Final answer:
Bromine does not have an isotope with a mass number of 80 because the neutron-to-proton ratio likely does not allow for a stable atomic nucleus for that isotopic mass, thus making it rare, highly unstable, or non-existent in nature.
Step-by-step explanation:
The question 'Why doesn't bromine have an isotope of 80?' relates to the stability of atomic nuclei and the natural occurrence of isotopes. Isotopes of an element have the same number of protons but differ in the number of neutrons. The stability of these isotopes depends on the balance between neutrons and protons. Stable isotopes are those that do not undergo radioactive decay within a short period and are commonly found in nature. While Bromine has two stable isotopes (79Br and 81Br), the ratio of neutrons to protons for a hypothetical 80Br isotope might not be favorable for stability, making such an isotope too short-lived or not existing naturally in significant amounts to be detected or have practical relevance. This means that in practice, or within present scientific knowledge, bromine does not have a stable isotope with a mass number of 80.
Bromine doesn't have an isotope with a mass of 80 because the naturally occurring isotopes of bromine have masses of 78.9183 and 80.9163 amu, respectively. These isotopes, 79Br and 81Br, have abundances of 50.69% and 49.31%, respectively. When calculating the average atomic mass of bromine based on these isotopes and their abundances, we find that the weighted average is 79.904 amu. Therefore, there is no naturally occurring isotope of bromine with a mass of 80 amu.