Calculate average atomic mass in amu from the masses and percent abundances of up to five isotopes, with weighted mass steps shown.

Enter isotope masses in u (amu) and percent abundances totaling 100%. Totals within 0.01 percentage point are accepted and normalized.

Average Atomic Mass Formula

The following formula is used to calculate the average atomic mass of a substance.

AM = f₁M₁ + f₂M₂ + … + fₙMₙ
  • Where AM is the average atomic mass
  • fn is the fractional abundance of isotope n (percentage divided by 100)
  • M is the mass of the isotope

To calculate the average atomic mass, multiply the fractional abundance of each isotope by the mass of the isotope, then sum these values together.

Average Atomic Mass Definition

Average atomic mass is the abundance-weighted mean isotope mass for an element in a specified sample.

How to calculate average atomic mass?

How to calculate average atomic mass?

  1. First, determine each isotope percentage in the sample

    For example, NIST lists representative chlorine abundances of 75.76% chlorine-35 and 24.24% chlorine-37. Their fractional abundances are 0.7576 and 0.2424.

  2. Next, determine the masses of each isotope

    Using the same example, the isotope masses are approximately 34.968852682 u and 36.965902602 u. The integers 35 and 37 are mass numbers, not precise isotope masses.

  3. Finally, calculate the average atomic mass

    The weighted average is 0.7576 × 34.968852682 + 0.2424 × 36.965902602 ≈ 35.452938 u. Natural sample compositions can vary.

FAQ

What is average atomic mass?

Average atomic mass is the abundance-weighted mean isotope mass for an element in a specified sample.

What is fractional abundance?

Fractional abundance is the isotope percentage divided by 100. For example, 75.76% corresponds to 0.7576.