Calculate the average atomic mass of the bromine atom.

1. If an element exists in the form of isotopes, for example, ${\mathrm{Cl}}^{35}$and ${\mathrm{Cl}}^{37}$. In that case, we take the average atomic mass of that element.
2. For example, in the case of chlorine, its average atomic mass is 35.5u.
3. To calculate the average atomic mass of the Bromine atom, we should have an idea about the isotopes of Bromine.
4. Bromine has 2 isotopes ${\mathrm{Br}}^{79}$ and ${\mathrm{Br}}^{81}$.
5. The percentage of ${\mathrm{Br}}^{79}$ available on earth is 49.7%. Therefore its contribution to atomic mass is ($49.7\mathrm{x}\frac{79}{10}$) = 39.26u
6. The percentage of ${\mathrm{Br}}^{81}$available on earth is 50.3%. Therefore its contribution to atomic mass is ($50.3\mathrm{x}\frac{81}{100}$) = 40.64u
7. The average atomic mass of Bromine is $39.26+40.64=79.9\mathrm{u}.$

The average atomic mass of Bromine is found to be $79.9\mathrm{u}\mathrm{or}79.9\mathrm{g}{\mathrm{mol}}^{-1}.$