Question

Bromine atoms are available in two isotopes, $_{35}^{79}Br$ (49.7 % ) and $_{35}^{81}Br$ (50.3 % ). The average atomic mass of bromine atom is:
(A) 79.016
(B) 80.076
(C) 80.006
(D) 81.016

Answer 1

The atomic mass is the mass of an atom and the average atomic mass of an element is the sum of the masses of its isotopes each multiplied by its natural abundance.

Complete step by step answer:
Here use a formula of average atomic mass, i.e.
Average atomic mass of Br = $\dfrac{Atomic\; mass \;of\; Br_{35}^{79}\; \times\;Its\;natural \;abundance + Atomic\; mass\; of\; Br_{35}^{81} \;\times \;Its\;natural\; abundance}{100}$
Here is,
Percentage of $Br_{35}^{79}$ = 49.7 %
Atomic mass of $Br_{35}^{79}$ = 79
Percentage of $Br_{35}^{81}$ = 50.3
Atomic mass of $Br_{35}^{81}$ = 81
Apply a formula for putting a value.
Average atomic mass of Br = $\dfrac{Atomic\; mass \;of\; Br_{35}^{79}\; \times\;Its\;natural \;abundance + Atomic\; mass\; of\; Br_{35}^{81} \;\times \;Its\;natural\; abundance}{100}$
Average atomic mass of Br = $\left[ {\dfrac{{79 \times 49.7 + 81 \times 50.3}}{{100}}} \right]$
Average atomic mass of Br = $\left[ {\dfrac{{8000.6}}{{100}}} \right]$
Average atomic mass of Br = 80.006

Therefore, from the above explanation the correct option is (C) 80.006.

Note: Bromine occurs in nature mainly in the form of two isotopes. Both are used in nuclear medicine. Br-81 is used for the production of radioisotopes. Vapours of bromine are very toxic.