Question

Bromine occurs in nature mainly in the form of two isotopes `._(35)^(79)Br and ._(35)(81)Br`. If the abundance of `._(35)^(79)Br` isotope is 49.7 ^% and that of `._(35)^(81)Br` isotope is 50.3 %, calculate the average atomic mass of bromine.

Answer 1

We know that upper digit in the symbol of an isotope represent its mass (which is the same as its mass number) Now :
(i) The mass of `._(35)^(79)Br` isotope is 79 u and its abundance is 19.7 %
(ii) The mass of `._(35)^(81)Br` isotope is 81 u and its abundance is 50.3 %
So, Average atomic mass of bromine `= 79xx(49.7)/(100)+81xx(50.3)/(100)`
`=(3926.3)/(100)+(4074.3)/(100)`
`= 39.263 + 40.743`
`= 80.006`
`= 80 u`
Thus, the average atomic mass of bromine is 80 u.