## Chemistry and Chemical Reactivity (9th Edition)

The abundance of $Ar^{40}$ is $99.6\%$ The atomic weight of $Ar^{40}$ is $39.9627\ u$
Since these three are the only stable isotopes, the abundance of $Ar^{40}$ is the complement of the others: $100\% -0.337\%-0.063\%=99.6\%$ Argon's atomic weight is 39.948 u, which is the sum of the products of the isotopes' atomic masses by their abundances: $39.948= 35.967545\times0.337/100+37.96732\times0.063/100+x\times99.6/100$ So the atomic weight of $Ar^{40}$ is : $x=39.9627\ u$