Answer:
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 35Br79 to atomic mass = (79 × 49.7)/100
⇒ 39.26 u
Isotope of bromine with atomic mass 81 u = 50.3%
Contribution of 35Br81 to the atomic mass of bromine = (81 × 50.3)/100
⇒ 40.64u
Hence, average atomic mass of the bromine atom = 39.26 + 40.64 u = 79.9u
Answer
The average atomic mass of the bromine atom = 79.9u