Calculating Relative Atomic Mass (${\mathrm{A}}_{\mathrm{r}}$)

This section will better explain the importance weighted average.

Earlier you learned that ${}_{}{}^{35}\mathrm{Cl}$ is more abundant in nature than ${}_{}{}^{37}\mathrm{Cl}$ because out of all the naturally occurring chlorine atoms, ${}_{}{}^{35}\mathrm{Cl}$ is 75%. So, 75% and 25% are the percentage abundance of the two isotopes of chlorine.

Now, let’s calculate the A_r of chlorine using their masses and relative abundance.