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Atomic Weight Calculation

Calculating Atomic Weight of an Element with Isotopes


The electrons have little mass and are not counted in the atomic weight calculation.

The atomic weight is based on the weight of protons and neutrons (nucleons). The electrons have minimal mass.

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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.

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


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.

More About Atomic Mass/Weight Calculations

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