The atomic weight of an element depends on the abundance of its isotopes. If you know the mass of the isotopes and the fractional abundance of the isotopes, you can calculate the element's atomic weight. The atomic weight is calculated by adding the mass of each isotope multiplied by its fractional abundance. For example, for an element with 2 isotopes:

atomic weight = mass_{a} x fract_{a} + mass_{b} x fract_{b}

If there were three isotopes, you would add a 'c' entry. If there were four isotopes, you'd add a 'd', etc.

If chlorine has two naturally-occurring isotopes where:

Cl-35 mass is 34.968852 and fract is 0.7577

Cl-37 mass is 36.965303 and fract is 0.2423

atomic weight = mass_{a} x fract_{a} + mass_{b} x frac_{b}

atomic weight = 34.968852 x 0.7577 + 36.965303 x 0.2423

atomic weight = 26.496 amu + 8.9566 amu

atomic weight = 35.45 amu

**Tips**

The sum of the fractional abundance values must equal 1.

Be sure to use the mass or weight of each isotope and not its mass number.