Question

If bromine atom is available in the form of two isotopes ${}_{35}^{79}$Br (49.7 %) and ${}_{35}^{81}$Br (50.3 %), calculate the average atomic mass of bromine atom.

Answer 1

Isotope of bromine with atomic mass 79 u = 49.7 %
Therefore, Contribution of ${}_{35}^{79}$Br to atomic mass $=\frac{79\times 49.7}{100}=39.26u$
Isotope of bromine with atomic mass 81 u = 50.3 %
Contribution of ${}_{35}^{81}$Br to the atomic mass of bromine $=\frac{81\times 50.3}{100}=40.74u$
Hence atomic mass of the bromine atom $=39.26u+40.74u=80u$