# How does abundance contribute to the average atomic mass of an element?

Chlorine has ${A}_{r}$ value of 35.5 as there are two isotopes, Cl-35 and Cl-37. These do not occur in equal proportions. In every 100 atoms, there are 75 of Cl-35 and 25 of Cl-37. Therefore, the ${A}_{r}$ value is calculated as
$\frac{\left(35 \times 75\right) + \left(37 \times 25\right)}{100} = 35.5$