The atomic mass shown on a periodic table is a weighted average across an element’s naturally occurring isotopes, weighted by how common each one is.
How it works
Each isotope’s mass is multiplied by its fractional abundance, and those weighted values are summed and normalized by the total abundance given.
What this does not include
This does not include elements with more than two stable isotopes — this calculator handles the common two-isotope case directly.
How to use this calculator
- Enter each isotope’s mass and its percentage abundance.
A worked example
Chlorine-35 (mass 34.96885, abundance 75.76%) and chlorine-37 (mass 36.9659, abundance 24.24%): weighted average atomic mass = 35.4529 — very close to chlorine’s standard periodic table value of 35.45.
What the variables mean
| Variable | Meaning |
|---|---|
| Mass 1, Mass 2 | The atomic mass of each isotope |
| Abundance 1, Abundance 2 | Each isotope’s natural percentage abundance |
Edge cases worth knowing
This is a weighted average, not a simple average. A more abundant isotope pulls the result closer to its own mass — which is exactly why chlorine’s atomic weight (35.45) sits much closer to chlorine-35’s mass than to chlorine-37’s.
Zero total abundance makes the average undefined — there’s no meaningful weighting to apply if neither isotope is present at all.
Frequently asked questions
Why is chlorine’s atomic mass 35.45 instead of a whole number?
Because it’s a weighted average of chlorine-35 (about 76% abundant) and chlorine-37 (about 24% abundant), and the ratio lands closer to 35 than to a round number.
Where do isotope abundance values come from?
They’re measured experimentally and published in standard periodic table references, reflecting how each isotope actually occurs in nature.
Do the abundances need to add up to exactly 100%?
The calculator normalizes by whatever total is given, but real isotope abundances for an element are measured to sum to 100%.
