There are two stable isotopes of Bromine, Br-79, Br-81. If a sample of Br2 is fed into a mass spectrometer, how many peaks would be observed in the spectrum?

There should be five peaks. In the process of bombarding the Br2 molecule with electrons to charge it, the molecule can fragment into 2 Br atoms. We should therefore expect to see peaks at 79 and 81 m/z. Then we should observe three more peaks from Br2 molecules consisting of the three distinct combinations of the Br isotopes that could be present in the molecule; a peak at 158 m/z where both atoms are Br-79, one at 160 m/z with one atom of each isotope and a last one at 162 m/z with both atoms the Br-81 isotope.