How to Calculate Atomic Weight

Atom Particle
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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 = massa x fracta + massb x fractb

If there were three isotopes, you would add a 'c' entry. If there were four isotopes, you'd 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 0.7577
Cl-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.
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Helmenstine, Anne Marie, Ph.D. "How to Calculate Atomic Weight." ThoughtCo, Apr. 12, 2018, thoughtco.com/atomic-weight-calculation-606080. Helmenstine, Anne Marie, Ph.D. (2018, April 12). How to Calculate Atomic Weight. Retrieved from https://www.thoughtco.com/atomic-weight-calculation-606080 Helmenstine, Anne Marie, Ph.D. "How to Calculate Atomic Weight." ThoughtCo. https://www.thoughtco.com/atomic-weight-calculation-606080 (accessed May 26, 2018).