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The average atomic mass of an element represents the weighted average of the masses of its isotopes, considering their natural abundance.

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Here's how to calculate it:

1. Gather information:

  • You'll need the atomic masses (in amu) of each isotope of the element.
  • You'll also need the natural abundance (as a percentage) of each isotope. This represents the percentage of atoms of that isotope found in a naturally occurring sample of the element.

2. Convert abundance to decimal:

  • Percent abundance is usually given as a percentage. You'll need to convert this into a decimal for the calculation. Just divide the percentage by 100 (or move the decimal two places to the left).

3. Weighted average calculation:

  • For each isotope, multiply its atomic mass by its decimal abundance.
  • Add the products obtained for all the isotopes of the element.

Here's the formula:

Average atomic mass = (mass of isotope 1 x abundance of isotope 1) + (mass of isotope 2 x abundance of isotope 2) + ... (and so on for all isotopes)


Let's calculate the average atomic mass of chlorine (Cl), which has two main isotopes:

  • Chlorine-35 (Cl-35):
    • Mass = 34.96 amu
    • Abundance = 75.76% (or 0.7576 in decimal)
  • Chlorine-37 (Cl-37):
    • Mass = 36.97 amu
    • Abundance = 24.24% (or 0.2424 in decimal)


Average atomic mass (Cl) = (34.96 amu x 0.7576) + (36.97 amu x 0.2424) = 26.72 amu + 8.94 amu = 35.66 amu

Therefore, the average atomic mass of chlorine is approximately 35.66 amu.


  • You can find atomic masses and isotopic abundances in various chemistry resources, including periodic tables (sometimes listed), handbooks, and online databases.
  • If you're unsure about the number of isotopes for an element, refer to a periodic table or chemistry reference
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