Fractional abundance — Science definition
A percent abundance written as a decimal (percent abundance ÷ 100); it is what you actually multiply by in the average.
Where it is taught
- Isotopes & Average Atomic Mass — Atoms, Electrons & Periodicity, Physical Science: Chemistry, Grade 11
In use
Instead you multiply each isotope's mass by its fractional abundance (its percent abundance written as a decimal) and add the results: average atomic mass = (mass₁ × fraction₁) + (mass₂ × fraction₂) + … A rare isotope barely nudges the total, while an abundant one dominates it.
Isotopes & Average Atomic Mass