How to Calculate Atomic Weight

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 elements atomic weight in atomic mass units (expressed as u, Da, or amu). 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 massa x fracta massb x fractb If there were three isotopes, you would add a c entry. If there were four isotopes, youd add a d, etc. Atomic Weight Calculation Example If chlorine has two naturally-occurring isotopes where: Cl-35 mass is 34.968852 and fract is 0.7577Cl-37 mass is 36.965303 and fract is 0.2423 atomic weight massa x fracta massb x fracb 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 for Calculating Atomic Weight 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.