Bromine, with atomic mass 79.942 u, is composed of nearly equal amounts of two isotopes, one of which contains 79 nucleons per atom. The mass number of the other isotope is:
A) 78
B) 79
C) 80
D) 81
E) 82

Question

Quizplus · Accepted Answer

The average atomic mass of bromine is 79.942 u, which is close to 80. Since one isotope has a mass number of 79, the other must be 81 to average out to approximately 80 when both isotopes are present in nearly equal amounts.

Bromine, with Atomic Mass 79