# 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%.

May 10, 2014

The average atomic mass of Br is 79.91 u.

There are two ways to solve this problem.

Method 1

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 = $\left(799 100 \text{ u")/(10 000" atoms}\right)$ = 79.91 u/atom

Method 2

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.