How to Calculate Average Atomic Mass
Follow these steps to compute relative atomic mass from isotopes:
List Isotope Masses
Input exact isotope mass values in atomic mass units (amu).
Enter Percent Abundances
Enter percentage natural abundances for each isotope.
Multiply & Sum
Multiply mass by abundance fraction (fraction = % / 100) and sum products.
Check Periodic Table
Compare resulting weighted average against standard periodic table atomic weight.
Key Features & Principles
Dynamic Multi-Isotope Support
Add unlimited isotope rows for complex elements like Tin (10 stable isotopes).
Abundance Normalization
Automatically normalizes percentage abundances summing to 100%.
Step-by-Step Mathematical Trace
Displays individual product contributions for lab work and homework checks.
General Chemistry Guide: Isotope Abundance & Atomic Weight
The atomic weight listed on the Periodic Table of Elements is rarely an integer because it reflects the weighted mathematical average of all naturally occurring isotopes of that element based on their relative abundances.
For example, natural chlorine consists of ~75.78% Chlorine-35 (mass 34.969 amu) and ~24.22% Chlorine-37 (mass 36.966 amu). The weighted average calculation yields:Atomic Weight = (34.969 × 0.7578) + (36.966 × 0.2422) = 35.453 amu
Frequently Asked Questions
?Why is atomic weight not a whole number?
Atomic weight is a weighted average of multiple isotopes with different neutron counts, and individual isotope masses differ slightly from integer mass numbers due to nuclear binding energy mass defect.