic mass

First of all, the atomic mass (A) is obtained by adding the number of protons and neutrons in a given nucleus of a chemical element.

Definition of isotope

The same chemical element can be made up of different atoms, that is, their atomic numbers are the same, but the number of neutrons is different. These atoms are called isotopes of the element.

Definition of atomic mass

On the other hand, the atomic mass of an element is the weighted average mass of its natural isotopes. In other words, the atomic masses of chemical elements are usually calculated as the weighted average of the masses of the different isotopes of each element, taking into account the relative abundance of each of them.

Atomic mass of the element in this case

In this case, the first isotope has an atomic mass of 14.0031 amu and a percent natural abundance of 99.63 %. The second isotope has an atomic mass of 15.0001 amu and a percent natural abundance of 0.37 %.

Then, the average mass of lithium can be calculated as:

14.0031 amu×0.9963 + 15.0001 amu×0.0037= 14.0068 amu

Finally, the atomic mass of the element is 14.0068 amu.

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