If the bromine atom is available in the form of, say, two isotopes 35Br79 (49.7%) and 35Br81 (50.3%), calculate the average atomic mass of the Bromine atom.

Solution:

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, the average atomic mass of the bromine atom = 39.26 + 40.64 u = 79.9u