Structure of the Atom — Question 10

The atomic mass of an element is the mass of one atom of that element.

Average atomic mass takes into account the isotopic abundance.

Isotope of bromine with atomic mass 79 u = 49.7%

Therefore, Contribution of ₃₅⁷⁹Br to atomic mass = $\dfrac{79 \times 49.7}{100}$ = 39.263 u

Isotope of bromine with atomic mass 81 u = 50.3%

Contribution of ₃₅⁸¹Br to the atomic mass of bromine = $\dfrac{81 \times 50.3}{100}$ = 40.743u

Hence, the average atomic mass of the bromine atom = 39.263 + 40.743 = 80.006 u