The average atomic mass of an element is the sum of the masses of its isotopes, each multiplied by its natural abundance.

Average atomic mass can be found on the periodic table.

**Formula to calculate average atomic mass.**

**Example:**

Consider the chlorine isotopes, chlorine-35 has a mass of 34.969 , while chlorine-37 has a mass of 36.966 amu, if their natural abundance is 75.77% and 24.23% respectively, calculate their average atomic mass.

**Chlorine – 35**

**= 34.969 x 0.7577**

**= 26.50**

**Chlorine – 37**

**= 36.966 x 0.2423**

**= 8.957**

**Atomic Average Mass**

**= 26.50 + 8.957**

**= 35.457**

Therefore, your atomic average mass is 34.457 amu.