Final answer:
Bromine can have an atomic mass close to 80 amu without a mass number of 80 due to the weighted average of its isotopes' masses and abundances. Br-81 has a percent abundance of 49.31%. The mass of Br-79 is 78.9183 amu and the atomic mass of bromine is calculated using these values to be around 79.904 amu.
Step-by-step explanation:
To understand how bromine can have an atomic mass close to 80 amu without having an isotope with a mass number of 80, we need to consider the concept of weighted average atomic mass. Even if an element does not have an isotope with a mass number equal to the average atomic mass, the average can still be close to that number if the abundances and masses of the existing isotopes produce a weighted average close to that value.
If Br-79 has a percent abundance of 50.69%, the percent abundance of Br-81 must be the remainder of 100% since these are the only two isotopes, which is 100% - 50.69% = 49.31%. Using the given mass of Br-81 (80.9163 amu), we can calculate the mass of Br-79.
The atomic mass of bromine is calculated using the weighted average formula:
((mass of isotope 1 × abundance of isotope 1) + (mass of isotope 2 × abundance of isotope 2)) / 100. Based on the information given, we can substitute the values:
((78.9183 amu × 50.69%) + (80.9163 amu × 49.31%)) / 100. This shows that the atomic mass is indeed around 79.904 amu.