Isotopes & Average Atomic Mass

Select Element Mixture:

Isotope Abundance (%)

Atoms: 50

If a fictional element has Isotope-10 at 80% and Isotope-11 at 20%, what is its average mass?

Avg. Mass = Σ (Isotope Mass × Fractional Abundance)

Average Atomic Mass is a weighted average based on natural abundance.
It rarely equals a whole number because elements exist as a mixture of different isotopes.
To calculate, convert the percentage to a decimal (e.g., 75% → 0.75) before multiplying by the mass.
Try: Drag a slider. Watch how the formula updates, and see the number of colored particles instantly change.