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How Do You Calculate Weighted Average Atomic Mass?

Published in Atomic Mass Calculation 3 mins read

Calculating the weighted average atomic mass involves considering the relative abundance of each isotope of an element. It's a weighted average because some isotopes are more common than others.

The Calculation

The process is straightforward:

  1. Identify Isotopes and Abundances: Determine all the naturally occurring isotopes of the element and their respective percent abundances. These abundances represent the percentage of each isotope found in a naturally occurring sample of the element.

  2. Convert Percentages to Decimals: Change the percent abundances to decimal form. For example, 25% becomes 0.25.

  3. Multiply and Sum: Multiply the mass of each isotope (in atomic mass units, amu) by its decimal abundance. Then, add up all these products.

The result is the weighted average atomic mass of the element.

Formula:

Weighted Average Atomic Mass = (mass of isotope 1 × abundance of isotope 1) + (mass of isotope 2 × abundance of isotope 2) + ...

Example

Let's say an element has two isotopes:

  • Isotope 1: Mass = 10 amu, Abundance = 80% (0.80)
  • Isotope 2: Mass = 12 amu, Abundance = 20% (0.20)

Weighted Average Atomic Mass = (10 amu × 0.80) + (12 amu × 0.20) = 8 + 2.4 = 10.4 amu

This means the average atomic mass of this element is 10.4 amu. This reflects the fact that the lighter isotope (10 amu) is more abundant, pulling the average closer to its mass.

Resources and Further Learning

Several online resources offer more detailed explanations and examples of calculating weighted average atomic mass:

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