If bromine is available in the form of, say, two isotopes 7935Br (49.7%) 8135Br (5,3%) calculate the average atomic mass of bromine atom.
We know that upper digit in the symbol of an isotope represents its mass (which is the same as its mass number). Now:
(i) The mass of 
isotope is 79 u and its abundance is 49.7%.
(ii) The mass of 
isotope is 81 u and its abundance is 50.3%.
So, Average atomic mass of bromine = 79 × 
= 
= 39.263 + 40.743
= 80.006
= 80u
Thus, the average atomic mass of bromine is 80 u.
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