What is the average atomic mass of bromine? One isotope of bromine has an atomic mass of 78.92amu and a relative abundance of 50.69%. The other major isotope of bromine has an atomic mass of 80.92amu and a relative abundance of 49.31%.
The average atomic mass of Br is 79.91 u.
There are two ways to solve this problem.
Let's assume we have 10 000 atoms of Br. Then 5069 atoms will have mass 78.92 u, and 4931 atoms will have mass 80.92 u.
Mass of the 5069 atoms = 5069 × 78.92 u = 400 000 u
Mass of the 4931 atoms = 4931 × 80.92 u = 399 000 u
Mass of 10 000 atoms = 799 100 u
Average mass of an atom =
The average mass is the mass of each isotope multiplied by its percentage. Thus,
Average mass = 50.69 % × 78.92 u + 49.31 % × 80.92 u = 40.00 u + 39.90 u = 79.91 u
The second method is easier, but it doesn't explain intuitively why the answer is the average atomic mass.
Here is a video which summarizes the steps needed to calculate average atomic mass.